[PATCH] D69224: [InstCombine] Fold uadd.sat(a, b) == 0 and usub.sat(a, b) == 0

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, lebedev.ri, dmgreen.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

This adds folds for comparing uadd.sat/usub.sat with zero:

- `uadd.sat(a, b) == 0` => `a == 0 && b == 0` => `(a | b) == 0`
- `usub.sat(a, b) == 0` => `a <= b`

And inverted forms for `!=`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D69224

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/saturating-add-sub.ll


Index: llvm/test/Transforms/InstCombine/saturating-add-sub.ll
===================================================================
--- llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -1158,9 +1158,9 @@
 
 define i1 @scalar_uadd_eq_zero(i8 %a, i8 %b) {
 ; CHECK-LABEL: @scalar_uadd_eq_zero(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.uadd.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[SAT]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    [[TMP1:%.*]] = or i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    ret i1 [[TMP2]]
 ;
   %sat = call i8 @llvm.uadd.sat.i8(i8 %a, i8 %b)
   %cmp = icmp eq i8 %sat, 0
@@ -1169,9 +1169,9 @@
 
 define i1 @scalar_uadd_ne_zero(i8 %a, i8 %b) {
 ; CHECK-LABEL: @scalar_uadd_ne_zero(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.uadd.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[SAT]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    [[TMP1:%.*]] = or i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp ne i8 [[TMP1]], 0
+; CHECK-NEXT:    ret i1 [[TMP2]]
 ;
   %sat = call i8 @llvm.uadd.sat.i8(i8 %a, i8 %b)
   %cmp = icmp ne i8 %sat, 0
@@ -1180,8 +1180,7 @@
 
 define i1 @scalar_usub_eq_zero(i8 %a, i8 %b) {
 ; CHECK-LABEL: @scalar_usub_eq_zero(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[SAT]], 0
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ule i8 [[A:%.*]], [[B:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %sat = call i8 @llvm.usub.sat.i8(i8 %a, i8 %b)
@@ -1191,8 +1190,7 @@
 
 define i1 @scalar_usub_ne_zero(i8 %a, i8 %b) {
 ; CHECK-LABEL: @scalar_usub_ne_zero(
-; CHECK-NEXT:    [[SAT:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[A:%.*]], i8 [[B:%.*]])
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[SAT]], 0
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i8 [[A:%.*]], [[B:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %sat = call i8 @llvm.usub.sat.i8(i8 %a, i8 %b)
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3067,6 +3067,28 @@
     }
     break;
   }
+
+  case Intrinsic::uadd_sat: {
+    // uadd.sat(a, b) == 0  ->  (a | b) == 0
+    if (C.isNullValue()) {
+      Value *Or = Builder.CreateOr(II->getArgOperand(0), II->getArgOperand(1));
+      return replaceInstUsesWith(Cmp, Builder.CreateICmp(
+          Cmp.getPredicate(), Or, Constant::getNullValue(Ty)));
+
+    }
+    break;
+  }
+
+  case Intrinsic::usub_sat: {
+    // usub.sat(a, b) == 0  ->  a <= b
+    if (C.isNullValue()) {
+      ICmpInst::Predicate NewPred = Cmp.getPredicate() == ICmpInst::ICMP_EQ
+          ? ICmpInst::ICMP_ULE : ICmpInst::ICMP_UGT;
+      return ICmpInst::Create(Instruction::ICmp, NewPred,
+                              II->getArgOperand(0), II->getArgOperand(1));
+    }
+    break;
+  }
   default:
     break;
   }


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