From 2a7f1931d7272051592155619f21b338b5735734 Mon Sep 17 00:00:00 2001 From: Max Kazantsev Date: Fri, 17 Jul 2020 14:01:59 +0700 Subject: [PATCH] [InstCombine] Fix replace select with Phis when branch has the same labels ``` define i32 @test(i1 %cond) { entry: br i1 %cond, label %exit, label %exit exit: %result = select i1 %cond, i32 123, i32 456 ret i32 %result } ``` In this test, after applying transformation of replacing select with Phis, the result will be: ``` define i32 @test(i1 %cond) { entry: br i1 %cond, label %exit, label %exit exit: %result = i32 phi [123, %exit], [123, %exit] ret i32 %result } ``` That is, select is transformed into an invalid Phi, which will then be reduced to 123 and the second value will be lost. But it is worth noting that this problem will arise only if select is in the InstCombine worklist will be before the branch. Otherwise, InstCombine will replace the branch condition with false and transformation will not be applied. The fix is to check the target labels in the branch condition for equality. Patch By: Kirill Polushin Differential Revision: https://reviews.llvm.org/D84003 Reviewed By: mkazantsev (cherry picked from commit c98988107868db41c12b9d782fae25dea2a81c87) --- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp index 17124f717af79..db27711f29b17 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp @@ -2469,6 +2469,10 @@ static Instruction *foldSelectToPhiImpl(SelectInst &Sel, BasicBlock *BB, } else return nullptr; + // Make sure the branches are actually different. + if (TrueSucc == FalseSucc) + return nullptr; + // We want to replace select %cond, %a, %b with a phi that takes value %a // for all incoming edges that are dominated by condition `%cond == true`, // and value %b for edges dominated by condition `%cond == false`. If %a