Failure to spot `scmp(x, 0)` idiom

[Kmeakin]

https://godbolt.org/z/1xqx65vds

For

int signum(int x) {
    if (x < 0) return -1;
    if (x > 0) return +1;
    return 0;
}

clang produces

signum:
        cmp     w0, #0
        cset    w8, ne
        cmn     w0, #1
        csinv   w0, w8, wzr, gt
        ret

but GCC produces

signum:
        cmp     w0, 0
        cset    w0, ne
        csinv   w0, w0, wzr, ge
        ret

It seems that LLVM is not recognising that signum is equivalent to scmp(x, 0):
https://alive2.llvm.org/ce/z/EYQZwV

define dso_local range(i32 -1, 2) i32 @src(i32 noundef %0) local_unnamed_addr #0 {
  %2 = icmp ne i32 %0, 0
  %3 = zext i1 %2 to i32
  %4 = icmp sgt i32 %0, -1
  %5 = select i1 %4, i32 %3, i32 -1
  ret i32 %5
}

define dso_local range(i32 -1, 2) i32 @tgt(i32 noundef %0) local_unnamed_addr #0 {
  %2 = tail call i32 @llvm.scmp(i32 %0, i32 0)
  ret i32 %2
}

[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. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. 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.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

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: Karl Meakin (Kmeakin)

https://godbolt.org/z/1xqx65vds

For

int signum(int x) {
    if (x < 0) return -1;
    if (x > 0) return +1;
    return 0;
}

clang produces

signum:
        cmp     w0, #<!-- -->0
        cset    w8, ne
        cmn     w0, #<!-- -->1
        csinv   w0, w8, wzr, gt
        ret

but GCC produces

signum:
        cmp     w0, 0
        cset    w0, ne
        csinv   w0, w0, wzr, ge
        ret

It seems that LLVM is not recognising that signum is equivalent to scmp(x, 0):
https://alive2.llvm.org/ce/z/EYQZwV

define dso_local range(i32 -1, 2) i32 @<!-- -->src(i32 noundef %0) local_unnamed_addr #<!-- -->0 {
  %2 = icmp ne i32 %0, 0
  %3 = zext i1 %2 to i32
  %4 = icmp sgt i32 %0, -1
  %5 = select i1 %4, i32 %3, i32 -1
  ret i32 %5
}

define dso_local range(i32 -1, 2) i32 @<!-- -->tgt(i32 noundef %0) local_unnamed_addr #<!-- -->0 {
  %2 = tail call i32 @<!-- -->llvm.scmp(i32 %0, i32 0)
  ret i32 %2
}

[yashnator]

Hi, can I work on this?

[Kmeakin]

Hi, can I work on this?

I can't work on LLVM myself ATM, so feel free

[dtcxzyw]

Hi, can I work on this?

Please read https://llvm.org/docs/InstCombineContributorGuide.html before submitting a patch. Have fun!

[nikic]

The existing code for this fold is here:

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

Line 3563 in 3e1931d

Instruction *InstCombinerImpl::foldSelectToCmp(SelectInst &SI) {