[PATCH] D48508: [DAGCombiner] eliminate setcc bool math when input is low-bit of some value

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: craig.topper, nemanjai, RKSimon.
Herald added a subscriber: mcrosier.

This patch has the same motivating example as https://reviews.llvm.org/D48466, but I'm trying to kill the setcc bool math sooner rather than later.

By matching a larger pattern that includes both the low-bit mask and the trailing add/sub, we can create a universally good fold because we always eliminate the condition code intermediate value.

Here are Alive proofs for these (currently instcombine folds the 'add' variants, but misses the 'sub' patterns):
https://rise4fun.com/Alive/Gsyp

  Name: sub of zext cmp mask
    %a = and i8 %x, 1
    %c = icmp eq i8 %a, 0
    %z = zext i1 %c to i32
    %r = sub i32 C1, %z
    =>
    %optional_cast = zext i8 %a to i32
    %r = add i32 %optional_cast, C1-1
  
  Name: add of zext cmp mask
    %a = and i32 %x, 1
    %c = icmp eq i32 %a, 0
    %z = zext i1 %c to i8
    %r = add i8 %z, C1
    =>
    %optional_cast = trunc i32 %a to i8
    %r = sub i8 C1+1, %optional_cast

All of the tests look like improvements or neutral to me. But it is possible that x86 test+set+bitop is better than what we now show here. I suspect we could do better by adding another fold for the 'sub' variants in particular.

We start with select-of-constant in IR in the larger motivating test, so that's why I included tests with selects. Proofs for those variants:
https://rise4fun.com/Alive/Bx1

  Name: true const is bigger
  Pre: C2 == (C1 + 1)
    %a = and i8 %x, 1
    %c = icmp eq i8 %a, 0
    %r = select i1 %c, i64 C2, i64 C1
    =>
    %z = zext i8 %a to i64
    %r = sub i64 C2, %z
  
  Name: false const is bigger
  Pre: C2 == (C1 + 1)
    %a = and i8 %x, 1
    %c = icmp eq i8 %a, 0
    %r = select i1 %c, i64 C1, i64 C2
    =>
    %z = zext i8 %a to i64
    %r = add i64 C1, %z

I have not stepped through the PPC tests to see how the 3 unchanged select tests escaped any diffs. It's possible that those are not folded the same as x86 initially or something later reverses what we're doing here.


https://reviews.llvm.org/D48508

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/PowerPC/bool-math.ll
  test/CodeGen/X86/bool-math.ll

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