Commit 14ef87bd authored by Florian Hahn's avatar Florian Hahn
Browse files

[ValueTracking] usub(a, b) cannot overflow if a >= b. 

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
parent 74734e80

llvm/lib/Analysis/ValueTracking.cpp

+10 −0
Original line number Diff line number Diff line
@@ -4439,6 +4439,16 @@ 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(

llvm/test/Transforms/InstCombine/usub-overflow-known-by-implied-cond.ll

+18 −35
Original line number Diff line number Diff line
@@ -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

;
@@ -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

;
@@ -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

;
@@ -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

;
@@ -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

;
@@ -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

;
@@ -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

;