use crate::back::write::{
    self, save_temp_bitcode, to_llvm_opt_settings, with_llvm_pmb, DiagnosticHandlers,
};
use crate::llvm::archive_ro::ArchiveRO;
use crate::llvm::{self, False, True};
use crate::{LlvmCodegenBackend, ModuleLlvm};
use log::{debug, info};
use rustc_codegen_ssa::back::lto::{LtoModuleCodegen, SerializedModule, ThinModule, ThinShared};
use rustc_codegen_ssa::back::symbol_export;
use rustc_codegen_ssa::back::write::{CodegenContext, FatLTOInput, ModuleConfig};
use rustc_codegen_ssa::traits::*;
use rustc_codegen_ssa::{looks_like_rust_object_file, ModuleCodegen, ModuleKind};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_errors::{FatalError, Handler};
use rustc_hir::def_id::LOCAL_CRATE;
use rustc_middle::bug;
use rustc_middle::dep_graph::WorkProduct;
use rustc_middle::middle::exported_symbols::SymbolExportLevel;
use rustc_session::cgu_reuse_tracker::CguReuse;
use rustc_session::config::{self, Lto};

use std::ffi::{CStr, CString};
use std::fs::File;
use std::io;
use std::mem;
use std::path::Path;
use std::ptr;
use std::slice;
use std::sync::Arc;

/// We keep track of past LTO imports that were used to produce the current set
/// of compiled object files that we might choose to reuse during this
/// compilation session.
pub const THIN_LTO_IMPORTS_INCR_COMP_FILE_NAME: &str = "thin-lto-past-imports.bin";

pub fn crate_type_allows_lto(crate_type: config::CrateType) -> bool {
    match crate_type {
        config::CrateType::Executable
        | config::CrateType::Staticlib
        | config::CrateType::Cdylib => true,

        config::CrateType::Dylib | config::CrateType::Rlib | config::CrateType::ProcMacro => false,
    }
}

fn prepare_lto(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    diag_handler: &Handler,
) -> Result<(Vec<CString>, Vec<(SerializedModule<ModuleBuffer>, CString)>), FatalError> {
    let export_threshold = match cgcx.lto {
        // We're just doing LTO for our one crate
        Lto::ThinLocal => SymbolExportLevel::Rust,

        // We're doing LTO for the entire crate graph
        Lto::Fat | Lto::Thin => symbol_export::crates_export_threshold(&cgcx.crate_types),

        Lto::No => panic!("didn't request LTO but we're doing LTO"),
    };

    let symbol_filter = &|&(ref name, level): &(String, SymbolExportLevel)| {
        if level.is_below_threshold(export_threshold) {
            Some(CString::new(name.as_str()).unwrap())
        } else {
            None
        }
    };
    let exported_symbols = cgcx.exported_symbols.as_ref().expect("needs exported symbols for LTO");
    let mut symbol_white_list = {
        let _timer = cgcx.prof.generic_activity("LLVM_lto_generate_symbol_white_list");
        exported_symbols[&LOCAL_CRATE].iter().filter_map(symbol_filter).collect::<Vec<CString>>()
    };
    info!("{} symbols to preserve in this crate", symbol_white_list.len());

    // If we're performing LTO for the entire crate graph, then for each of our
    // upstream dependencies, find the corresponding rlib and load the bitcode
    // from the archive.
    //
    // We save off all the bytecode and LLVM module ids for later processing
    // with either fat or thin LTO
    let mut upstream_modules = Vec::new();
    if cgcx.lto != Lto::ThinLocal {
        if cgcx.opts.cg.prefer_dynamic {
            diag_handler
                .struct_err("cannot prefer dynamic linking when performing LTO")
                .note(
                    "only 'staticlib', 'bin', and 'cdylib' outputs are \
                               supported with LTO",
                )
                .emit();
            return Err(FatalError);
        }

        // Make sure we actually can run LTO
        for crate_type in cgcx.crate_types.iter() {
            if !crate_type_allows_lto(*crate_type) {
                let e = diag_handler.fatal(
                    "lto can only be run for executables, cdylibs and \
                                            static library outputs",
                );
                return Err(e);
            }
        }

        for &(cnum, ref path) in cgcx.each_linked_rlib_for_lto.iter() {
            let exported_symbols =
                cgcx.exported_symbols.as_ref().expect("needs exported symbols for LTO");
            {
                let _timer = cgcx.prof.generic_activity("LLVM_lto_generate_symbol_white_list");
                symbol_white_list.extend(exported_symbols[&cnum].iter().filter_map(symbol_filter));
            }

            let archive = ArchiveRO::open(&path).expect("wanted an rlib");
            let obj_files = archive
                .iter()
                .filter_map(|child| child.ok().and_then(|c| c.name().map(|name| (name, c))))
                .filter(|&(name, _)| looks_like_rust_object_file(name));
            for (name, child) in obj_files {
                info!("adding bitcode from {}", name);
                match get_bitcode_slice_from_object_data(child.data()) {
                    Ok(data) => {
                        let module = SerializedModule::FromRlib(data.to_vec());
                        upstream_modules.push((module, CString::new(name).unwrap()));
                    }
                    Err(msg) => return Err(diag_handler.fatal(&msg)),
                }
            }
        }
    }

    Ok((symbol_white_list, upstream_modules))
}

fn get_bitcode_slice_from_object_data(obj: &[u8]) -> Result<&[u8], String> {
    let mut len = 0;
    let data =
        unsafe { llvm::LLVMRustGetBitcodeSliceFromObjectData(obj.as_ptr(), obj.len(), &mut len) };
    if !data.is_null() {
        assert!(len != 0);
        let bc = unsafe { slice::from_raw_parts(data, len) };

        // `bc` must be a sub-slice of `obj`.
        assert!(obj.as_ptr() <= bc.as_ptr());
        assert!(bc[bc.len()..bc.len()].as_ptr() <= obj[obj.len()..obj.len()].as_ptr());

        Ok(bc)
    } else {
        assert!(len == 0);
        let msg = llvm::last_error().unwrap_or_else(|| "unknown LLVM error".to_string());
        Err(format!("failed to get bitcode from object file for LTO ({})", msg))
    }
}

/// Performs fat LTO by merging all modules into a single one and returning it
/// for further optimization.
pub(crate) fn run_fat(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    modules: Vec<FatLTOInput<LlvmCodegenBackend>>,
    cached_modules: Vec<(SerializedModule<ModuleBuffer>, WorkProduct)>,
) -> Result<LtoModuleCodegen<LlvmCodegenBackend>, FatalError> {
    let diag_handler = cgcx.create_diag_handler();
    let (symbol_white_list, upstream_modules) = prepare_lto(cgcx, &diag_handler)?;
    let symbol_white_list = symbol_white_list.iter().map(|c| c.as_ptr()).collect::<Vec<_>>();
    fat_lto(cgcx, &diag_handler, modules, cached_modules, upstream_modules, &symbol_white_list)
}

/// Performs thin LTO by performing necessary global analysis and returning two
/// lists, one of the modules that need optimization and another for modules that
/// can simply be copied over from the incr. comp. cache.
pub(crate) fn run_thin(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    modules: Vec<(String, ThinBuffer)>,
    cached_modules: Vec<(SerializedModule<ModuleBuffer>, WorkProduct)>,
) -> Result<(Vec<LtoModuleCodegen<LlvmCodegenBackend>>, Vec<WorkProduct>), FatalError> {
    let diag_handler = cgcx.create_diag_handler();
    let (symbol_white_list, upstream_modules) = prepare_lto(cgcx, &diag_handler)?;
    let symbol_white_list = symbol_white_list.iter().map(|c| c.as_ptr()).collect::<Vec<_>>();
    if cgcx.opts.cg.linker_plugin_lto.enabled() {
        unreachable!(
            "We should never reach this case if the LTO step \
                      is deferred to the linker"
        );
    }
    thin_lto(cgcx, &diag_handler, modules, upstream_modules, cached_modules, &symbol_white_list)
}

pub(crate) fn prepare_thin(module: ModuleCodegen<ModuleLlvm>) -> (String, ThinBuffer) {
    let name = module.name.clone();
    let buffer = ThinBuffer::new(module.module_llvm.llmod());
    (name, buffer)
}

fn fat_lto(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    diag_handler: &Handler,
    modules: Vec<FatLTOInput<LlvmCodegenBackend>>,
    cached_modules: Vec<(SerializedModule<ModuleBuffer>, WorkProduct)>,
    mut serialized_modules: Vec<(SerializedModule<ModuleBuffer>, CString)>,
    symbol_white_list: &[*const libc::c_char],
) -> Result<LtoModuleCodegen<LlvmCodegenBackend>, FatalError> {
    let _timer = cgcx.prof.generic_activity("LLVM_fat_lto_build_monolithic_module");
    info!("going for a fat lto");

    // Sort out all our lists of incoming modules into two lists.
    //
    // * `serialized_modules` (also and argument to this function) contains all
    //   modules that are serialized in-memory.
    // * `in_memory` contains modules which are already parsed and in-memory,
    //   such as from multi-CGU builds.
    //
    // All of `cached_modules` (cached from previous incremental builds) can
    // immediately go onto the `serialized_modules` modules list and then we can
    // split the `modules` array into these two lists.
    let mut in_memory = Vec::new();
    serialized_modules.extend(cached_modules.into_iter().map(|(buffer, wp)| {
        info!("pushing cached module {:?}", wp.cgu_name);
        (buffer, CString::new(wp.cgu_name).unwrap())
    }));
    for module in modules {
        match module {
            FatLTOInput::InMemory(m) => in_memory.push(m),
            FatLTOInput::Serialized { name, buffer } => {
                info!("pushing serialized module {:?}", name);
                let buffer = SerializedModule::Local(buffer);
                serialized_modules.push((buffer, CString::new(name).unwrap()));
            }
        }
    }

    // Find the "costliest" module and merge everything into that codegen unit.
    // All the other modules will be serialized and reparsed into the new
    // context, so this hopefully avoids serializing and parsing the largest
    // codegen unit.
    //
    // Additionally use a regular module as the base here to ensure that various
    // file copy operations in the backend work correctly. The only other kind
    // of module here should be an allocator one, and if your crate is smaller
    // than the allocator module then the size doesn't really matter anyway.
    let costliest_module = in_memory
        .iter()
        .enumerate()
        .filter(|&(_, module)| module.kind == ModuleKind::Regular)
        .map(|(i, module)| {
            let cost = unsafe { llvm::LLVMRustModuleCost(module.module_llvm.llmod()) };
            (cost, i)
        })
        .max();

    // If we found a costliest module, we're good to go. Otherwise all our
    // inputs were serialized which could happen in the case, for example, that
    // all our inputs were incrementally reread from the cache and we're just
    // re-executing the LTO passes. If that's the case deserialize the first
    // module and create a linker with it.
    let module: ModuleCodegen<ModuleLlvm> = match costliest_module {
        Some((_cost, i)) => in_memory.remove(i),
        None => {
            assert!(!serialized_modules.is_empty(), "must have at least one serialized module");
            let (buffer, name) = serialized_modules.remove(0);
            info!("no in-memory regular modules to choose from, parsing {:?}", name);
            ModuleCodegen {
                module_llvm: ModuleLlvm::parse(cgcx, &name, buffer.data(), diag_handler)?,
                name: name.into_string().unwrap(),
                kind: ModuleKind::Regular,
            }
        }
    };
    let mut serialized_bitcode = Vec::new();
    {
        let (llcx, llmod) = {
            let llvm = &module.module_llvm;
            (&llvm.llcx, llvm.llmod())
        };
        info!("using {:?} as a base module", module.name);

        // The linking steps below may produce errors and diagnostics within LLVM
        // which we'd like to handle and print, so set up our diagnostic handlers
        // (which get unregistered when they go out of scope below).
        let _handler = DiagnosticHandlers::new(cgcx, diag_handler, llcx);

        // For all other modules we codegened we'll need to link them into our own
        // bitcode. All modules were codegened in their own LLVM context, however,
        // and we want to move everything to the same LLVM context. Currently the
        // way we know of to do that is to serialize them to a string and them parse
        // them later. Not great but hey, that's why it's "fat" LTO, right?
        for module in in_memory {
            let buffer = ModuleBuffer::new(module.module_llvm.llmod());
            let llmod_id = CString::new(&module.name[..]).unwrap();
            serialized_modules.push((SerializedModule::Local(buffer), llmod_id));
        }
        // Sort the modules to ensure we produce deterministic results.
        serialized_modules.sort_by(|module1, module2| module1.1.cmp(&module2.1));

        // For all serialized bitcode files we parse them and link them in as we did
        // above, this is all mostly handled in C++. Like above, though, we don't
        // know much about the memory management here so we err on the side of being
        // save and persist everything with the original module.
        let mut linker = Linker::new(llmod);
        for (bc_decoded, name) in serialized_modules {
            let _timer = cgcx
                .prof
                .generic_activity_with_arg("LLVM_fat_lto_link_module", format!("{:?}", name));
            info!("linking {:?}", name);
            let data = bc_decoded.data();
            linker.add(&data).map_err(|()| {
                let msg = format!("failed to load bc of {:?}", name);
                write::llvm_err(&diag_handler, &msg)
            })?;
            serialized_bitcode.push(bc_decoded);
        }
        drop(linker);
        save_temp_bitcode(&cgcx, &module, "lto.input");

        // Internalize everything that *isn't* in our whitelist to help strip out
        // more modules and such
        unsafe {
            let ptr = symbol_white_list.as_ptr();
            llvm::LLVMRustRunRestrictionPass(
                llmod,
                ptr as *const *const libc::c_char,
                symbol_white_list.len() as libc::size_t,
            );
            save_temp_bitcode(&cgcx, &module, "lto.after-restriction");
        }

        if cgcx.no_landing_pads {
            unsafe {
                llvm::LLVMRustMarkAllFunctionsNounwind(llmod);
            }
            save_temp_bitcode(&cgcx, &module, "lto.after-nounwind");
        }
    }

    Ok(LtoModuleCodegen::Fat { module: Some(module), _serialized_bitcode: serialized_bitcode })
}

struct Linker<'a>(&'a mut llvm::Linker<'a>);

impl Linker<'a> {
    fn new(llmod: &'a llvm::Module) -> Self {
        unsafe { Linker(llvm::LLVMRustLinkerNew(llmod)) }
    }

    fn add(&mut self, bytecode: &[u8]) -> Result<(), ()> {
        unsafe {
            if llvm::LLVMRustLinkerAdd(
                self.0,
                bytecode.as_ptr() as *const libc::c_char,
                bytecode.len(),
            ) {
                Ok(())
            } else {
                Err(())
            }
        }
    }
}

impl Drop for Linker<'a> {
    fn drop(&mut self) {
        unsafe {
            llvm::LLVMRustLinkerFree(&mut *(self.0 as *mut _));
        }
    }
}

/// Prepare "thin" LTO to get run on these modules.
///
/// The general structure of ThinLTO is quite different from the structure of
/// "fat" LTO above. With "fat" LTO all LLVM modules in question are merged into
/// one giant LLVM module, and then we run more optimization passes over this
/// big module after internalizing most symbols. Thin LTO, on the other hand,
/// avoid this large bottleneck through more targeted optimization.
///
/// At a high level Thin LTO looks like:
///
///     1. Prepare a "summary" of each LLVM module in question which describes
///        the values inside, cost of the values, etc.
///     2. Merge the summaries of all modules in question into one "index"
///     3. Perform some global analysis on this index
///     4. For each module, use the index and analysis calculated previously to
///        perform local transformations on the module, for example inlining
///        small functions from other modules.
///     5. Run thin-specific optimization passes over each module, and then code
///        generate everything at the end.
///
/// The summary for each module is intended to be quite cheap, and the global
/// index is relatively quite cheap to create as well. As a result, the goal of
/// ThinLTO is to reduce the bottleneck on LTO and enable LTO to be used in more
/// situations. For example one cheap optimization is that we can parallelize
/// all codegen modules, easily making use of all the cores on a machine.
///
/// With all that in mind, the function here is designed at specifically just
/// calculating the *index* for ThinLTO. This index will then be shared amongst
/// all of the `LtoModuleCodegen` units returned below and destroyed once
/// they all go out of scope.
fn thin_lto(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    diag_handler: &Handler,
    modules: Vec<(String, ThinBuffer)>,
    serialized_modules: Vec<(SerializedModule<ModuleBuffer>, CString)>,
    cached_modules: Vec<(SerializedModule<ModuleBuffer>, WorkProduct)>,
    symbol_white_list: &[*const libc::c_char],
) -> Result<(Vec<LtoModuleCodegen<LlvmCodegenBackend>>, Vec<WorkProduct>), FatalError> {
    let _timer = cgcx.prof.generic_activity("LLVM_thin_lto_global_analysis");
    unsafe {
        info!("going for that thin, thin LTO");

        let green_modules: FxHashMap<_, _> =
            cached_modules.iter().map(|&(_, ref wp)| (wp.cgu_name.clone(), wp.clone())).collect();

        let full_scope_len = modules.len() + serialized_modules.len() + cached_modules.len();
        let mut thin_buffers = Vec::with_capacity(modules.len());
        let mut module_names = Vec::with_capacity(full_scope_len);
        let mut thin_modules = Vec::with_capacity(full_scope_len);

        for (i, (name, buffer)) in modules.into_iter().enumerate() {
            info!("local module: {} - {}", i, name);
            let cname = CString::new(name.clone()).unwrap();
            thin_modules.push(llvm::ThinLTOModule {
                identifier: cname.as_ptr(),
                data: buffer.data().as_ptr(),
                len: buffer.data().len(),
            });
            thin_buffers.push(buffer);
            module_names.push(cname);
        }

        // FIXME: All upstream crates are deserialized internally in the
        //        function below to extract their summary and modules. Note that
        //        unlike the loop above we *must* decode and/or read something
        //        here as these are all just serialized files on disk. An
        //        improvement, however, to make here would be to store the
        //        module summary separately from the actual module itself. Right
        //        now this is store in one large bitcode file, and the entire
        //        file is deflate-compressed. We could try to bypass some of the
        //        decompression by storing the index uncompressed and only
        //        lazily decompressing the bytecode if necessary.
        //
        //        Note that truly taking advantage of this optimization will
        //        likely be further down the road. We'd have to implement
        //        incremental ThinLTO first where we could actually avoid
        //        looking at upstream modules entirely sometimes (the contents,
        //        we must always unconditionally look at the index).
        let mut serialized = Vec::with_capacity(serialized_modules.len() + cached_modules.len());

        let cached_modules =
            cached_modules.into_iter().map(|(sm, wp)| (sm, CString::new(wp.cgu_name).unwrap()));

        for (module, name) in serialized_modules.into_iter().chain(cached_modules) {
            info!("upstream or cached module {:?}", name);
            thin_modules.push(llvm::ThinLTOModule {
                identifier: name.as_ptr(),
                data: module.data().as_ptr(),
                len: module.data().len(),
            });
            serialized.push(module);
            module_names.push(name);
        }

        // Sanity check
        assert_eq!(thin_modules.len(), module_names.len());

        // Delegate to the C++ bindings to create some data here. Once this is a
        // tried-and-true interface we may wish to try to upstream some of this
        // to LLVM itself, right now we reimplement a lot of what they do
        // upstream...
        let data = llvm::LLVMRustCreateThinLTOData(
            thin_modules.as_ptr(),
            thin_modules.len() as u32,
            symbol_white_list.as_ptr(),
            symbol_white_list.len() as u32,
        )
        .ok_or_else(|| write::llvm_err(&diag_handler, "failed to prepare thin LTO context"))?;

        info!("thin LTO data created");

        let (import_map_path, prev_import_map, curr_import_map) =
            if let Some(ref incr_comp_session_dir) = cgcx.incr_comp_session_dir {
                let path = incr_comp_session_dir.join(THIN_LTO_IMPORTS_INCR_COMP_FILE_NAME);
                // If previous imports have been deleted, or we get an IO error
                // reading the file storing them, then we'll just use `None` as the
                // prev_import_map, which will force the code to be recompiled.
                let prev = if path.exists() {
                    ThinLTOImportMaps::load_from_file(&path).ok()
                } else {
                    None
                };
                let curr = ThinLTOImportMaps::from_thin_lto_data(data);
                (Some(path), prev, curr)
            } else {
                // If we don't compile incrementally, we don't need to load the
                // import data from LLVM.
                assert!(green_modules.is_empty());
                let curr = ThinLTOImportMaps::default();
                (None, None, curr)
            };
        info!("thin LTO import map loaded");

        let data = ThinData(data);

        // Throw our data in an `Arc` as we'll be sharing it across threads. We
        // also put all memory referenced by the C++ data (buffers, ids, etc)
        // into the arc as well. After this we'll create a thin module
        // codegen per module in this data.
        let shared = Arc::new(ThinShared {
            data,
            thin_buffers,
            serialized_modules: serialized,
            module_names,
        });

        let mut copy_jobs = vec![];
        let mut opt_jobs = vec![];

        info!("checking which modules can be-reused and which have to be re-optimized.");
        for (module_index, module_name) in shared.module_names.iter().enumerate() {
            let module_name = module_name_to_str(module_name);

            // If (1.) the module hasn't changed, and (2.) none of the modules
            // it imports from have changed, *and* (3.) the import and export
            // sets themselves have not changed from the previous compile when
            // it was last ThinLTO'ed, then we can re-use the post-ThinLTO
            // version of the module. Otherwise, freshly perform LTO
            // optimization.
            //
            // (Note that globally, the export set is just the inverse of the
            // import set.)
            //
            // For further justification of why the above is necessary and sufficient,
            // see the LLVM blog post on ThinLTO:
            //
            // http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
            //
            // which states the following:
            //
            // ```quote
            // any particular ThinLTO backend must be redone iff:
            //
            // 1. The corresponding (primary) module’s bitcode changed
            // 2. The list of imports into or exports from the module changed
            // 3. The bitcode for any module being imported from has changed
            // 4. Any global analysis result affecting either the primary module
            //    or anything it imports has changed.
            // ```
            //
            // This strategy means we can always save the computed imports as
            // canon: when we reuse the post-ThinLTO version, condition (3.)
            // ensures that the current import set is the same as the previous
            // one. (And of course, when we don't reuse the post-ThinLTO
            // version, the current import set *is* the correct one, since we
            // are doing the ThinLTO in this current compilation cycle.)
            //
            // For more discussion, see rust-lang/rust#59535 (where the import
            // issue was discovered) and rust-lang/rust#69798 (where the
            // analogous export issue was discovered).
            if let (Some(prev_import_map), true) =
                (prev_import_map.as_ref(), green_modules.contains_key(module_name))
            {
                assert!(cgcx.incr_comp_session_dir.is_some());

                let prev_imports = prev_import_map.imports_of(module_name);
                let curr_imports = curr_import_map.imports_of(module_name);
                let prev_exports = prev_import_map.exports_of(module_name);
                let curr_exports = curr_import_map.exports_of(module_name);
                let imports_all_green = curr_imports
                    .iter()
                    .all(|imported_module| green_modules.contains_key(imported_module));
                if imports_all_green
                    && equivalent_as_sets(prev_imports, curr_imports)
                    && equivalent_as_sets(prev_exports, curr_exports)
                {
                    let work_product = green_modules[module_name].clone();
                    copy_jobs.push(work_product);
                    info!(" - {}: re-used", module_name);
                    assert!(cgcx.incr_comp_session_dir.is_some());
                    cgcx.cgu_reuse_tracker.set_actual_reuse(module_name, CguReuse::PostLto);
                    continue;
                }
            }

            info!(" - {}: re-compiled", module_name);
            opt_jobs.push(LtoModuleCodegen::Thin(ThinModule {
                shared: shared.clone(),
                idx: module_index,
            }));
        }

        // Save the current ThinLTO import information for the next compilation
        // session, overwriting the previous serialized imports (if any).
        if let Some(path) = import_map_path {
            if let Err(err) = curr_import_map.save_to_file(&path) {
                let msg = format!("Error while writing ThinLTO import data: {}", err);
                return Err(write::llvm_err(&diag_handler, &msg));
            }
        }

        Ok((opt_jobs, copy_jobs))
    }
}

/// Given two slices, each with no repeat elements. returns true if and only if
/// the two slices have the same contents when considered as sets (i.e. when
/// element order is disregarded).
fn equivalent_as_sets(a: &[String], b: &[String]) -> bool {
    // cheap path: unequal lengths means cannot possibly be set equivalent.
    if a.len() != b.len() {
        return false;
    }
    // fast path: before building new things, check if inputs are equivalent as is.
    if a == b {
        return true;
    }
    // slow path: general set comparison.
    let a: FxHashSet<&str> = a.iter().map(|s| s.as_str()).collect();
    let b: FxHashSet<&str> = b.iter().map(|s| s.as_str()).collect();
    a == b
}

pub(crate) fn run_pass_manager(
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    module: &ModuleCodegen<ModuleLlvm>,
    config: &ModuleConfig,
    thin: bool,
) {
    let _timer = cgcx.prof.extra_verbose_generic_activity("LLVM_lto_optimize", &module.name[..]);

    // Now we have one massive module inside of llmod. Time to run the
    // LTO-specific optimization passes that LLVM provides.
    //
    // This code is based off the code found in llvm's LTO code generator:
    //      tools/lto/LTOCodeGenerator.cpp
    debug!("running the pass manager");
    unsafe {
        if write::should_use_new_llvm_pass_manager(config) {
            let opt_stage = if thin { llvm::OptStage::ThinLTO } else { llvm::OptStage::FatLTO };
            let opt_level = config.opt_level.unwrap_or(config::OptLevel::No);
            // See comment below for why this is necessary.
            let opt_level = if let config::OptLevel::No = opt_level {
                config::OptLevel::Less
            } else {
                opt_level
            };
            write::optimize_with_new_llvm_pass_manager(cgcx, module, config, opt_level, opt_stage);
            debug!("lto done");
            return;
        }

        let pm = llvm::LLVMCreatePassManager();
        llvm::LLVMAddAnalysisPasses(module.module_llvm.tm, pm);

        if config.verify_llvm_ir {
            let pass = llvm::LLVMRustFindAndCreatePass("verify\0".as_ptr().cast());
            llvm::LLVMRustAddPass(pm, pass.unwrap());
        }

        // When optimizing for LTO we don't actually pass in `-O0`, but we force
        // it to always happen at least with `-O1`.
        //
        // With ThinLTO we mess around a lot with symbol visibility in a way
        // that will actually cause linking failures if we optimize at O0 which
        // notable is lacking in dead code elimination. To ensure we at least
        // get some optimizations and correctly link we forcibly switch to `-O1`
        // to get dead code elimination.
        //
        // Note that in general this shouldn't matter too much as you typically
        // only turn on ThinLTO when you're compiling with optimizations
        // otherwise.
        let opt_level = config
            .opt_level
            .map(|x| to_llvm_opt_settings(x).0)
            .unwrap_or(llvm::CodeGenOptLevel::None);
        let opt_level = match opt_level {
            llvm::CodeGenOptLevel::None => llvm::CodeGenOptLevel::Less,
            level => level,
        };
        with_llvm_pmb(module.module_llvm.llmod(), config, opt_level, false, &mut |b| {
            if thin {
                llvm::LLVMRustPassManagerBuilderPopulateThinLTOPassManager(b, pm);
            } else {
                llvm::LLVMPassManagerBuilderPopulateLTOPassManager(
                    b, pm, /* Internalize = */ False, /* RunInliner = */ True,
                );
            }
        });

        // We always generate bitcode through ThinLTOBuffers,
        // which do not support anonymous globals
        if config.bitcode_needed() {
            let pass = llvm::LLVMRustFindAndCreatePass("name-anon-globals\0".as_ptr().cast());
            llvm::LLVMRustAddPass(pm, pass.unwrap());
        }

        if config.verify_llvm_ir {
            let pass = llvm::LLVMRustFindAndCreatePass("verify\0".as_ptr().cast());
            llvm::LLVMRustAddPass(pm, pass.unwrap());
        }

        llvm::LLVMRunPassManager(pm, module.module_llvm.llmod());

        llvm::LLVMDisposePassManager(pm);
    }
    debug!("lto done");
}

pub struct ModuleBuffer(&'static mut llvm::ModuleBuffer);

unsafe impl Send for ModuleBuffer {}
unsafe impl Sync for ModuleBuffer {}

impl ModuleBuffer {
    pub fn new(m: &llvm::Module) -> ModuleBuffer {
        ModuleBuffer(unsafe { llvm::LLVMRustModuleBufferCreate(m) })
    }
}

impl ModuleBufferMethods for ModuleBuffer {
    fn data(&self) -> &[u8] {
        unsafe {
            let ptr = llvm::LLVMRustModuleBufferPtr(self.0);
            let len = llvm::LLVMRustModuleBufferLen(self.0);
            slice::from_raw_parts(ptr, len)
        }
    }
}

impl Drop for ModuleBuffer {
    fn drop(&mut self) {
        unsafe {
            llvm::LLVMRustModuleBufferFree(&mut *(self.0 as *mut _));
        }
    }
}

pub struct ThinData(&'static mut llvm::ThinLTOData);

unsafe impl Send for ThinData {}
unsafe impl Sync for ThinData {}

impl Drop for ThinData {
    fn drop(&mut self) {
        unsafe {
            llvm::LLVMRustFreeThinLTOData(&mut *(self.0 as *mut _));
        }
    }
}

pub struct ThinBuffer(&'static mut llvm::ThinLTOBuffer);

unsafe impl Send for ThinBuffer {}
unsafe impl Sync for ThinBuffer {}

impl ThinBuffer {
    pub fn new(m: &llvm::Module) -> ThinBuffer {
        unsafe {
            let buffer = llvm::LLVMRustThinLTOBufferCreate(m);
            ThinBuffer(buffer)
        }
    }
}

impl ThinBufferMethods for ThinBuffer {
    fn data(&self) -> &[u8] {
        unsafe {
            let ptr = llvm::LLVMRustThinLTOBufferPtr(self.0) as *const _;
            let len = llvm::LLVMRustThinLTOBufferLen(self.0);
            slice::from_raw_parts(ptr, len)
        }
    }
}

impl Drop for ThinBuffer {
    fn drop(&mut self) {
        unsafe {
            llvm::LLVMRustThinLTOBufferFree(&mut *(self.0 as *mut _));
        }
    }
}

pub unsafe fn optimize_thin_module(
    thin_module: &mut ThinModule<LlvmCodegenBackend>,
    cgcx: &CodegenContext<LlvmCodegenBackend>,
) -> Result<ModuleCodegen<ModuleLlvm>, FatalError> {
    let diag_handler = cgcx.create_diag_handler();
    let tm = (cgcx.tm_factory.0)().map_err(|e| write::llvm_err(&diag_handler, &e))?;

    // Right now the implementation we've got only works over serialized
    // modules, so we create a fresh new LLVM context and parse the module
    // into that context. One day, however, we may do this for upstream
    // crates but for locally codegened modules we may be able to reuse
    // that LLVM Context and Module.
    let llcx = llvm::LLVMRustContextCreate(cgcx.fewer_names);
    let llmod_raw = parse_module(
        llcx,
        &thin_module.shared.module_names[thin_module.idx],
        thin_module.data(),
        &diag_handler,
    )? as *const _;
    let module = ModuleCodegen {
        module_llvm: ModuleLlvm { llmod_raw, llcx, tm },
        name: thin_module.name().to_string(),
        kind: ModuleKind::Regular,
    };
    {
        let llmod = module.module_llvm.llmod();
        save_temp_bitcode(&cgcx, &module, "thin-lto-input");

        // Before we do much else find the "main" `DICompileUnit` that we'll be
        // using below. If we find more than one though then rustc has changed
        // in a way we're not ready for, so generate an ICE by returning
        // an error.
        let mut cu1 = ptr::null_mut();
        let mut cu2 = ptr::null_mut();
        llvm::LLVMRustThinLTOGetDICompileUnit(llmod, &mut cu1, &mut cu2);
        if !cu2.is_null() {
            let msg = "multiple source DICompileUnits found";
            return Err(write::llvm_err(&diag_handler, msg));
        }

        // Like with "fat" LTO, get some better optimizations if landing pads
        // are disabled by removing all landing pads.
        if cgcx.no_landing_pads {
            let _timer = cgcx
                .prof
                .generic_activity_with_arg("LLVM_thin_lto_remove_landing_pads", thin_module.name());
            llvm::LLVMRustMarkAllFunctionsNounwind(llmod);
            save_temp_bitcode(&cgcx, &module, "thin-lto-after-nounwind");
        }

        // Up next comes the per-module local analyses that we do for Thin LTO.
        // Each of these functions is basically copied from the LLVM
        // implementation and then tailored to suit this implementation. Ideally
        // each of these would be supported by upstream LLVM but that's perhaps
        // a patch for another day!
        //
        // You can find some more comments about these functions in the LLVM
        // bindings we've got (currently `PassWrapper.cpp`)
        {
            let _timer =
                cgcx.prof.generic_activity_with_arg("LLVM_thin_lto_rename", thin_module.name());
            if !llvm::LLVMRustPrepareThinLTORename(thin_module.shared.data.0, llmod) {
                let msg = "failed to prepare thin LTO module";
                return Err(write::llvm_err(&diag_handler, msg));
            }
            save_temp_bitcode(cgcx, &module, "thin-lto-after-rename");
        }

        {
            let _timer = cgcx
                .prof
                .generic_activity_with_arg("LLVM_thin_lto_resolve_weak", thin_module.name());
            if !llvm::LLVMRustPrepareThinLTOResolveWeak(thin_module.shared.data.0, llmod) {
                let msg = "failed to prepare thin LTO module";
                return Err(write::llvm_err(&diag_handler, msg));
            }
            save_temp_bitcode(cgcx, &module, "thin-lto-after-resolve");
        }

        {
            let _timer = cgcx
                .prof
                .generic_activity_with_arg("LLVM_thin_lto_internalize", thin_module.name());
            if !llvm::LLVMRustPrepareThinLTOInternalize(thin_module.shared.data.0, llmod) {
                let msg = "failed to prepare thin LTO module";
                return Err(write::llvm_err(&diag_handler, msg));
            }
            save_temp_bitcode(cgcx, &module, "thin-lto-after-internalize");
        }

        {
            let _timer =
                cgcx.prof.generic_activity_with_arg("LLVM_thin_lto_import", thin_module.name());
            if !llvm::LLVMRustPrepareThinLTOImport(thin_module.shared.data.0, llmod) {
                let msg = "failed to prepare thin LTO module";
                return Err(write::llvm_err(&diag_handler, msg));
            }
            save_temp_bitcode(cgcx, &module, "thin-lto-after-import");
        }

        // Ok now this is a bit unfortunate. This is also something you won't
        // find upstream in LLVM's ThinLTO passes! This is a hack for now to
        // work around bugs in LLVM.
        //
        // First discovered in #45511 it was found that as part of ThinLTO
        // importing passes LLVM will import `DICompileUnit` metadata
        // information across modules. This means that we'll be working with one
        // LLVM module that has multiple `DICompileUnit` instances in it (a
        // bunch of `llvm.dbg.cu` members). Unfortunately there's a number of
        // bugs in LLVM's backend which generates invalid DWARF in a situation
        // like this:
        //
        //  https://bugs.llvm.org/show_bug.cgi?id=35212
        //  https://bugs.llvm.org/show_bug.cgi?id=35562
        //
        // While the first bug there is fixed the second ended up causing #46346
        // which was basically a resurgence of #45511 after LLVM's bug 35212 was
        // fixed.
        //
        // This function below is a huge hack around this problem. The function
        // below is defined in `PassWrapper.cpp` and will basically "merge"
        // all `DICompileUnit` instances in a module. Basically it'll take all
        // the objects, rewrite all pointers of `DISubprogram` to point to the
        // first `DICompileUnit`, and then delete all the other units.
        //
        // This is probably mangling to the debug info slightly (but hopefully
        // not too much) but for now at least gets LLVM to emit valid DWARF (or
        // so it appears). Hopefully we can remove this once upstream bugs are
        // fixed in LLVM.
        {
            let _timer = cgcx
                .prof
                .generic_activity_with_arg("LLVM_thin_lto_patch_debuginfo", thin_module.name());
            llvm::LLVMRustThinLTOPatchDICompileUnit(llmod, cu1);
            save_temp_bitcode(cgcx, &module, "thin-lto-after-patch");
        }

        // Alright now that we've done everything related to the ThinLTO
        // analysis it's time to run some optimizations! Here we use the same
        // `run_pass_manager` as the "fat" LTO above except that we tell it to
        // populate a thin-specific pass manager, which presumably LLVM treats a
        // little differently.
        {
            info!("running thin lto passes over {}", module.name);
            let config = cgcx.config(module.kind);
            run_pass_manager(cgcx, &module, config, true);
            save_temp_bitcode(cgcx, &module, "thin-lto-after-pm");
        }
    }
    Ok(module)
}

/// Summarizes module import/export relationships used by LLVM's ThinLTO pass.
///
/// Note that we tend to have two such instances of `ThinLTOImportMaps` in use:
/// one loaded from a file that represents the relationships used during the
/// compilation associated with the incremetnal build artifacts we are
/// attempting to reuse, and another constructed via `from_thin_lto_data`, which
/// captures the relationships of ThinLTO in the current compilation.
#[derive(Debug, Default)]
pub struct ThinLTOImportMaps {
    // key = llvm name of importing module, value = list of modules it imports from
    imports: FxHashMap<String, Vec<String>>,
    // key = llvm name of exporting module, value = list of modules it exports to
    exports: FxHashMap<String, Vec<String>>,
}

impl ThinLTOImportMaps {
    /// Returns modules imported by `llvm_module_name` during some ThinLTO pass.
    fn imports_of(&self, llvm_module_name: &str) -> &[String] {
        self.imports.get(llvm_module_name).map(|v| &v[..]).unwrap_or(&[])
    }

    /// Returns modules exported by `llvm_module_name` during some ThinLTO pass.
    fn exports_of(&self, llvm_module_name: &str) -> &[String] {
        self.exports.get(llvm_module_name).map(|v| &v[..]).unwrap_or(&[])
    }

    fn save_to_file(&self, path: &Path) -> io::Result<()> {
        use std::io::Write;
        let file = File::create(path)?;
        let mut writer = io::BufWriter::new(file);
        for (importing_module_name, imported_modules) in &self.imports {
            writeln!(writer, "{}", importing_module_name)?;
            for imported_module in imported_modules {
                writeln!(writer, " {}", imported_module)?;
            }
            writeln!(writer)?;
        }
        Ok(())
    }

    fn load_from_file(path: &Path) -> io::Result<ThinLTOImportMaps> {
        use std::io::BufRead;
        let mut imports = FxHashMap::default();
        let mut exports: FxHashMap<_, Vec<_>> = FxHashMap::default();
        let mut current_module: Option<String> = None;
        let mut current_imports: Vec<String> = vec![];
        let file = File::open(path)?;
        for line in io::BufReader::new(file).lines() {
            let line = line?;
            if line.is_empty() {
                let importing_module = current_module.take().expect("Importing module not set");
                for imported in &current_imports {
                    exports.entry(imported.clone()).or_default().push(importing_module.clone());
                }
                imports.insert(importing_module, mem::replace(&mut current_imports, vec![]));
            } else if line.starts_with(' ') {
                // Space marks an imported module
                assert_ne!(current_module, None);
                current_imports.push(line.trim().to_string());
            } else {
                // Otherwise, beginning of a new module (must be start or follow empty line)
                assert_eq!(current_module, None);
                current_module = Some(line.trim().to_string());
            }
        }
        Ok(ThinLTOImportMaps { imports, exports })
    }

    /// Loads the ThinLTO import map from ThinLTOData.
    unsafe fn from_thin_lto_data(data: *const llvm::ThinLTOData) -> ThinLTOImportMaps {
        unsafe extern "C" fn imported_module_callback(
            payload: *mut libc::c_void,
            importing_module_name: *const libc::c_char,
            imported_module_name: *const libc::c_char,
        ) {
            let map = &mut *(payload as *mut ThinLTOImportMaps);
            let importing_module_name = CStr::from_ptr(importing_module_name);
            let importing_module_name = module_name_to_str(&importing_module_name);
            let imported_module_name = CStr::from_ptr(imported_module_name);
            let imported_module_name = module_name_to_str(&imported_module_name);

            if !map.imports.contains_key(importing_module_name) {
                map.imports.insert(importing_module_name.to_owned(), vec![]);
            }

            map.imports
                .get_mut(importing_module_name)
                .unwrap()
                .push(imported_module_name.to_owned());

            if !map.exports.contains_key(imported_module_name) {
                map.exports.insert(imported_module_name.to_owned(), vec![]);
            }

            map.exports
                .get_mut(imported_module_name)
                .unwrap()
                .push(importing_module_name.to_owned());
        }

        let mut map = ThinLTOImportMaps::default();
        llvm::LLVMRustGetThinLTOModuleImports(
            data,
            imported_module_callback,
            &mut map as *mut _ as *mut libc::c_void,
        );
        map
    }
}

fn module_name_to_str(c_str: &CStr) -> &str {
    c_str.to_str().unwrap_or_else(|e| {
        bug!("Encountered non-utf8 LLVM module name `{}`: {}", c_str.to_string_lossy(), e)
    })
}

pub fn parse_module<'a>(
    cx: &'a llvm::Context,
    name: &CStr,
    data: &[u8],
    diag_handler: &Handler,
) -> Result<&'a llvm::Module, FatalError> {
    unsafe {
        llvm::LLVMRustParseBitcodeForLTO(cx, data.as_ptr(), data.len(), name.as_ptr()).ok_or_else(
            || {
                let msg = "failed to parse bitcode for LTO module";
                write::llvm_err(&diag_handler, msg)
            },
        )
    }
}
