[llvm] r177933 - Disable some unsafe-fp-math DAG-combine transformation after legalization.

[Shuxin Yang]

Author: shuxin_yang
Date: Mon Mar 25 17:52:29 2013
New Revision: 177933

URL: http://llvm.org/viewvc/llvm-project?rev=177933&view=rev
Log:
Disable some unsafe-fp-math DAG-combine transformation after legalization.
For instance, following transformation will be disabled:
    x + x + x => 3.0f * x;

The problem of these transformations is that it introduces a FP constant, which
following Instruction-Selection pass cannot handle.

Reviewed by Nadav, thanks a lot!

rdar://13445387

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/trunk/test/CodeGen/X86/dagcombine_unsafe_math.ll

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=177933&r1=177932&r2=177933&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Mon Mar 25 17:52:29 2013
@@ -5835,14 +5835,25 @@ SDValue DAGCombiner::visitFADD(SDNode *N
                        DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
                                    N0.getOperand(1), N1));
 
+  // No FP constant should be created after legalization as Instruction
+  // Selection pass has hard time in dealing with FP constant.
+  //
+  // We don't need test this condition for transformation like following, as
+  // the DAG being transformed implies it is legal to take FP constant as
+  // operand.
+  // 
+  //  (fadd (fmul c, x), x) -> (fmul c+1, x)
+  // 
+  bool AllowNewFpConst = (Level < AfterLegalizeDAG);
+
   // If allow, fold (fadd (fneg x), x) -> 0.0
-  if (DAG.getTarget().Options.UnsafeFPMath &&
+  if (AllowNewFpConst && DAG.getTarget().Options.UnsafeFPMath &&
       N0.getOpcode() == ISD::FNEG && N0.getOperand(0) == N1) {
     return DAG.getConstantFP(0.0, VT);
   }
 
     // If allow, fold (fadd x, (fneg x)) -> 0.0
-  if (DAG.getTarget().Options.UnsafeFPMath &&
+  if (AllowNewFpConst && DAG.getTarget().Options.UnsafeFPMath &&
       N1.getOpcode() == ISD::FNEG && N1.getOperand(0) == N0) {
     return DAG.getConstantFP(0.0, VT);
   }
@@ -5944,7 +5955,7 @@ SDValue DAGCombiner::visitFADD(SDNode *N
       }
     }
 
-    if (N0.getOpcode() == ISD::FADD) {
+    if (N0.getOpcode() == ISD::FADD && AllowNewFpConst) {
       ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N0.getOperand(0));
       // (fadd (fadd x, x), x) -> (fmul 3.0, x)
       if (!CFP && N0.getOperand(0) == N0.getOperand(1) &&
@@ -5954,7 +5965,7 @@ SDValue DAGCombiner::visitFADD(SDNode *N
       }
     }
 
-    if (N1.getOpcode() == ISD::FADD) {
+    if (N1.getOpcode() == ISD::FADD && AllowNewFpConst) {
       ConstantFPSDNode *CFP10 = dyn_cast<ConstantFPSDNode>(N1.getOperand(0));
       // (fadd x, (fadd x, x)) -> (fmul 3.0, x)
       if (!CFP10 && N1.getOperand(0) == N1.getOperand(1) &&
@@ -5965,7 +5976,8 @@ SDValue DAGCombiner::visitFADD(SDNode *N
     }
 
     // (fadd (fadd x, x), (fadd x, x)) -> (fmul 4.0, x)
-    if (N0.getOpcode() == ISD::FADD && N1.getOpcode() == ISD::FADD &&
+    if (AllowNewFpConst &&
+        N0.getOpcode() == ISD::FADD && N1.getOpcode() == ISD::FADD &&
         N0.getOperand(0) == N0.getOperand(1) &&
         N1.getOperand(0) == N1.getOperand(1) &&
         N0.getOperand(0) == N1.getOperand(0)) {

Modified: llvm/trunk/test/CodeGen/X86/dagcombine_unsafe_math.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/dagcombine_unsafe_math.ll?rev=177933&r1=177932&r2=177933&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/dagcombine_unsafe_math.ll (original)
+++ llvm/trunk/test/CodeGen/X86/dagcombine_unsafe_math.ll Mon Mar 25 17:52:29 2013
@@ -40,3 +40,17 @@ define float @test4(float %x, float %y)
 ; CHECK: test4
 ; CHECK: vaddss
 }
+
+; rdar://13445387
+; "x + x + x => 3.0 * x" should be disabled after legalization because 
+; Instruction-Selection dosen't know how to handle "3.0"
+; 
+define float @test5() {
+  %mul.i.i151 = fmul <4 x float> zeroinitializer, zeroinitializer
+  %vecext.i8.i152 = extractelement <4 x float> %mul.i.i151, i32 1
+  %vecext1.i9.i153 = extractelement <4 x float> %mul.i.i151, i32 0
+  %add.i10.i154 = fadd float %vecext1.i9.i153, %vecext.i8.i152
+  %vecext.i7.i155 = extractelement <4 x float> %mul.i.i151, i32 2
+  %add.i.i156 = fadd float %vecext.i7.i155, %add.i10.i154
+  ret float %add.i.i156
+}