Fix a README item: when doing a comparison with the result
of a select instruction, see if doing the compare with the
true and false values of the select gives the same result.
If so, that can be used as the value of the comparison.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118378 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index b49b4d0..1955802 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -252,8 +252,27 @@
break;
}
}
-
-
+
+ // If the comparison is with the result of a select instruction, check whether
+ // comparing with either branch of the select always yields the same value.
+ if (isa<SelectInst>(LHS) || isa<SelectInst>(RHS)) {
+ // Make sure the select is on the LHS.
+ if (!isa<SelectInst>(LHS)) {
+ std::swap(LHS, RHS);
+ Pred = CmpInst::getSwappedPredicate(Pred);
+ }
+ SelectInst *SI = cast<SelectInst>(LHS);
+ // Now that we have "icmp select(cond, TV, FV), RHS", analyse it.
+ // Does "icmp TV, RHS" simplify?
+ if (Value *TCmp = SimplifyICmpInst(Pred, SI->getTrueValue(), RHS, TD))
+ // It does! Does "icmp FV, RHS" simplify?
+ if (Value *FCmp = SimplifyICmpInst(Pred, SI->getFalseValue(), RHS, TD))
+ // It does! If they simplified to the same value, then use it as the
+ // result of the original comparison.
+ if (TCmp == FCmp)
+ return TCmp;
+ }
+
return 0;
}