[LLVMdev] Doubt with GVNPRE

[ambika]

Hi,

I have a program as:

int main(int argc, char **argv)
{
        int a,b,k,l;
    scanf("%d%d",&a,&b);
        if (argc > 1) {
          k=a+b;
        } else {
           k=5;
        }
        l=a+b;
        printf("%d,%d",k,l);
        return 0;
}
 

Now i run the following on it:

    llvm-gcc -O1 -emit-llvm -c -o 1.bc 1.c
    llvm-dis 1.bc -o 1.ll
    opt -gvnpre 1.bc -o 1p.bc            // I believe this line writes 
the code to 1p.bc after applying gvnpre to 1.bc
    llvm-dis 1p.bc -o 1p.ll

Now 1.ll is as :
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
entry:
  %b = alloca i32, align 4                        ; <i32*> [#uses=3]
  %a = alloca i32, align 4                        ; <i32*> [#uses=3]
  %0 = call i32 (i8*, ...)* @scanf(i8* getelementptr inbounds ([5 x i8]* 
@.str, i32 0, i32 0), i32* %a, i32* %b) nounwind ; <i32> [#uses=0]
  %1 = icmp sgt i32 %argc, 1                      ; <i1> [#uses=1]
  br i1 %1, label %bb, label %bb2

bb:                                               ; preds = %entry
  %2 = load i32* %a, align 4                      ; <i32> [#uses=1]
  %3 = load i32* %b, align 4                      ; <i32> [#uses=1]
  %4 = add nsw i32 %3, %2                         ; <i32> [#uses=1]
  br label %bb2

bb2:                                              ; preds = %bb, %entry
  %k.0 = phi i32 [ %4, %bb ], [ 5, %entry ]       ; <i32> [#uses=1]
  %5 = load i32* %a, align 4                      ; <i32> [#uses=1]
  %6 = load i32* %b, align 4                      ; <i32> [#uses=1]
  %7 = add nsw i32 %6, %5                         ; <i32> [#uses=1]
  %8 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x 
i8]* @.str1, i32 0, i32 0), i32 %k.0, i32 %7) nounwind ; <i32> [#uses=0]
  ret i32 0
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


And 1p.ll as:
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
entry:
  %b = alloca i32, align 4                        ; <i32*> [#uses=3]
  %a = alloca i32, align 4                        ; <i32*> [#uses=3]
  %0 = call i32 (i8*, ...)* @scanf(i8* getelementptr inbounds ([5 x i8]* 
@.str, i32 0, i32 0), i32* %a, i32* %b) nounwind ; <i32> [#uses=0]
  %1 = icmp sgt i32 %argc, 1                      ; <i1> [#uses=1]
  br i1 %1, label %bb, label %entry.bb2_crit_edge

entry.bb2_crit_edge:                              ; preds = %entry
  br label %bb2

bb:                                               ; preds = %entry
  %2 = load i32* %a, align 4                      ; <i32> [#uses=1]
  %3 = load i32* %b, align 4                      ; <i32> [#uses=1]
  %4 = add nsw i32 %3, %2                         ; <i32> [#uses=1]
  br label %bb2

bb2:                                              ; preds = %bb, 
%entry.bb2_crit_edge
  %k.0 = phi i32 [ %4, %bb ], [ 5, %entry.bb2_crit_edge ] ; <i32> [#uses=1]
  %5 = load i32* %a, align 4                      ; <i32> [#uses=1]
  %6 = load i32* %b, align 4                      ; <i32> [#uses=1]
  %7 = add nsw i32 %6, %5                         ; <i32> [#uses=1]
  %8 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x 
i8]* @.str1, i32 0, i32 0), i32 %k.0, i32 %7) nounwind ; <i32> [#uses=0]
  ret i32 0
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

I believe GVNPRE should have remove partial redundancy by introducing an 
extra computation in entry.bb2_crit_edge.
But this does not happen, redundancy still remains.

Can someone tell me where am I going wrong.

Thanks and Regards,
Ambika