From a90c8e690bd9103bb4a5d943f98279a55bf42ad1 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Fri, 23 Jan 2009 19:10:37 +0000
Subject: [PATCH] Fold x-0 to x in unsafe-fp-math mode. This comes up in the
 testcase from PR3376, and in fact is sufficient to completely avoid the
 problem in that testcase.

There's an underlying problem though; TLI.isOperationLegal
considers Custom to be Legal, which might be ok in some
cases, but that's what DAGCombiner is using in many places
to test if something is legal when LegalOperations is true.
When DAGCombiner is running after legalize, this isn't
sufficient. I'll address this in a separate commit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62860 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/SelectionDAG/DAGCombiner.cpp  |  3 +++
 lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 25 +++++++++++++++--------
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7e8e46fba50..e4b24f1aaff 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3853,6 +3853,9 @@ SDValue DAGCombiner::visitFSUB(SDNode *N) {
   // fold (fsub c1, c2) -> c1-c2
   if (N0CFP && N1CFP && VT != MVT::ppcf128)
     return DAG.getNode(ISD::FSUB, VT, N0, N1);
+  // fold (A-0) -> A
+  if (UnsafeFPMath && N1CFP && N1CFP->getValueAPF().isZero())
+    return N0;
   // fold (0-B) -> -B
   if (UnsafeFPMath && N0CFP && N0CFP->getValueAPF().isZero()) {
     if (isNegatibleForFree(N1, LegalOperations))
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 29744862089..6d1fdd23407 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2402,15 +2402,22 @@ SDValue SelectionDAG::getNode(unsigned Opcode, MVT VT,
   case ISD::FMUL:
   case ISD::FDIV:
   case ISD::FREM:
-    if (UnsafeFPMath && Opcode == ISD::FADD) {
-      // 0+x --> x
-      if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N1))
-        if (CFP->getValueAPF().isZero())
-          return N2;
-      // x+0 --> x
-      if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N2))
-        if (CFP->getValueAPF().isZero())
-          return N1;
+    if (UnsafeFPMath) {
+      if (Opcode == ISD::FADD) {
+        // 0+x --> x
+        if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N1))
+          if (CFP->getValueAPF().isZero())
+            return N2;
+        // x+0 --> x
+        if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N2))
+          if (CFP->getValueAPF().isZero())
+            return N1;
+      } else if (Opcode == ISD::FSUB) {
+        // x-0 --> x
+        if (ConstantFPSDNode *CFP = dyn_cast<ConstantFPSDNode>(N2))
+          if (CFP->getValueAPF().isZero())
+            return N1;
+      }
     }
     assert(N1.getValueType() == N2.getValueType() &&
            N1.getValueType() == VT && "Binary operator types must match!");