[InstCombine] ssubo X, C -> saddo X, -C

ssubo X, C is equivalent to saddo X, -C. Make the transformation in
InstCombine and allow the logic implemented for saddo to fold prior
usages of add nsw or sub nsw with constants.

Patch by Dan Robertson.

Differential Revision: https://reviews.llvm.org/D60061

llvm-svn: 358099

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 08f4be9b737..2cc2e824cfb 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2125,6 +2125,7 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
     }
     break;
   }
+
   case Intrinsic::umul_with_overflow:
   case Intrinsic::smul_with_overflow:
     if (Instruction *I = canonicalizeConstantArg0ToArg1(CI))
@@ -2132,10 +2133,30 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
     LLVM_FALLTHROUGH;
 
   case Intrinsic::usub_with_overflow:
+    if (Instruction *I = foldIntrinsicWithOverflowCommon(II))
+      return I;
+    break;
+
   case Intrinsic::ssub_with_overflow: {
     if (Instruction *I = foldIntrinsicWithOverflowCommon(II))
       return I;
 
+    Constant *C;
+    Value *Arg0 = II->getArgOperand(0);
+    Value *Arg1 = II->getArgOperand(1);
+    // Given a constant C that is not the minimum signed value
+    // for an integer of a given bit width:
+    //
+    // ssubo X, C -> saddo X, -C
+    if (match(Arg1, m_Constant(C)) && C->isNotMinSignedValue()) {
+      Value *NegVal = ConstantExpr::getNeg(C);
+      // Build a saddo call that is equivalent to the discovered
+      // ssubo call.
+      return replaceInstUsesWith(
+          *II, Builder.CreateBinaryIntrinsic(Intrinsic::sadd_with_overflow,
+                                             Arg0, NegVal));
+    }
+
     break;
   }