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.