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[InstCombine] Simplify @llvm.usub.with.overflow+non-zero check (PR43251)

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Summary:
This is again motivated by D67122 sanitizer check enhancement.
That patch seemingly worsens `-fsanitize=pointer-overflow`
overhead from 25% to 50%, which strongly implies missing folds.

In this particular case, given
```
char* test(char& base, unsigned long offset) {
  return &base - offset;
}
```
it will end up producing something like
https://godbolt.org/z/luGEju
which after optimizations reduces down to roughly
```
declare void @use64(i64)
define i1 @test(i8* dereferenceable(1) %base, i64 %offset) {
  %base_int = ptrtoint i8* %base to i64
  %adjusted = sub i64 %base_int, %offset
  call void @use64(i64 %adjusted)
  %not_null = icmp ne i64 %adjusted, 0
  %no_underflow = icmp ule i64 %adjusted, %base_int
  %no_underflow_and_not_null = and i1 %not_null, %no_underflow
  ret i1 %no_underflow_and_not_null
}
```
Without D67122 there was no `%not_null`,
and in this particular case we can "get rid of it", by merging two checks:
Here we are checking: `Base u>= Offset && (Base u- Offset) != 0`, but that is simply `Base u> Offset`

Alive proofs:
https://rise4fun.com/Alive/QOs

The `@llvm.usub.with.overflow` pattern itself is not handled here
because this is the main pattern, that we currently consider canonical.

https://bugs.llvm.org/show_bug.cgi?id=43251

Reviewers: spatel, nikic, xbolva00, majnemer

Reviewed By: xbolva00, majnemer

Subscribers: vsk, majnemer, xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67356

llvm-svn: 372341
This commit is contained in:
Roman Lebedev 2019-09-19 17:25:19 +00:00
parent e15136d802
commit 472f610901
2 changed files with 39 additions and 18 deletions
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21
lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -1084,6 +1084,27 @@ static Value *foldUnsignedUnderflowCheck(ICmpInst *ZeroICmp,
return Builder.CreateICmpULE(Base, Offset);
}
if (!match(UnsignedICmp,
m_c_ICmp(UnsignedPred, m_Specific(Base), m_Specific(Offset))) ||
!ICmpInst::isUnsigned(UnsignedPred))
return nullptr;
if (UnsignedICmp->getOperand(0) != Base)
UnsignedPred = ICmpInst::getSwappedPredicate(UnsignedPred);
// Base >=/> Offset && (Base - Offset) != 0 <--> Base > Offset
// (no overflow and not null)
if ((UnsignedPred == ICmpInst::ICMP_UGE ||
UnsignedPred == ICmpInst::ICMP_UGT) &&
EqPred == ICmpInst::ICMP_NE && IsAnd)
return Builder.CreateICmpUGT(Base, Offset);
// Base <=/< Offset || (Base - Offset) == 0 <--> Base <= Offset
// (overflow or null)
if ((UnsignedPred == ICmpInst::ICMP_ULE ||
UnsignedPred == ICmpInst::ICMP_ULT) &&
EqPred == ICmpInst::ICMP_EQ && !IsAnd)
return Builder.CreateICmpULE(Base, Offset);
return nullptr;
}

36
test/Transforms/InstCombine/result-of-usub-is-non-zero-and-no-overflow.ll
View File

@ -24,8 +24,8 @@ define i1 @t0_noncanonical_ignoreme(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NOT_NULL]], [[NO_UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -45,8 +45,8 @@ define i1 @t1(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NOT_NULL]], [[NO_UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -65,8 +65,8 @@ define i1 @t1_strict(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NOT_NULL]], [[NO_UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -115,8 +115,8 @@ define i1 @t3_commutability0(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NOT_NULL]], [[NO_UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -135,8 +135,8 @@ define i1 @t4_commutability1(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NO_UNDERFLOW]], [[NOT_NULL]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -155,8 +155,8 @@ define i1 @t5_commutability2(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[NO_UNDERFLOW]])
; CHECK-NEXT: [[NOT_NULL:%.*]] = icmp ne i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NOT_NULL]])
; CHECK-NEXT: [[R:%.*]] = and i1 [[NO_UNDERFLOW]], [[NOT_NULL]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ugt i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -206,8 +206,8 @@ define i1 @t7(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[UNDERFLOW]])
; CHECK-NEXT: [[NULL:%.*]] = icmp eq i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NULL]])
; CHECK-NEXT: [[R:%.*]] = or i1 [[NULL]], [[UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ule i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -226,8 +226,8 @@ define i1 @t7_nonstrict(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[UNDERFLOW]])
; CHECK-NEXT: [[NULL:%.*]] = icmp eq i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NULL]])
; CHECK-NEXT: [[R:%.*]] = or i1 [[NULL]], [[UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ule i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)
@ -272,8 +272,8 @@ define i1 @t9_commutative(i8 %base, i8 %offset) {
; CHECK-NEXT: call void @use1(i1 [[UNDERFLOW]])
; CHECK-NEXT: [[NULL:%.*]] = icmp eq i8 [[ADJUSTED]], 0
; CHECK-NEXT: call void @use1(i1 [[NULL]])
; CHECK-NEXT: [[R:%.*]] = or i1 [[NULL]], [[UNDERFLOW]]
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ule i8 [[BASE]], [[OFFSET]]
; CHECK-NEXT: ret i1 [[TMP1]]
;
%adjusted = sub i8 %base, %offset
call void @use8(i8 %adjusted)