[LLVMdev] Another missed optimization opportunity?

[Scott Pakin]

I was suprised to find that some bitcode I'm generating isn't getting
optimized.  Here, I'm doing the equivalent of "myarray[5]++" (on an
"extern int *myarray"), repeated three times:

     @myarray = external global i32*

     define void @update_array() #0 {
       %1 = load i32** @myarray, align 8
       %2 = getelementptr inbounds i32* %1, i64 5
       %3 = load i32* %2, align 4
       %4 = add nsw i32 %3, 1
       store i32 %4, i32* %2, align 4
       %5 = load i32** @myarray, align 8
       %6 = getelementptr inbounds i32* %5, i64 5
       %7 = load i32* %6, align 4
       %8 = add nsw i32 %7, 1
       store i32 %8, i32* %6, align 4
       %9 = load i32** @myarray, align 8
       %10 = getelementptr inbounds i32* %9, i64 5
       %11 = load i32* %10, align 4
       %12 = add nsw i32 %11, 1
       store i32 %12, i32* %10, align 4
       ret void
     }

Running "opt -std-compile-opts" or even "opt -O3" doesn't seem to
change the bitcode any.  I had expected the three increments by 1 to
be collapsed into a single increment by 3:

     @myarray = external global i32*

     define void @update_array() #0 {
       %1 = load i32** @myarray, align 8
       %2 = load i32* %1, align 4
       %3 = add nsw i32 %2, 3
       store i32 %3, i32* %1, align 4
       ret void
     }

Even the (x86-64) code generator doesn't do any last-minute
optimizations:

     movq    myarray(%rip), %rax
     incl    20(%rax)
     movq    myarray(%rip), %rax
     incl    20(%rax)
     movq    myarray(%rip), %rax
     incl    20(%rax)

This is with LLVM revision 180116.

Is there some semantic reason that the increments aren't allowed to be
combined, or is this a missed optimization opportunity in LLVM?

Thanks,
-- Scott