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

[Craig Topper via cfe-commits]

topperc 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!
> 
> Might have been related to: [1283646](https://github.com/llvm/llvm-project/commit/12836467b76c56872b4c22a6fd44bcda696ea720)

Doesn't that program call `abs` on 0x80000000 which is poison?

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