[PATCH] D57453: [InstCombine] try to reduce x86 addcarry to generic uaddo intrinsic

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: RKSimon, craig.topper.
Herald added a subscriber: mcrosier.

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 I think 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.


https://reviews.llvm.org/D57453

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/Transforms/InstCombine/X86/addcarry.ll


Index: test/Transforms/InstCombine/X86/addcarry.ll
===================================================================
--- test/Transforms/InstCombine/X86/addcarry.ll
+++ test/Transforms/InstCombine/X86/addcarry.ll
@@ -6,11 +6,12 @@
 
 define i32 @no_carryin_i32(i32 %x, i32 %y, i8* %p) {
 ; CHECK-LABEL: @no_carryin_i32(
-; CHECK-NEXT:    [[S:%.*]] = call { i8, i32 } @llvm.x86.addcarry.32(i8 0, i32 [[X:%.*]], i32 [[Y:%.*]])
-; CHECK-NEXT:    [[OV:%.*]] = extractvalue { i8, i32 } [[S]], 0
-; CHECK-NEXT:    store i8 [[OV]], i8* [[P:%.*]], align 1
-; CHECK-NEXT:    [[R:%.*]] = extractvalue { i8, i32 } [[S]], 1
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = extractvalue { i32, i1 } [[TMP1]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = extractvalue { i32, i1 } [[TMP1]], 1
+; CHECK-NEXT:    [[TMP4:%.*]] = zext i1 [[TMP3]] to i8
+; CHECK-NEXT:    store i8 [[TMP4]], i8* [[P:%.*]], align 1
+; CHECK-NEXT:    ret i32 [[TMP2]]
 ;
   %s = call { i8, i32 } @llvm.x86.addcarry.32(i8 0, i32 %x, i32 %y)
   %ov = extractvalue { i8, i32 } %s, 0
@@ -21,11 +22,12 @@
 
 define i64 @no_carryin_i64(i64 %x, i64 %y, i8* %p) {
 ; CHECK-LABEL: @no_carryin_i64(
-; CHECK-NEXT:    [[S:%.*]] = call { i8, i64 } @llvm.x86.addcarry.64(i8 0, i64 [[X:%.*]], i64 [[Y:%.*]])
-; CHECK-NEXT:    [[OV:%.*]] = extractvalue { i8, i64 } [[S]], 0
-; CHECK-NEXT:    store i8 [[OV]], i8* [[P:%.*]], align 1
-; CHECK-NEXT:    [[R:%.*]] = extractvalue { i8, i64 } [[S]], 1
-; CHECK-NEXT:    ret i64 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 [[X:%.*]], i64 [[Y:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = extractvalue { i64, i1 } [[TMP1]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = extractvalue { i64, i1 } [[TMP1]], 1
+; CHECK-NEXT:    [[TMP4:%.*]] = zext i1 [[TMP3]] to i8
+; CHECK-NEXT:    store i8 [[TMP4]], i8* [[P:%.*]], align 1
+; CHECK-NEXT:    ret i64 [[TMP2]]
 ;
   %s = call { i8, i64 } @llvm.x86.addcarry.64(i8 0, i64 %x, i64 %y)
   %ov = extractvalue { i8, i64 } %s, 0
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -752,6 +752,33 @@
   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));
@@ -3128,6 +3155,12 @@
       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


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