[PATCH] D35340: [x86] use more shift or LEA for select-of-constants

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
Herald added a subscriber: mcrosier.

We can convert any select-of-constants to math ops:
http://rise4fun.com/Alive/d7d

For this patch, I'm proposing to enhance an existing x86 transform that uses fake multiplies (they always become shl/lea) to avoid cmov or branching. The current code misses cases where we have a negative constant and a positive constant, so this is just trying to plug that hole.

The DAGCombiner diff prevents us from hitting a terrible inefficiency: we can start with a select in IR, create a select DAG node, convert it into a sext, convert it back into a select, and then lower it to sext machine code.

Some notes about the test diffs:

1. 2010-08-04-MaskedSignedCompare.ll - We were creating control flow that didn't exist in the IR.
2. memcmp.ll - Choose -1 or 1 is the case that got me looking at this again. I think we could avoid the push/pop in some cases if we used 'movzbl %al' instead of an xor on a different reg? That's a post-DAG problem though.
3. mul-constant-result.ll - The trade-off between sbb+not vs. setne+neg could be addressed if that's a regression, but I think those would always be nearly equivalent.
4. pr22338.ll and sext-i1.ll - These tests have undef operands, so I don't think we actually care about these diffs.
5. sbb.ll - This shows a win for what I think is a common case: choose -1 or 0.
6. select.ll - There's another borderline case here: cmp+sbb+or vs. test+set+lea? Also, sbb+not vs. setae+neg shows up again.
7. select_const.ll - These are motivating cases for the enhancement; replace cmov with cheaper ops.


https://reviews.llvm.org/D35340

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/2010-08-04-MaskedSignedCompare.ll
  test/CodeGen/X86/memcmp.ll
  test/CodeGen/X86/merge-consecutive-stores.ll
  test/CodeGen/X86/mul-constant-result.ll
  test/CodeGen/X86/pr22338.ll
  test/CodeGen/X86/sbb.ll
  test/CodeGen/X86/select.ll
  test/CodeGen/X86/select_const.ll
  test/CodeGen/X86/sext-i1.ll

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