[META][X86] Avoid lowering C intrinsics calls to target-specific LLVM IR intrinsics

[ZviRackover]

Bugzilla Link	30624
Version	trunk
OS	All
Depends On	#11397 #27233 #27714 #28375 #29909 #29911 #29981 #30333 #31060 #32931 #36902 #39274 #39366 #39403 #39605 #39606 #46195
Attachments	List of C intrinsics that are lowered to llvm.x86.* LLVM IR intrinsics
CC	@topperc,@igor-breger,@RKSimon,@rotateright

Extended Description

This bug is for tracking the effort of lowering all X86 C intrinsics to target-independent LLVM IR, for all cases that it can be done reasonably.

The attached CSV file was created by Ayman Musa. It should contain a list of all intrinsics which are lowered to calls to llvm.x86.* intrinsics.
The list was created by parsing the Clang tests.

All intrinsics that cannot be lowered to a reasonable LLVM-IR code such that the semantics of the original function are easy to identify at Isel, are marked as so.

Of the 3142 intrinsics functions that appear in the CSV:

• 139 were identified as can be lowered to target-independent IR
• 55 were identified as cannot be lowered to target-independent IR
• 2948 are TBD. (Yes, lots of more work ahead of us).

[RKSimon]

Nice! I think it'd be useful to split the 'Can lower to IR' into several subtypes:

1 - replace with generic IR in headers
2 - replace with generic IR in clang frontend
3 - replace with generic llvm intrinsic
4 - available as x86 ISD opcode

It should be noted that there are ops that need extra handling:
e.g NAN handling for maxps is different from llvm.maxnum
e.g out of range shift values for PSRA/PSRL/PSLL etc.

[rotateright]

This is great. To paraphrase a blurb from https://reviews.llvm.org/D24815 about the general steps for converting these:

1. Prepare the backend (DAG) to handle the expected new IR patterns and add tests for those.
2. Auto-upgrade the IR, remove deprecated handling of the old IR intrinsics, and change/remove existing tests.
3. Update clang to not produce the deprecated intrinsics.

If someone is actively working on these, it would be great to have an annotated list that shows what is underway at any given time so we don't have any duplicated effort. Or just open a specific bug report for an intrinsic, assign it to an engineer, and link that report to this master list?

[RKSimon]

mentioned in issue #29981

[RKSimon]

mentioned in issue #30333

[RKSimon]

mentioned in issue #31060

[ZviRackover]

mentioned in issue #32931

[RKSimon]

mentioned in issue #36902

[RKSimon]

mentioned in issue #39274

[RKSimon]

mentioned in issue #39366

[RKSimon]

mentioned in issue #39403

[topperc]

mentioned in issue #39606

[topperc]

mentioned in issue #39605

[RKSimon]

mentioned in issue #46195