[llvm-dev] Legality of transformation

[Roman Lebedev via llvm-dev]

This refinement is correct as per the LLVM IR semantics,
since the memory is uninitialized.

https://llvm.org/docs/LangRef.html#alloca-instruction

> Semantics:
> Memory is allocated; a pointer is returned. The allocated memory is uninitialized,
> and loading from uninitialized memory produces an undefined value.

$ /repositories/alive2/build-Clang-release/alive-tv /tmp/old.ll /tmp/new.ll

----------------------------------------
define i32 @main() {
%entry:
 %A = alloca i64 4, align 16
 %t = load i32, * %A, align 16
 ret i32 %t
}
=>
define i32 @main() {
%entry:
 ret i32 undef
}
Transformation seems to be correct!

Summary:
 1 correct transformations
 0 incorrect transformations
 0 Alive2 errors

Roman

On Sat, Apr 4, 2020 at 7:34 PM Akash Banerjee via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Please consider the following C code:
>     #define SZ 2048
>     int main(void) {
>       int A[SZ];
>       int B[SZ];
>       int i, tmp;
>       for (i = 0; i < SZ; i++) {
>         tmp = A[i];
>         B[i] = tmp;
>       }
>       assert(A[SZ/2] == B[SZ/2]);
>     }
>
> On running -O1 followed by -reg2mem I get the following IR:
> define dso_local i32 @main() local_unnamed_addr #0 {
> entry:
>   %A = alloca [2048 x i32], align 16
>   %B = alloca [2048 x i32], align 16
>   %"reg2mem alloca point" = bitcast i32 0 to i32
>   %arrayidx3 = getelementptr inbounds [2048 x i32], [2048 x i32]* %A, i64 0, i64 1024
>   %0 = load i32, i32* %arrayidx3, align 16
>   %arrayidx4 = getelementptr inbounds [2048 x i32], [2048 x i32]* %B, i64 0, i64 1024
>   %1 = load i32, i32* %arrayidx4, align 16
>   %cmp5 = icmp eq i32 %0, %1
>   %conv = zext i1 %cmp5 to i32
>   %call = call i32 (i32, ...) bitcast (i32 (...)* @assert to i32 (i32, ...)*)(i32 %conv) #2
>   ret i32 0
> }
>
> It is my understanding that in the original C code the assert would never fail, however in the optimized IR the assert might fail.
> I tried using clang to generate the executable, and the assert doesn't fail when using -O1 but fails with -O2 and -O3 in my setting. I also tried GCC and could not get it to hit an assert fail.
> Please help me in understanding the above transformation and its legality.
>
> Thanks,
> Akash.
>
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