[PATCH] D42951: [CGP] Strength reduce cmp (xor (a, -1), xor(b, -1)) => cmp (b, a)

[Dave Green via Phabricator via llvm-commits]

dmgreen created this revision.
dmgreen added reviewers: samparker, john.brawn, spatel.
Herald added a subscriber: javed.absar.

This is (hopefully) the last part of PR35875. The code sequence we currently have
is slightly worse than the original form, due to a higher register pressure causing
more spills on thumb1 cores with few registers.

The sequence:

  %an = xor i8 %a, -1
  %bn = xor i8 %b, -1
  %cmp15 = icmp ult i8 %an, %bn
  %cond = select i1 %cmp15, i8 %an, i8 %bn

is strength-reduced to

  %an = xor i8 %a, -1
  %bn = xor i8 %b, -1
  %cmp15 = icmp ult i8 %b, %a
  %cond = select i1 %cmp15, i8 %an, i8 %bn

I originally tried to do this during ISEL optimisation, but constant-hoist has
transformed the -1 to constants and pulled them out into higher blocks. So it
would need to look through truncate/zext/register chains into different basic
blocks. Instead it is done here in codegen prepare, late in the pipeline so as to
not break the representation of min/max.


https://reviews.llvm.org/D42951

Files:
  lib/CodeGen/CodeGenPrepare.cpp
  test/Transforms/CodeGenPrepare/ARM/xorcmp.ll

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