[clang] [llvm] [InstCombine] Canonicalize `(sitofp x)` -> `(uitofp x)` if `x >= 0` (PR #82404)

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goldsteinn wrote:

> Well, I'm not sure how proper that wold be as a reproducer,
> 
> I extracted the mentioned test to a program:
> 
> ```
> #include <cmath>
> 
> #include "third_party/swiftshader/src/Reactor/Coroutine.hpp"
> #include "third_party/swiftshader/src/Reactor/Reactor.hpp"
> 
> using float4 = float[4];
> using int4 = int[4];
> 
> static std::string testName() { return "test name"; }
> 
> using namespace rr;
> 
> int main() {
>   Function<Void(Pointer<Int4>, Pointer<Int4>)> function;
>   {
>     Pointer<Int4> x = function.Arg<0>();
>     Pointer<Int4> y = function.Arg<1>();
> 
>     *y = Abs(*x);
>   }
> 
>   auto routine = function(testName().c_str());
>   auto callable = (void (*)(int4 *, int4 *))routine->getEntry();
> 
>   int input[] = {1, -1, 0, (int)0x80000000};
> 
>   for (int x : input) {
>     int4 v_in = {x, x, x, x};
>     int4 v_out = {0};
> 
>     callable(&v_in, &v_out);
> 
>     float expected = abs(x);
>     if (v_out[0] != expected) {
>       return 10;
>     }
>   }
> }
> ```
> 
> Before the change program exits with 0, after it exists with 10. More precisely: `v_out[0]` is: -2147483648, `expected` is: 2.14748365e+09
> 
> [out_at.txt](https://github.com/llvm/llvm-project/files/14670227/out_at.txt) [out_before.txt](https://github.com/llvm/llvm-project/files/14670228/out_before.txt)
> 
> I attached the generated IR before and after the change. It's only one line diff:
> 
> ```
>    call void @llvm.memset.p0.i64(ptr noundef nonnull align 16 dereferenceable(16) %9, i8 0, i64 16, i1 false)
>    call void %49(ptr noundef nonnull %8, ptr noundef nonnull %9) #16
>    %61 = call i32 @llvm.abs.i32(i32 %60, i1 true)
> -  %62 = sitofp i32 %61 to float
> +  %62 = uitofp i32 %61 to float
>    %63 = load i32, ptr %9, align 16
>    %64 = sitofp i32 %63 to float
>    %65 = fcmp une float %64, %62
> ```
> 
> generation command is `clang -O1 -fsized-deallocation '-std=gnu++20' -c pre.ii -emit-llvm -S -o /tmp/ir_before.ll`
> 
> I'm trying to reduce the preprocessed file, tho not sure i'll keep the semantics of the failure.

This should be sufficient, thank you!

https://github.com/llvm/llvm-project/pull/82404