[Support] Add KnownBits implementations for avgFloor and avgCeil

[RKSimon]

Followup to #84211

These are implemented as part of #76644 but in SelectionDAG not in the KnownBits class directly

• Add KnownBIts::avgfloors/avgflooru/avgceils/avgceilu implementations based off [AArch64] SimplifyDemandedBitsForTargetNode - add AArch64ISD::BICi handling #76644
• Add exhaustive KnownBits test coverage (can it be optimal or just correct?)
• Depending upon whether [AArch64] SimplifyDemandedBitsForTargetNode - add AArch64ISD::BICi handling #76644 lands before or after this, you might need to update the SelectionDAG code or coordinate with @snikitav

[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:

1. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Followup to #84211

These are implemented as part of #76644 but in SelectionDAG not in the KnownBits class directly

• Add KnownBIts::avgfloors/avgflooru/avgceils/avgceilu implementations based off #76644
• Add exhaustive KnownBits test coverage (can it be optimal or just correct?)
• Depending upon whether #76644 lands before or after this, you might need to update the SelectionDAG code or coordinate with @snikitav

[Sh0g0-1758]

Hey @RKSimon, I would like to work on this.

[changkhothuychung]

Hi I would like to work on this if this is available

[EugeneZelenko]

@changkhothuychung: Just create pull request and mention it on this page.

[RKSimon]

@changkhothuychung @Sh0g0-1758 Both of you have requested assignment for #84640 and #84639 - do either of you have a preference? I'd prefer to keep the good-first-issue tickets distributed.

[Sh0g0-1758]

I have started working on #84639

[changkhothuychung]

@RKSimon RK
Thank you for assigning this ticket to me. I have a few questions for direction.

• I am supposed to add the implementations in the file KnownBits.cpp right?
• Is there any documentation which explains what KnownBits are for and the use case of it? So I can have a better understanding of what I am doing. Thanks.

[RKSimon]

KnownBits are for analysis to allow us to track whether any specific bits of a variable are guaranteed to be zero or one, and help considerably with codegen simplification.

The implementations need moving into KnownBits.cpp - #76644 has the 4 implementations merged into 1 case, but ideally we'd want them separate.

KnownBits::avgCeilS
KnownBits::avgCeilU
KnownBits::avgFloorS
KnownBits::avglFoorU

The exhaustive tests will need to be added around here:

llvm-project/llvm/unittests/Support/KnownBitsTest.cpp

Lines 601 to 609 in 995d1d1

	testBinaryOpExhaustive(
	[](const KnownBits &Known1, const KnownBits &Known2) {
	return KnownBits::mulhu(Known1, Known2);
	},
	[](const APInt &N1, const APInt &N2) {
	unsigned Bits = N1.getBitWidth();
	return (N1.zext(2 * Bits) * N2.zext(2 * Bits)).extractBits(Bits, Bits);
	},
	checkCorrectnessOnlyBinary);

You will need to wait until #84431 is completed so the equivalent APInt constant methods are available.

[RKSimon]

@changkhothuychung reverse-ping

[changkhothuychung]

@RKSimon Hi, I am figuring out how to do the implementations, I will make a PR as soon as I have something.