[LLVMdev] Redundant Add Operation in Code Generation?

[Andrew Trick]

On Oct 17, 2012, at 1:22 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:

> I'm curious why I am seeing this: 
> 
>   %uglygep18.sum = add i32 %lsr_iv8, %tmp45
>   %scevgep19 = getelementptr i8* %parBits_017, i32 %uglygep18_sum
>   %scevgep1920 = bitcast i8* %scevgep19 to i16*
>   %tmp78 = load i16* %scevgep1920, align 2
>   %uglygep14.sum = add i32 %lsr_iv8, %tmp45
>   %scevgep15 = getelementptr i8* %extIn_013, i32 %uglygep14_sum
>   %scevgep1516 = bitcast i8* %scevgep15 to i16*
>   %tmp79 = load i16* %scevgep1516, align 2
>   %conv93.i.i = sext i16 %tmp79 to i32
>   %uglygep.sum = add i32 %lsr_iv8, %tmp45
>   %scevgep11 = getelementptr i8* %sysBits_010, i32 %uglygep_sum
> 
> You can see here that "add i32 %lsr_iv8, %tmp45" is done multiple times, appearing that there are two redundant add operations that are not needed yet are generated?

That's LSR, as you can see from the variable names ;) It might think that load(Base + Index) is a legal addressing mode for your target. -disable-lsr might be the right thing for you anyway.

Incidentally, MachineCSE could clean this up if it doesn't get folded into the address, but like LSR, it tries hard not to increase register pressure.

-Andy
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