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[InstCombine] mark ADD with nuw if no unsigned overflow

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Summary:
As a starting step, we only use one simple heuristic: if the sign bits
of both a and b are zero, we can prove "add a, b" do not unsigned
overflow, and thus convert it to "add nuw a, b".

Updated all affected tests and added two new tests (@zero_sign_bit and
@zero_sign_bit2) in AddOverflow.ll

Test Plan: make check-all

Reviewers: eliben, rafael, meheff, chandlerc

Reviewed By: chandlerc

Subscribers: chandlerc, llvm-commits

Differential Revision: http://reviews.llvm.org/D4144

llvm-svn: 211084
This commit is contained in:
Jingyue Wu 2014-06-17 00:42:07 +00:00
parent c1b16f1be2
commit 089dd5d55e
6 changed files with 54 additions and 9 deletions
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1
lib/Transforms/InstCombine/InstCombine.h
View File

@ -247,6 +247,7 @@ private:
bool DoXform = true);
Instruction *transformSExtICmp(ICmpInst *ICI, Instruction &CI);
bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS);
bool WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS);
Value *EmitGEPOffset(User *GEP);
Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);
Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);

22
lib/Transforms/InstCombine/InstCombineAddSub.cpp
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@ -965,6 +965,21 @@ bool InstCombiner::WillNotOverflowSignedAdd(Value *LHS, Value *RHS) {
return false;
}
/// WillNotOverflowUnsignedAdd - Return true if we can prove that:
/// (zext (add LHS, RHS)) === (add (zext LHS), (zext RHS))
bool InstCombiner::WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS) {
// There are different heuristics we can use for this. Here is a simple one.
// If the sign bit of LHS and that of RHS are both zero, no unsigned wrap.
bool LHSKnownNonNegative, LHSKnownNegative;
bool RHSKnownNonNegative, RHSKnownNegative;
ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);
ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);
if (LHSKnownNonNegative && RHSKnownNonNegative)
return true;
return false;
}
Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
bool Changed = SimplifyAssociativeOrCommutative(I);
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
@ -1240,10 +1255,17 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
return BinaryOperator::CreateOr(A, B);
}
// TODO(jingyue): Consider WillNotOverflowSignedAdd and
// WillNotOverflowUnsignedAdd to reduce the number of invocations of
// computeKnownBits.
if (!I.hasNoSignedWrap() && WillNotOverflowSignedAdd(LHS, RHS)) {
Changed = true;
I.setHasNoSignedWrap(true);
}
if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedAdd(LHS, RHS)) {
Changed = true;
I.setHasNoUnsignedWrap(true);
}
return Changed ? &I : nullptr;
}

24
test/Transforms/InstCombine/AddOverFlow.ll
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@ -12,6 +12,28 @@ define i16 @oppositesign(i16 %x, i16 %y) {
ret i16 %c
}
define i16 @zero_sign_bit(i16 %a) {
; CHECK-LABEL: @zero_sign_bit(
; CHECK-NEXT: and
; CHECK-NEXT: add nuw
; CHECK-NEXT: ret
%1 = and i16 %a, 32767
%2 = add i16 %1, 512
ret i16 %2
}
define i16 @zero_sign_bit2(i16 %a, i16 %b) {
; CHECK-LABEL: @zero_sign_bit2(
; CHECK-NEXT: and
; CHECK-NEXT: and
; CHECK-NEXT: add nuw
; CHECK-NEXT: ret
%1 = and i16 %a, 32767
%2 = and i16 %b, 32767
%3 = add i16 %1, %2
ret i16 %3
}
; CHECK-LABEL: @ripple_nsw1
; CHECK: add nsw i16 %a, %b
define i16 @ripple_nsw1(i16 %x, i16 %y) {
@ -45,7 +67,7 @@ define i32 @ripple_no_nsw1(i32 %x, i32 %y) {
}
; CHECK-LABEL: @ripple_no_nsw2
; CHECK: add i16 %a, %b
; CHECK: add nuw i16 %a, %b
define i16 @ripple_no_nsw2(i16 %x, i16 %y) {
; %a has at most one bit set
%a = and i16 %y, 1

2
test/Transforms/InstCombine/add-sitofp.ll
View File

@ -1,4 +1,4 @@
; RUN: opt < %s -instcombine -S | grep "add nsw i32"
; RUN: opt < %s -instcombine -S | grep "add nuw nsw i32"
define double @x(i32 %a, i32 %b) nounwind {
%m = lshr i32 %a, 24

6
test/Transforms/InstCombine/ffs-1.ll
View File

@ -103,7 +103,7 @@ define i32 @test_simplify13(i32 %x) {
; CHECK-LABEL: @test_simplify13(
%ret = call i32 @ffs(i32 %x)
; CHECK-NEXT: [[CTTZ:%[a-z0-9]+]] = call i32 @llvm.cttz.i32(i32 %x, i1 false)
; CHECK-NEXT: [[INC:%[a-z0-9]+]] = add nsw i32 [[CTTZ]], 1
; CHECK-NEXT: [[INC:%[a-z0-9]+]] = add nuw nsw i32 [[CTTZ]], 1
; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp ne i32 %x, 0
; CHECK-NEXT: [[RET:%[a-z0-9]+]] = select i1 [[CMP]], i32 [[INC]], i32 0
ret i32 %ret
@ -114,7 +114,7 @@ define i32 @test_simplify14(i32 %x) {
; CHECK-LINUX-LABEL: @test_simplify14(
%ret = call i32 @ffsl(i32 %x)
; CHECK-LINUX-NEXT: [[CTTZ:%[a-z0-9]+]] = call i32 @llvm.cttz.i32(i32 %x, i1 false)
; CHECK-LINUX-NEXT: [[INC:%[a-z0-9]+]] = add nsw i32 [[CTTZ]], 1
; CHECK-LINUX-NEXT: [[INC:%[a-z0-9]+]] = add nuw nsw i32 [[CTTZ]], 1
; CHECK-LINUX-NEXT: [[CMP:%[a-z0-9]+]] = icmp ne i32 %x, 0
; CHECK-LINUX-NEXT: [[RET:%[a-z0-9]+]] = select i1 [[CMP]], i32 [[INC]], i32 0
ret i32 %ret
@ -125,7 +125,7 @@ define i32 @test_simplify15(i64 %x) {
; CHECK-LINUX-LABEL: @test_simplify15(
%ret = call i32 @ffsll(i64 %x)
; CHECK-LINUX-NEXT: [[CTTZ:%[a-z0-9]+]] = call i64 @llvm.cttz.i64(i64 %x, i1 false)
; CHECK-LINUX-NEXT: [[INC:%[a-z0-9]+]] = add nsw i64 [[CTTZ]], 1
; CHECK-LINUX-NEXT: [[INC:%[a-z0-9]+]] = add nuw nsw i64 [[CTTZ]], 1
; CHECK-LINUX-NEXT: [[TRUNC:%[a-z0-9]+]] = trunc i64 [[INC]] to i32
; CHECK-LINUX-NEXT: [[CMP:%[a-z0-9]+]] = icmp ne i64 %x, 0
; CHECK-LINUX-NEXT: [[RET:%[a-z0-9]+]] = select i1 [[CMP]], i32 [[TRUNC]], i32 0

8
test/Transforms/InstCombine/select.ll
View File

@ -281,7 +281,7 @@ define i32 @test15i(i32 %X) {
; CHECK-NEXT: %t1 = shl i32 %X, 8
; CHECK-NEXT: %1 = and i32 %t1, 512
; CHECK-NEXT: %2 = xor i32 %1, 512
; CHECK-NEXT: %3 = add nsw i32 %2, 577
; CHECK-NEXT: %3 = add nuw nsw i32 %2, 577
; CHECK-NEXT: ret i32 %3
}
@ -294,7 +294,7 @@ define i32 @test15j(i32 %X) {
; CHECK-LABEL: @test15j(
; CHECK-NEXT: %t1 = shl i32 %X, 8
; CHECK-NEXT: %1 = and i32 %t1, 512
; CHECK-NEXT: %2 = add nsw i32 %1, 577
; CHECK-NEXT: %2 = add nuw nsw i32 %1, 577
; CHECK-NEXT: ret i32 %2
}
@ -521,7 +521,7 @@ define i32 @test35(i32 %x) {
; CHECK-LABEL: @test35(
; CHECK: ashr i32 %x, 31
; CHECK: and i32 {{.*}}, 40
; CHECK: add nsw i32 {{.*}}, 60
; CHECK: add nuw nsw i32 {{.*}}, 60
; CHECK: ret
}
@ -1235,4 +1235,4 @@ define i32 @test75(i32 %x) {
; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp ult i32 %x, 68
; CHECK-NEXT: [[SEL:%[a-z0-9]+]] = select i1 [[CMP]], i32 68, i32 %x
; CHECK-NEXT: ret i32 [[SEL]]
}
}