[llvm-dev] Legality of transformation

[Akash Banerjee via llvm-dev]

Hi, in your example instead of returning A, returning undef is fine, but in
my case, although all the elements of array A are undef, B is an exact copy
of A , and this notion is left out in the transformed IR, whereas the
assertion which depends on it is left intact.
Please help me understand this better.

Thanks,
Akash.
On Sat, Apr 4, 2020 at 10:19 PM, Roman Lebedev <lebedev.ri at gmail.com> wrote:

> 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.
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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