[llvm-dev] [GVN] same sequence of instructions in if and else branch

[Taewook Oh via llvm-dev]

Hello,


I found that GVN doesn't promote identical sequence of instructions in if and else branch to their common predecessors. For example, for the following code snippet


pred:

…

br i1 %cmp, label %if, label %else

if:

%incdec.ptr.1 = getelementptr inbounds i8, i8* %ptr, i64 1

%cast1 = ptrtoint i8* %incdec.ptr.1 to i64

    …

else:

%incdec.ptr.2 = getelementptr inbounds i8, i8* %ptr, i64 1

%cast2 = ptrtoint i8* %incdec.ptr.2 to i64


GVN doesn't move instructions in 'if' and 'else' blocks to 'pred' block even though it knows that incdec.ptr.1/case1 has a same value number with incdec.ptr.2/cast2. I see a case where this kind of redundancy confuses following optimization passes and ends up generating suboptimal code.


>From the GVN implementation, it seems that transformation is performed only when the "leader" value dominates the other value, so it cannot handle a case like the above example. I wonder if this is by design or just a missing feature.


Thanks,

Taewook
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