[InstCombine] Infer exact for lshr by cttz

[nikic]

From rust-lang/rust#138497:

define i8 @src(i8 %x) {
  %cttz = call i8 @llvm.cttz.i8(i8 %x, i1 false)
  %lshr = lshr i8 %x, %cttz
  ret i8 %lshr
}

define i8 @tgt(i8 %x) {
  %cttz = call i8 @llvm.cttz.i8(i8 %x, i1 false)
  %lshr = lshr exact i8 %x, %cttz
  ret i8 %lshr
}

https://alive2.llvm.org/ce/z/KFemjy

This then allows folding icmp eq %lshr, 0.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Nikita Popov (nikic)

From https://github.com/rust-lang/rust/issues/138497:

define i8 @<!-- -->src(i8 %x) {
  %cttz = call i8 @<!-- -->llvm.cttz.i8(i8 %x, i1 false)
  %lshr = lshr i8 %x, %cttz
  ret i8 %lshr
}

define i8 @<!-- -->tgt(i8 %x) {
  %cttz = call i8 @<!-- -->llvm.cttz.i8(i8 %x, i1 false)
  %lshr = lshr exact i8 %x, %cttz
  ret i8 %lshr
}

https://alive2.llvm.org/ce/z/KFemjy

This then allows folding icmp eq %lshr, 0.

[nikic]

Should be able to handle this with an extra special case similar to this one:

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

Lines 992 to 996 in 2549517

	// shr (shl X, Y), Y
	if (match(I.getOperand(0), m_Shl(m_Value(), m_Specific(I.getOperand(1))))) {
	I.setIsExact();
	return true;
	}

If you're new to InstCombine development, please take a look at https://llvm.org/docs/InstCombineContributorGuide.html first.

[aayush-tripathi]

I would like to work on this!

[houngkoungting]

Hi @aayush-tripathi , I see you’re assigned to #131444. I’m a new contributor and have a working solution with updated tests ready. Are you currently working on this? If not, I’d like to submit a PR. Happy to coordinate if you’re already on it!

[aayush-tripathi]

Hi @aayush-tripathi , I see you’re assigned to #131444. I’m a new contributor and have a working solution with updated tests ready. Are you currently working on this? If not, I’d like to submit a PR. Happy to coordinate if you’re already on it!

Hey @houngkoungting ! I am actually currently working on this issue and have made some progress. We can collaborate on this, maybe you could review my PR, I plan to submit one soon.

Thanks!