[X86] Prevent the creation of redundant ops for sadd and ssub with overflow.

This commit teaches the X86 backend to create the same X86 instructions when it
lowers an sadd/ssub with overflow intrinsic and a conditional branch that uses
that overflow result. This allows SelectionDAG to recognize and remove one of
the redundant operations.

This fixes <rdar://problem/15874016> and <rdar://problem/15661073>.

Reviewed by Nadav

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199976 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 17 insertions, 2 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 4b30ff67c9..03eb57a408 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -10715,11 +10715,26 @@ SDValue X86TargetLowering::LowerBRCOND(SDValue Op, SelectionDAG &DAG) const {
     unsigned X86Opcode;
     unsigned X86Cond;
     SDVTList VTs;
+    // Keep this in sync with LowerXALUO, otherwise we might create redundant
+    // instructions that can't be removed afterwards (i.e. X86ISD::ADD and
+    // X86ISD::INC).
     switch (CondOpcode) {
     case ISD::UADDO: X86Opcode = X86ISD::ADD; X86Cond = X86::COND_B; break;
-    case ISD::SADDO: X86Opcode = X86ISD::ADD; X86Cond = X86::COND_O; break;
+    case ISD::SADDO:
+      if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(RHS))
+        if (C->isOne()) {
+          X86Opcode = X86ISD::INC; X86Cond = X86::COND_O;
+          break;
+        }
+      X86Opcode = X86ISD::ADD; X86Cond = X86::COND_O; break;
     case ISD::USUBO: X86Opcode = X86ISD::SUB; X86Cond = X86::COND_B; break;
-    case ISD::SSUBO: X86Opcode = X86ISD::SUB; X86Cond = X86::COND_O; break;
+    case ISD::SSUBO:
+      if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(RHS))
+        if (C->isOne()) {
+          X86Opcode = X86ISD::DEC; X86Cond = X86::COND_O;
+          break;
+        }
+      X86Opcode = X86ISD::SUB; X86Cond = X86::COND_O; break;
     case ISD::UMULO: X86Opcode = X86ISD::UMUL; X86Cond = X86::COND_O; break;
     case ISD::SMULO: X86Opcode = X86ISD::SMUL; X86Cond = X86::COND_O; break;
     default: llvm_unreachable("unexpected overflowing operator");