[DAG] convert vector select-of-constants to logic/math

This goes back to a discussion about IR canonicalization. We'd like to preserve and convert
more IR to 'select' than we currently do because that's likely the best choice in IR:
http://lists.llvm.org/pipermail/llvm-dev/2016-September/105335.html
...but that's often not true for codegen, so we need to account for this pattern coming in
to the backend and transform it to better DAG ops.

Steps in this patch:

  1. Add an EVT param to the existing convertSelectOfConstantsToMath() TLI hook to more finely
     enable this transform. Other targets will probably want that anyway to distinguish scalars
     from vectors. We're using that here to exclude AVX512 targets, but it may not be necessary.

  2. Convert a vselect to ext+add. This eliminates a constant load/materialization, and the
     vector ext is often free.

Implementing a more general fold using xor+and can be a follow-up for targets that don't have
a legal vselect. It's also possible that we can remove the TLI hook for the special case fold
implemented here because we're eliminating a constant, but it needs to be tested on other
targets.

Differential Revision: https://reviews.llvm.org/D36840

llvm-svn: 311731

2 files changed, 10 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index fdfdde3767c..f570f1991b3 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -4574,6 +4574,15 @@ bool X86TargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   return true;
 }
 
+bool X86TargetLowering::convertSelectOfConstantsToMath(EVT VT) const {
+  // TODO: It might be a win to ease or lift this restriction, but the generic
+  // folds in DAGCombiner conflict with vector folds for an AVX512 target.
+  if (VT.isVector() && Subtarget.hasAVX512())
+    return false;
+
+  return true;
+}
+
 bool X86TargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
                                                 unsigned Index) const {
   if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 7307af100a6..53cd8ca5361 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -1030,9 +1030,7 @@ namespace llvm {
     bool shouldConvertConstantLoadToIntImm(const APInt &Imm,
                                            Type *Ty) const override;
 
-    bool convertSelectOfConstantsToMath() const override {
-      return true;
-    }
+    bool convertSelectOfConstantsToMath(EVT VT) const override;
 
     /// Return true if EXTRACT_SUBVECTOR is cheap for this result type
     /// with this index.