[llvm] a5f7bc0 - [InstCombine] Canonicalize uadd.with.overflow to uadd.sat

[David Green via llvm-commits]

Author: David Green
Date: 2019-10-31T12:45:38Z
New Revision: a5f7bc0de72f1c631ef13d2cccf2b77c9a030e7d

URL: https://github.com/llvm/llvm-project/commit/a5f7bc0de72f1c631ef13d2cccf2b77c9a030e7d
DIFF: https://github.com/llvm/llvm-project/commit/a5f7bc0de72f1c631ef13d2cccf2b77c9a030e7d.diff

LOG: [InstCombine] Canonicalize uadd.with.overflow to uadd.sat

This adds some patterns to transform uadd.with.overflow to uadd.sat
(with usub.with.overflow to usub.sat too). The patterns selects from
UINTMAX (or 0 for subs) depending on whether the operation overflowed.

Signed patterns are a little more involved (they can wrap in two
directions), but can be added here in a followup patch too.

Differential Revision: https://reviews.llvm.org/D69245

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
    llvm/test/Transforms/InstCombine/overflow_to_sat.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index b06d31a3fa2d..738bd966ac46 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1732,6 +1732,36 @@ static Instruction *foldAddSubSelect(SelectInst &SI,
   return nullptr;
 }
 
+/// Turn X + Y overflows ? -1 : X + Y -> uadd_sat X, Y
+/// And X - Y overflows ? 0 : X - Y -> usub_sat X, Y
+static Instruction *
+foldOverflowingAddSubSelect(SelectInst &SI, InstCombiner::BuilderTy &Builder) {
+  Value *CondVal = SI.getCondition();
+  Value *TrueVal = SI.getTrueValue();
+  Value *FalseVal = SI.getFalseValue();
+
+  WithOverflowInst *II;
+  if (!match(CondVal, m_ExtractValue<1>(m_WithOverflowInst(II))) ||
+      !match(FalseVal, m_ExtractValue<0>(m_Specific(II))))
+    return nullptr;
+
+  Intrinsic::ID NewIntrinsicID;
+  if (II->getIntrinsicID() == Intrinsic::uadd_with_overflow &&
+      match(TrueVal, m_AllOnes()))
+    // X + Y overflows ? -1 : X + Y -> uadd_sat X, Y
+    NewIntrinsicID = Intrinsic::uadd_sat;
+  else if (II->getIntrinsicID() == Intrinsic::usub_with_overflow &&
+           match(TrueVal, m_Zero()))
+    // X - Y overflows ? 0 : X - Y -> usub_sat X, Y
+    NewIntrinsicID = Intrinsic::usub_sat;
+  else
+    return nullptr;
+
+  Function *F =
+      Intrinsic::getDeclaration(SI.getModule(), NewIntrinsicID, SI.getType());
+  return CallInst::Create(F, {II->getArgOperand(0), II->getArgOperand(1)});
+}
+
 Instruction *InstCombiner::foldSelectExtConst(SelectInst &Sel) {
   Constant *C;
   if (!match(Sel.getTrueValue(), m_Constant(C)) &&
@@ -2398,6 +2428,8 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
 
   if (Instruction *Add = foldAddSubSelect(SI, Builder))
     return Add;
+  if (Instruction *Add = foldOverflowingAddSubSelect(SI, Builder))
+    return Add;
 
   // Turn (select C, (op X, Y), (op X, Z)) -> (op X, (select C, Y, Z))
   auto *TI = dyn_cast<Instruction>(TrueVal);

diff  --git a/llvm/test/Transforms/InstCombine/overflow_to_sat.ll b/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
index 4bc989648e69..ddafc43e89fd 100644
--- a/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
+++ b/llvm/test/Transforms/InstCombine/overflow_to_sat.ll
@@ -3,10 +3,7 @@
 
 define i32 @uadd(i32 %x, i32 %y) {
 ; CHECK-LABEL: @uadd(
-; CHECK-NEXT:    [[AO:%.*]] = tail call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
-; CHECK-NEXT:    [[O:%.*]] = extractvalue { i32, i1 } [[AO]], 1
-; CHECK-NEXT:    [[A:%.*]] = extractvalue { i32, i1 } [[AO]], 0
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[O]], i32 -1, i32 [[A]]
+; CHECK-NEXT:    [[S:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
   %ao = tail call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 %x, i32 %y)
@@ -18,10 +15,7 @@ define i32 @uadd(i32 %x, i32 %y) {
 
 define i32 @usub(i32 %x, i32 %y) {
 ; CHECK-LABEL: @usub(
-; CHECK-NEXT:    [[AO:%.*]] = tail call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
-; CHECK-NEXT:    [[O:%.*]] = extractvalue { i32, i1 } [[AO]], 1
-; CHECK-NEXT:    [[A:%.*]] = extractvalue { i32, i1 } [[AO]], 0
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[O]], i32 0, i32 [[A]]
+; CHECK-NEXT:    [[S:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
   %ao = tail call { i32, i1 } @llvm.usub.with.overflow.i32(i32 %x, i32 %y)