[PATCH] D22011: [SystemZ] Generate fewer instructions for (sub <constant>, x)

Ulrich Weigand via llvm-commits llvm-commits at lists.llvm.org
Wed Jul 6 07:17:07 PDT 2016

uweigand added a comment.

So I see that we get less instructions in this particular case.  However, I'm wondering whether execution is also *faster*, given that the new code seems to create longer dependency chains.  Normally I'd like to compare benchmark runs before making a change like that ...

Also, I'm wondering why we can't have the best of both worlds: just two instructions *and* the shorter dependency chain, by creating instead something like:

  lghi %r0, 63
  sgrk %r2, %r0, %r2

(assuming we're on z196 and higher.  But then I don't think z10 is a big concern these days anyway.)


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