[LLVMdev] why gvn does not work for global variables

[Xiangyang Guo]

Hi, All,

Suppose I have a very simple code like this and variable 'a' is a global
variable:
***************************************************************************
*double ** a;*
*int main(){*
*    a[5][10] = 0.1;*
*    a[5][20] = 0.5;*
*    return 0;*
*}*
***************************************************************************

Then I can get the IR without any optimization like this:
***************************************************************************
*@a = global double** null, align 8*

*; Function Attrs: nounwind uwtable*
*define i32 @main() #0 {*
*  %1 = alloca i32, align 4*
*  store i32 0, i32* %1*
*  %2 = load double*** @a, align 8*
*  %3 = getelementptr inbounds double** %2, i64 5*
*  %4 = load double** %3, align 8*
*  %5 = getelementptr inbounds double* %4, i64 10*
*  store double 1.000000e-01, double* %5, align 8*
*  %6 = load double*** @a, align 8*
*  %7 = getelementptr inbounds double** %6, i64 5*
*  %8 = load double** %7, align 8*
*  %9 = getelementptr inbounds double* %8, i64 20*
*  store double 5.000000e-01, double* %9, align 8*
*  ret i32 0*
*}*
***************************************************************************

I hope GVN can get rid of redundant 'load' and 'getelementptr' instructions
such as '*%2 = load double*** @a, align 8*' and '*%6 = load double*** @a,
align 8*'. So I use 'opt input.ll -basicaa -gvn -S -o output.ll ' , I still
get :
***************************************************************************
*  %2 = load double*** @a, align 8*
*  %3 = getelementptr inbounds double** %2, i64 5*
*  %4 = load double** %3, align 8*
*  %5 = getelementptr inbounds double* %4, i64 10*
*  store double 1.000000e-01, double* %5, align 8*
*  %6 = load double*** @a, align 8*
*  %7 = getelementptr inbounds double** %6, i64 5*
*  %8 = load double** %7, align 8*
*  %9 = getelementptr inbounds double* %8, i64 20*
*  store double 5.000000e-01, double* %9, align 8*
***************************************************************************

It seems GVN doesn't work. Then I use '-aa-eval', it seems LLVM thinks that
is "*may alias*". But '%2' and '%6' are alias, right?
***************************************************************************
*===== Alias Analysis Evaluator Report =====*
*  45 Total Alias Queries Performed*
*  13 no alias responses (28.8%)*
*  32 may alias responses (71.1%)*
*  0 partial alias responses (0.0%)*
*  0 must alias responses (0.0%)*
*  Alias Analysis Evaluator Pointer Alias Summary: 28%/71%/0%/0%*
*  Alias Analysis Mod/Ref Evaluator Summary: no mod/ref!*
***************************************************************************

Can anyone help me to use GVN in this situation? Thanks

Regards,

Xiangyang
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