[X86][SSE] Improve recognition of uitofp conversions that can be performed as sitofp

With D24253 we can now use SelectionDAG::SignBitIsZero with vector operations.

This patch uses SelectionDAG::SignBitIsZero to recognise that a zero sign bit means that we can use a sitofp instead of a uitofp (which is not directly support on pre-AVX512 hardware).

While AVX512 does provide support for uitofp, the conversion to sitofp should not cause any regressions.

Differential Revision: https://reviews.llvm.org/D24343

llvm-svn: 281852

2 files changed, 9 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 309be2c2253..570c26a42ed 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1980,10 +1980,6 @@ SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,
 /// SignBitIsZero - Return true if the sign bit of Op is known to be zero.  We
 /// use this predicate to simplify operations downstream.
 bool SelectionDAG::SignBitIsZero(SDValue Op, unsigned Depth) const {
-  // This predicate is not safe for vector operations.
-  if (Op.getValueType().isVector())
-    return false;
-
   unsigned BitWidth = Op.getScalarValueSizeInBits();
   return MaskedValueIsZero(Op, APInt::getSignBit(BitWidth), Depth);
 }
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index a9051886251..bbd84dca16a 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -13871,15 +13871,15 @@ SDValue X86TargetLowering::LowerUINT_TO_FP(SDValue Op,
   SDLoc dl(Op);
   auto PtrVT = getPointerTy(DAG.getDataLayout());
 
-  if (Op.getSimpleValueType().isVector())
-    return lowerUINT_TO_FP_vec(Op, DAG);
-
   // Since UINT_TO_FP is legal (it's marked custom), dag combiner won't
   // optimize it to a SINT_TO_FP when the sign bit is known zero. Perform
   // the optimization here.
   if (DAG.SignBitIsZero(N0))
     return DAG.getNode(ISD::SINT_TO_FP, dl, Op.getValueType(), N0);
 
+  if (Op.getSimpleValueType().isVector())
+    return lowerUINT_TO_FP_vec(Op, DAG);
+
   MVT SrcVT = N0.getSimpleValueType();
   MVT DstVT = Op.getSimpleValueType();
 
@@ -31204,6 +31204,12 @@ static SDValue combineUIntToFP(SDNode *N, SelectionDAG &DAG,
     return DAG.getNode(ISD::SINT_TO_FP, dl, VT, P);
   }
 
+  // Since UINT_TO_FP is legal (it's marked custom), dag combiner won't
+  // optimize it to a SINT_TO_FP when the sign bit is known zero. Perform
+  // the optimization here.
+  if (DAG.SignBitIsZero(Op0))
+    return DAG.getNode(ISD::SINT_TO_FP, SDLoc(N), VT, Op0);
+
   return SDValue();
 }