Rename ValueRequiresCast to ShouldOptimizeCast, to better reflect
what it does. Enhance it to return false to optimizing vector
sign extensions from vector comparisions, which is the idiom used
to get a splatted vector for a vector comparison.
Doing this breaks vector-casts.ll, add some compensating
transformations to handle the important case they cover without
depending on this canonicalization.
This fixes rdar://7434900 a serious pessimization of vector compares.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95855 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 09cd21f..68e17e5 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -255,17 +255,26 @@
return Instruction::CastOps(Res);
}
-/// ValueRequiresCast - Return true if the cast from "V to Ty" actually results
-/// in any code being generated. It does not require codegen if V is simple
-/// enough or if the cast can be folded into other casts.
-bool InstCombiner::ValueRequiresCast(Instruction::CastOps opcode,const Value *V,
- const Type *Ty) {
+/// ShouldOptimizeCast - Return true if the cast from "V to Ty" actually
+/// results in any code being generated and is interesting to optimize out. If
+/// the cast can be eliminated by some other simple transformation, we prefer
+/// to do the simplification first.
+bool InstCombiner::ShouldOptimizeCast(Instruction::CastOps opc, const Value *V,
+ const Type *Ty) {
+ // Noop casts and casts of constants should be eliminated trivially.
if (V->getType() == Ty || isa<Constant>(V)) return false;
- // If this is another cast that can be eliminated, it isn't codegen either.
+ // If this is another cast that can be eliminated, we prefer to have it
+ // eliminated.
if (const CastInst *CI = dyn_cast<CastInst>(V))
- if (isEliminableCastPair(CI, opcode, Ty, TD))
+ if (isEliminableCastPair(CI, opc, Ty, TD))
return false;
+
+ // If this is a vector sext from a compare, then we don't want to break the
+ // idiom where each element of the extended vector is either zero or all ones.
+ if (opc == Instruction::SExt && isa<CmpInst>(V) && isa<VectorType>(Ty))
+ return false;
+
return true;
}