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[ValueTracking] usub(a, b) cannot overflow if a >= b.
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If we know that a >= b (unsigned), usub.with.overflow(a, b) cannot
overflow. Similarly, if b > a, the same expression overflows.

Reviewers: nikic, RKSimon, lebedev.ri, spatel

Reviewed By: nikic, Gerolf

Differential Revision: https://reviews.llvm.org/D74066

(Cherry-picked from 14ef87b)
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@fhahn
fhahn committed Feb 11, 2020
1 parent 67c42f7 commit b14f09d
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Showing 2 changed files with 28 additions and 35 deletions.
10 changes: 10 additions & 0 deletions llvm/lib/Analysis/ValueTracking.cpp
View file
Expand Up @@ -4309,6 +4309,16 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
AssumptionCache *AC,
const Instruction *CxtI,
const DominatorTree *DT) {
// Checking for conditions implied by dominating conditions may be expensive.
// Limit it to usub_with_overflow calls for now.
if (match(CxtI,
m_Intrinsic<Intrinsic::usub_with_overflow>(m_Value(), m_Value())))
if (auto C =
isImpliedByDomCondition(CmpInst::ICMP_UGE, LHS, RHS, CxtI, DL)) {
if (*C)
return OverflowResult::NeverOverflows;
return OverflowResult::AlwaysOverflowsLow;
}
ConstantRange LHSRange = computeConstantRangeIncludingKnownBits(
LHS, /*ForSigned=*/false, DL, /*Depth=*/0, AC, CxtI, DT);
ConstantRange RHSRange = computeConstantRangeIncludingKnownBits(
Expand Down
53 changes: 18 additions & 35 deletions llvm/test/Transforms/InstCombine/usub-overflow-known-by-implied-cond.ll
View file
Expand Up @@ -9,12 +9,9 @@ define i32 @test1(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp ult i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB3:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB2:%.*]], label [[BB3]]
; CHECK-NEXT: br i1 false, label [[BB2:%.*]], label [[BB3]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: ret i32 undef
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand All @@ -39,12 +36,10 @@ define i32 @test2(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp ult i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB3:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB3]], label [[BB2:%.*]]
; CHECK-NEXT: br i1 false, label [[BB3]], label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: [[SUB1:%.*]] = sub nuw i32 [[A]], [[B]]
; CHECK-NEXT: ret i32 [[SUB1]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand All @@ -70,12 +65,9 @@ define i32 @test3(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp ugt i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB2:%.*]], label [[BB3]]
; CHECK-NEXT: br i1 false, label [[BB2:%.*]], label [[BB3]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: ret i32 undef
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand All @@ -100,12 +92,10 @@ define i32 @test4(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp ugt i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB3]], label [[BB2:%.*]]
; CHECK-NEXT: br i1 false, label [[BB3]], label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: [[SUB1:%.*]] = sub nuw i32 [[A]], [[B]]
; CHECK-NEXT: ret i32 [[SUB1]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand All @@ -131,12 +121,10 @@ define i32 @test5(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB3]], label [[BB2:%.*]]
; CHECK-NEXT: br i1 false, label [[BB3]], label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: [[SUB1:%.*]] = sub nuw i32 [[A]], [[B]]
; CHECK-NEXT: ret i32 [[SUB1]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand All @@ -161,12 +149,9 @@ define i32 @test6(i32 %a, i32 %b) {
; CHECK-NEXT: [[COND:%.*]] = icmp ult i32 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB3]], label [[BB2:%.*]]
; CHECK-NEXT: br i1 true, label [[BB3]], label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: ret i32 undef
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand Down Expand Up @@ -252,12 +237,10 @@ define i32 @test9(i32 %a, i32 %b, i1 %cond2) {
; CHECK-NEXT: [[AND:%.*]] = and i1 [[COND]], [[COND2:%.*]]
; CHECK-NEXT: br i1 [[AND]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[SUB1:%.*]] = call { i32, i1 } @llvm.usub.with.overflow.i32(i32 [[A]], i32 [[B]])
; CHECK-NEXT: [[C1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 1
; CHECK-NEXT: br i1 [[C1]], label [[BB3]], label [[BB2:%.*]]
; CHECK-NEXT: br i1 false, label [[BB3]], label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[R1:%.*]] = extractvalue { i32, i1 } [[SUB1]], 0
; CHECK-NEXT: ret i32 [[R1]]
; CHECK-NEXT: [[SUB1:%.*]] = sub nuw i32 [[A]], [[B]]
; CHECK-NEXT: ret i32 [[SUB1]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
Expand Down

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