[llvm-dev] [GVN] same sequence of instructions in if and else branch
Taewook Oh via llvm-dev
llvm-dev at lists.llvm.org
Tue Feb 9 11:30:20 PST 2016
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
br i1 %cmp, label %if, label %else
%incdec.ptr.1 = getelementptr inbounds i8, i8* %ptr, i64 1
%cast1 = ptrtoint i8* %incdec.ptr.1 to i64
%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.
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