[InstCombine] try to reduce x86 addcarry to generic uaddo intrinsic

If we can reduce the x86-specific intrinsic to the generic op, it allows existing 
simplifications and value tracking folds. AFAICT, this always results in identical 
x86 codegen in the non-reduced case...which should be true because we semi-generically 
(too aggressively IMO) convert to llvm.uadd.with.overflow in CGP, so the DAG/isel must 
already combine/lower this intrinsic as expected.

This isn't quite what was requested in:
https://bugs.llvm.org/show_bug.cgi?id=40486
...but we want to have these kinds of folds early for efficiency and to enable greater 
simplifications. For the case in the bug report where we have:
_addcarry_u64(0, ahi, 0, &ahi)
...this gets completely simplified away in IR.

Differential Revision: https://reviews.llvm.org/D57453

llvm-svn: 352870

1 files changed, 33 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 7b09fe615f1..80d1144ee1b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -751,6 +751,33 @@ static Value *simplifyX86movmsk(const IntrinsicInst &II,
   return nullptr;
 }
 
+static Value *simplifyX86addcarry(const IntrinsicInst &II,
+                                  InstCombiner::BuilderTy &Builder) {
+  Value *CarryIn = II.getArgOperand(0);
+  Value *Op1 = II.getArgOperand(1);
+  Value *Op2 = II.getArgOperand(2);
+  Type *RetTy = II.getType();
+  Type *OpTy = Op1->getType();
+  assert(RetTy->getStructElementType(0)->isIntegerTy(8) &&
+         RetTy->getStructElementType(1) == OpTy && OpTy == Op2->getType() &&
+         "Unexpected types for x86 addcarry");
+
+  // If carry-in is zero, this is just an unsigned add with overflow.
+  if (match(CarryIn, m_ZeroInt())) {
+    Value *UAdd = Builder.CreateIntrinsic(Intrinsic::uadd_with_overflow, OpTy,
+                                          { Op1, Op2 });
+    // The types have to be adjusted to match the x86 call types.
+    Value *UAddResult = Builder.CreateExtractValue(UAdd, 0);
+    Value *UAddOV = Builder.CreateZExt(Builder.CreateExtractValue(UAdd, 1),
+                                       Builder.getInt8Ty());
+    Value *Res = UndefValue::get(II.getType());
+    Res = Builder.CreateInsertValue(Res, UAddOV, 0);
+    return Builder.CreateInsertValue(Res, UAddResult, 1);
+  }
+
+  return nullptr;
+}
+
 static Value *simplifyX86insertps(const IntrinsicInst &II,
                                   InstCombiner::BuilderTy &Builder) {
   auto *CInt = dyn_cast<ConstantInt>(II.getArgOperand(2));
@@ -3109,6 +3136,12 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
       return nullptr;
     break;
 
+  case Intrinsic::x86_addcarry_32:
+  case Intrinsic::x86_addcarry_64:
+    if (Value *V = simplifyX86addcarry(*II, Builder))
+      return replaceInstUsesWith(*II, V);
+    break;
+
   case Intrinsic::ppc_altivec_vperm:
     // Turn vperm(V1,V2,mask) -> shuffle(V1,V2,mask) if mask is a constant.
     // Note that ppc_altivec_vperm has a big-endian bias, so when creating