[InstCombine] KnownBits::isNonNegative should recognize b - a after a <= b

llvm/lib/Analysis/ValueTracking.cpp

@@ -278,7 +278,16 @@ static bool isKnownNonZero(const Value *V, const APInt &DemandedElts,
 
 bool llvm::isKnownNonNegative(const Value *V, const SimplifyQuery &SQ,
                               unsigned Depth) {
-  return computeKnownBits(V, SQ, Depth).isNonNegative();
+  if (computeKnownBits(V, SQ, Depth).isNonNegative())
+    return true;
+
+  Value *X, *Y;
+  if (match(V, m_NSWSub(m_Value(X), m_Value(Y))))
+    if (std::optional<bool> result =
+            isImpliedByDomCondition(ICmpInst::ICMP_SLE, Y, X, SQ.CxtI, SQ.DL))
+      return *result;
+
+  return false;
 }
 
 bool llvm::isKnownPositive(const Value *V, const SimplifyQuery &SQ,

llvm/test/Transforms/InstCombine/sub-after-sle-is-non-negative.ll

@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+define void @test_as_arg(i8 %a, i8 %b) {
+; CHECK-LABEL: define void @test_as_arg(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i8 [[A]], [[B]]
+; CHECK-NEXT:    br i1 [[CMP]], label %[[COND_END:.*]], label %[[COND_FALSE:.*]]
+; CHECK:       [[COND_FALSE]]:
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i8 [[B]], [[A]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext nneg i8 [[SUB]] to i16
+; CHECK-NEXT:    call void @subroutine(i16 [[CONV]])
+; CHECK-NEXT:    br label %[[COND_END]]
+; CHECK:       [[COND_END]]:
+; CHECK-NEXT:    ret void
+;
+  %cmp = icmp sgt i8 %a, %b
+  br i1 %cmp, label %cond.end, label %cond.false
+
+cond.false:
+  %sub = sub nsw i8 %b, %a
+  %conv = sext i8 %sub to i16
+  call void @subroutine(i16 %conv)
+  br label %cond.end
+
+cond.end:
+  ret void
+}
+
+declare void @subroutine(i16)
+
+define i16 @test_as_retval(i8 %a, i8 %b) {
+; CHECK-LABEL: define i16 @test_as_retval(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i8 [[A]], [[B]]
+; CHECK-NEXT:    br i1 [[CMP]], label %[[COND_TRUE:.*]], label %[[COND_FALSE:.*]]
+; CHECK:       [[COND_TRUE]]:
+; CHECK-NEXT:    ret i16 0
+; CHECK:       [[COND_FALSE]]:
+; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i8 [[B]], [[A]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext nneg i8 [[SUB]] to i16
+; CHECK-NEXT:    ret i16 [[CONV]]
+;
+  %cmp = icmp sgt i8 %a, %b
+  br i1 %cmp, label %cond.true, label %cond.false
+
+cond.true:
+  ret i16 0
+
+cond.false:
+  %sub = sub nsw i8 %b, %a
+  %conv = sext i8 %sub to i16
+  ret i16 %conv
+}