Disable shrinking of SNaN constants

When expanding FP constants, we attempt to shrink doubles to floats and perform an extending load.
However, on SystemZ, and possibly on other targets (I've only confirmed the problem on SystemZ), the FP extending load instruction may convert SNaN into QNaN, or may cause an exception. So in the general case, we would still like to shrink FP constants, but SNaNs should be left as doubles.

Differential Revision: https://reviews.llvm.org/D22685

llvm-svn: 277602

1 files changed, 17 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 609ff86c8c4..87fb80cea23 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -259,19 +259,25 @@ SelectionDAGLegalize::ExpandConstantFP(ConstantFPSDNode *CFP, bool UseCP) {
                            (VT == MVT::f64) ? MVT::i64 : MVT::i32);
   }
 
+  APFloat APF = CFP->getValueAPF();
   EVT OrigVT = VT;
   EVT SVT = VT;
-  while (SVT != MVT::f32 && SVT != MVT::f16) {
-    SVT = (MVT::SimpleValueType)(SVT.getSimpleVT().SimpleTy - 1);
-    if (ConstantFPSDNode::isValueValidForType(SVT, CFP->getValueAPF()) &&
-        // Only do this if the target has a native EXTLOAD instruction from
-        // smaller type.
-        TLI.isLoadExtLegal(ISD::EXTLOAD, OrigVT, SVT) &&
-        TLI.ShouldShrinkFPConstant(OrigVT)) {
-      Type *SType = SVT.getTypeForEVT(*DAG.getContext());
-      LLVMC = cast<ConstantFP>(ConstantExpr::getFPTrunc(LLVMC, SType));
-      VT = SVT;
-      Extend = true;
+
+  // We don't want to shrink SNaNs. Converting the SNaN back to its real type
+  // can cause it to be changed into a QNaN on some platforms (e.g. on SystemZ).
+  if (!APF.isSignaling()) {
+    while (SVT != MVT::f32 && SVT != MVT::f16) {
+      SVT = (MVT::SimpleValueType)(SVT.getSimpleVT().SimpleTy - 1);
+      if (ConstantFPSDNode::isValueValidForType(SVT, APF) &&
+          // Only do this if the target has a native EXTLOAD instruction from
+          // smaller type.
+          TLI.isLoadExtLegal(ISD::EXTLOAD, OrigVT, SVT) &&
+          TLI.ShouldShrinkFPConstant(OrigVT)) {
+        Type *SType = SVT.getTypeForEVT(*DAG.getContext());
+        LLVMC = cast<ConstantFP>(ConstantExpr::getFPTrunc(LLVMC, SType));
+        VT = SVT;
+        Extend = true;
+      }
     }
   }