De-llvm some integer intrinsics that on the Rust side always use `u32`

[WaffleLapkin]

Proposal

There is a number of integer operations which currently use an intrinsic which has T (type of the integer being operated on) in place where Rust methods use u32. All those methods have as u32s in the implementation.

The only reason why those signatures are the way they are, is because LLVM intrinsics expect them as such.

I propose to change the intrinsics to accept/return u32 and make LLVM backend insert necessary casts.

Here is the list of intrinsics I propose to change:

• Right-hand side (shift amount)
  • rotate_left (.rotate_left())
  • rotate_right (.rotate_right)
  • unchecked_shl (basis of shl methods)
  • unchecked_shr (basis of shr methods)
• Return type (some type of count)
  • ctlz (.leading_zeros())
  • cttz (.trailing_zeros())
  • ctlz_nonzero (.leading_zeros())
  • cttz_nonzero (.trailing_zeros())
  • ctpop (.count_ones(), .count_zeros())

I would also like to change codegen methods when applicable (for example the shl/ashr, lshr).

Disclosure: at my {job} we are working on a new backend for rustc, those changes would make my work a little bit easier.

(still, I think this makes sense even without the context of my work)

Mentors or Reviewers

@scottmcm

Process

The main points of the Major Change Process are as follows:

• [x] File an issue describing the proposal.
• [ ] A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[davidtwco]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted