[TargetLowering] expandFP_TO_UINT - avoid FPE due to out of range conversion (PR17686)

PR17686 demonstrates that for some targets FP exceptions can fire in cases where the FP_TO_UINT is expanded using a FP_TO_SINT instruction.

The existing code converts both the inrange and outofrange cases using FP_TO_SINT and then selects the result, this patch changes this for 'strict' cases to pre-select the FP_TO_SINT input and the offset adjustment.

The X87 cases don't need the strict flag but generates much nicer code with it....

Differential Revision: https://reviews.llvm.org/D53794

llvm-svn: 348251

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index f8102002b72..809c5a149a0 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -4812,6 +4812,12 @@ bool X86TargetLowering::decomposeMulByConstant(EVT VT, SDValue C) const {
          (1 - MulC).isPowerOf2() || (-(MulC + 1)).isPowerOf2();
 }
 
+bool X86TargetLowering::shouldUseStrictFP_TO_INT(EVT FpVT, EVT IntVT,
+                                                 bool IsSigned) const {
+  // f80 UINT_TO_FP is more efficient using Strict code if FCMOV is available.
+  return !IsSigned && FpVT == MVT::f80 && Subtarget.hasCMov();
+}
+
 bool X86TargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
                                                 unsigned Index) const {
   if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))