[llvm-dev] Legality of transformation

[Akash Banerjee via llvm-dev]

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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