[InstCombine] Missing fold for `fsqrt(select(b, c1, c2)) => select(b, fsqrt(c1), fsqrt(c2))`

[Kmeakin]

float src(bool b) { return sqrt(b ? 4.0 : 1.0); }
float tgt(bool b) { return b ? sqrt(4.0) : sqrt(1.0); }

https://godbolt.org/z/xnvc5q7nx
https://alive2.llvm.org/ce/z/9tiFV4

[RKSimon]

InstCombinerImpl::FoldOpIntoSelect might be able to handle this?

[llvmbot]

Hi!

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[llvmbot]

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

Author: Karl Meakin (Kmeakin)

```c float src(bool b) { return sqrt(b ? 4.0 : 1.0); } float tgt(bool b) { return b ? sqrt(4.0) : sqrt(1.0); } ```

https://godbolt.org/z/xnvc5q7nx
https://alive2.llvm.org/ce/z/9tiFV4

[timmeywerk]

Hi, I would like to work on this one.

[nikic]

Could you please clarify what you're trying to report here? As far as I can tell, this optimization already occurs. Your src and tgt generate the same assembly.

[Kmeakin]

I guess it must have been fixed since I posted the issue. Previously (on clang 18.1.0) it was:

src:
        fmov    d0, #1.00000000
        fmov    d1, #4.00000000
        tst     w0, #0x1
        fcsel   d0, d1, d0, ne
        fsqrt   d0, d0
        fcvt    s0, d0
        ret

tgt:
        fmov    s0, #1.00000000
        fmov    s1, #2.00000000
        tst     w0, #0x1
        fcsel   s0, s1, s0, ne
        ret