Add a target lowering hook to control whether it's worthwhile to compress fp constant.

For x86, if sse2 is available, it's not a good idea since cvtss2sd is slower than a movsd load and it prevents load folding. On x87, it's important to shrink fp constant since fldt is very expensive.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47931 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 12 insertions, 1 deletions

diff --git a/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index c1bf2188b1..24dd2fd3bd 100644
--- a/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -504,7 +504,8 @@ static SDOperand ExpandConstantFP(ConstantFPSDNode *CFP, bool UseCP,
     if (CFP->isValueValidForType(SVT, CFP->getValueAPF()) &&
         // Only do this if the target has a native EXTLOAD instruction from
         // smaller type.
-        TLI.isLoadXLegal(ISD::EXTLOAD, SVT)) {
+        TLI.isLoadXLegal(ISD::EXTLOAD, SVT) &&
+        TLI.ShouldShrinkFPConstant(VT)) {
       const Type *SType = MVT::getTypeForValueType(SVT);
       LLVMC = cast<ConstantFP>(ConstantExpr::getFPTrunc(LLVMC, SType));
       VT = SVT;
diff --git a/lib/Target/X86/X86ISelLowering.h b/lib/Target/X86/X86ISelLowering.h
index d9593a4ef6..e0ddf63bf0 100644
--- a/lib/Target/X86/X86ISelLowering.h
+++ b/lib/Target/X86/X86ISelLowering.h
@@ -434,6 +434,16 @@ namespace llvm {
     virtual bool isVectorClearMaskLegal(std::vector<SDOperand> &BVOps,
                                         MVT::ValueType EVT,
                                         SelectionDAG &DAG) const;
+
+    /// ShouldShrinkFPConstant - If true, then instruction selection should
+    /// seek to shrink the FP constant of the specified type to a smaller type
+    /// in order to save space and / or reduce runtime.
+    virtual bool ShouldShrinkFPConstant(MVT::ValueType VT) const {
+      // Don't shrink FP constpool if SSE2 is available since cvtss2sd is more
+      // expensive than a straight movsd. On the other hand, it's important to
+      // shrink long double fp constant since fldt is very slow.
+      return !X86ScalarSSEf64 || VT == MVT::f80;
+    }
     
     /// IsEligibleForTailCallOptimization - Check whether the call is eligible
     /// for tail call optimization. Target which want to do tail call