[DAG] use SDNode flags 'nsz' to enable fadd/fsub with zero folds

As discussed in D24815, let's start the process of killing off the broken fast-math global
state housed in TargetOptions and eliminate the need for function-level fast-math attributes.

Here we enable two similar folds that are possible when we don't care about signed-zero:
fadd nsz x, 0 --> x
fsub nsz 0, x --> -x

Note that although the test cases include a 'sin' function call, I'm side-stepping the 
FMF-on-calls question (and lack of support in the DAG) for now. It's not needed for these
tests - isNegatibleForFree/GetNegatedExpression just look through a ISD::FSIN node.

Also, when we create an FNEG node and propagate the Flags of the FSUB to it, this doesn't
actually do anything today because Flags are silently dropped for any node that is not a
binary operator.

Differential Revision: https://reviews.llvm.org/D25297

llvm-svn: 284824

1 files changed, 20 insertions, 16 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 81886941009..5ae47c93936 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -622,7 +622,8 @@ static char isNegatibleForFree(SDValue Op, bool LegalOperations,
                               Depth + 1);
   case ISD::FSUB:
     // We can't turn -(A-B) into B-A when we honor signed zeros.
-    if (!Options->UnsafeFPMath) return 0;
+    if (!Options->UnsafeFPMath && !Op.getNode()->getFlags()->hasNoSignedZeros())
+      return 0;
 
     // fold (fneg (fsub A, B)) -> (fsub B, A)
     return 1;
@@ -685,9 +686,6 @@ static SDValue GetNegatedExpression(SDValue Op, SelectionDAG &DAG,
                                             LegalOperations, Depth+1),
                        Op.getOperand(0), Flags);
   case ISD::FSUB:
-    // We can't turn -(A-B) into B-A when we honor signed zeros.
-    assert(Options.UnsafeFPMath);
-
     // fold (fneg (fsub 0, B)) -> B
     if (ConstantFPSDNode *N0CFP = dyn_cast<ConstantFPSDNode>(Op.getOperand(0)))
       if (N0CFP->isZero())
@@ -8462,17 +8460,20 @@ SDValue DAGCombiner::visitFADD(SDNode *N) {
     return DAG.getNode(ISD::FSUB, DL, VT, N1,
                        GetNegatedExpression(N0, DAG, LegalOperations), Flags);
 
+  // FIXME: Auto-upgrade the target/function-level option.
+  if (Options.UnsafeFPMath || N->getFlags()->hasNoSignedZeros()) {
+    // fold (fadd A, 0) -> A
+    if (ConstantFPSDNode *N1C = isConstOrConstSplatFP(N1))
+      if (N1C->isZero())
+        return N0;
+  }
+
   // If 'unsafe math' is enabled, fold lots of things.
   if (Options.UnsafeFPMath) {
     // No FP constant should be created after legalization as Instruction
     // Selection pass has a hard time dealing with FP constants.
     bool AllowNewConst = (Level < AfterLegalizeDAG);
 
-    // fold (fadd A, 0) -> A
-    if (ConstantFPSDNode *N1C = isConstOrConstSplatFP(N1))
-      if (N1C->isZero())
-        return N0;
-
     // fold (fadd (fadd x, c1), c2) -> (fadd x, (fadd c1, c2))
     if (N1CFP && N0.getOpcode() == ISD::FADD && N0.getNode()->hasOneUse() &&
         isConstantFPBuildVectorOrConstantFP(N0.getOperand(1)))
@@ -8599,19 +8600,22 @@ SDValue DAGCombiner::visitFSUB(SDNode *N) {
     return DAG.getNode(ISD::FADD, DL, VT, N0,
                        GetNegatedExpression(N1, DAG, LegalOperations), Flags);
 
-  // If 'unsafe math' is enabled, fold lots of things.
-  if (Options.UnsafeFPMath) {
-    // (fsub A, 0) -> A
-    if (N1CFP && N1CFP->isZero())
-      return N0;
-
+  // FIXME: Auto-upgrade the target/function-level option.
+  if (Options.UnsafeFPMath || N->getFlags()->hasNoSignedZeros()) {
     // (fsub 0, B) -> -B
     if (N0CFP && N0CFP->isZero()) {
       if (isNegatibleForFree(N1, LegalOperations, TLI, &Options))
         return GetNegatedExpression(N1, DAG, LegalOperations);
       if (!LegalOperations || TLI.isOperationLegal(ISD::FNEG, VT))
-        return DAG.getNode(ISD::FNEG, DL, VT, N1);
+        return DAG.getNode(ISD::FNEG, DL, VT, N1, Flags);
     }
+  }
+
+  // If 'unsafe math' is enabled, fold lots of things.
+  if (Options.UnsafeFPMath) {
+    // (fsub A, 0) -> A
+    if (N1CFP && N1CFP->isZero())
+      return N0;
 
     // (fsub x, x) -> 0.0
     if (N0 == N1)