Big endian vector elements

Tim Northover t.p.northover at
Mon Mar 3 07:21:10 PST 2014

Hi all,

What's LLVM's position on where element 0 exists when a vector is
stored on a big-endian machine? As far as I can see there are three
1. At the high address.
2. At the low address.
3. Target decision, no opts that depend on it happen.

I think there's code assuming both 1 and 2.:

1. In DAGCombiner.cpp, visitEXTRACT_VECTOR_ELT counts lanes backwards
for addressing purposes.
2. SROA converts the attached .ll file (union type-punning,
essentially) into an insertvector on element 1.

One of those has to be a bug (and the documentation should probably be
clarified), but I'm not sure which. Any hints?



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