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[InstCombine] Handle ceil division idiom #100977

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[InstCombine] Handle ceil division idiom #100977

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91d8aea
[InstCombine] Introduce test for PR100977 (NFC)
antoniofrighetto Jul 29, 2024
af0d68c
[InstCombine] Handle ceil division idiom
antoniofrighetto Jul 29, 2024
b9593d2
!fixup change to m_value
antoniofrighetto Jul 29, 2024
cdf886f
!fixup pow2 no zero
antoniofrighetto Jul 29, 2024
8552378
!fixup drop oneuse in shift amount
antoniofrighetto Jul 30, 2024
4e5cd1a
!fixup style
antoniofrighetto Jul 30, 2024
cca259c
!fixup style
antoniofrighetto Jul 30, 2024
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94 changes: 70 additions & 24 deletions llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1250,6 +1250,74 @@ static Instruction *foldToUnsignedSaturatedAdd(BinaryOperator &I) {
return nullptr;
}

static Value *foldCeilIdioms(BinaryOperator &I, InstCombinerImpl &IC) {
assert(I.getOpcode() == Instruction::Add && "Expecting add instruction.");
Value *A, *B;
auto &ICB = IC.Builder;

// Fold the log2 ceil idiom:
// zext (ctpop(A) >u/!= 1) + (ctlz (A, true) ^ (BW - 1))
// -> BW - ctlz (A - 1, false)
const APInt *XorC;
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Why did you merge the log2 ceil part into this function? I don't think they share common code.

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They do not, but they do both involve the ceil part, thus I thought it was nice to batch them together under a single foldCeilIdioms.

ICmpInst::Predicate Pred;
if (match(&I,
m_c_Add(
m_ZExt(m_ICmp(Pred, m_Intrinsic<Intrinsic::ctpop>(m_Value(A)),
m_One())),
m_OneUse(m_ZExtOrSelf(m_OneUse(m_Xor(
m_OneUse(m_TruncOrSelf(m_OneUse(
m_Intrinsic<Intrinsic::ctlz>(m_Deferred(A), m_One())))),
m_APInt(XorC))))))) &&
(Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_NE) &&
*XorC == A->getType()->getScalarSizeInBits() - 1) {
Value *Sub = ICB.CreateAdd(A, Constant::getAllOnesValue(A->getType()));
Value *Ctlz = ICB.CreateIntrinsic(Intrinsic::ctlz, {A->getType()},
{Sub, ICB.getFalse()});
Value *Ret = ICB.CreateSub(
ConstantInt::get(A->getType(), A->getType()->getScalarSizeInBits()),
Ctlz, "", /*HasNUW*/ true, /*HasNSW*/ true);
return ICB.CreateZExtOrTrunc(Ret, I.getType());
}

// Fold the ceil division idiom:
// add (udiv (sub A, Bias), B), Bias
// -> udiv (add A, B - 1), B)
// with Bias = A != 0; A + B not to overflow
auto MatchDivision = [&IC](Instruction *Div, Value *&DivOp0, Value *&DivOp1) {
if (match(Div, m_UDiv(m_Value(DivOp0), m_Value(DivOp1))))
return true;

Value *N;
if (match(Div, m_LShr(m_Value(DivOp0), m_Value(N))) &&
match(N,
m_Sub(m_SpecificInt(Div->getType()->getScalarSizeInBits() - 1),
m_Intrinsic<Intrinsic::ctlz>(m_Value(DivOp1), m_Zero()))) &&
IC.isKnownToBeAPowerOfTwo(DivOp1, /*OrZero*/ false, 0, Div))
return true;

return false;
};

Instruction *Div;
Value *Bias, *Sub;
if (match(&I, m_c_Add(m_Instruction(Div), m_Value(Bias))) &&
MatchDivision(Div, Sub, B) &&
match(Sub, m_Sub(m_Value(A), m_Value(Bias))) &&
match(Bias, m_ZExt(m_SpecificICmp(ICmpInst::ICMP_NE, m_Specific(A),
m_ZeroInt()))) &&
Bias->hasNUses(2)) {
WithCache<const Value *> LHSCache(A), RHSCache(B);
auto OR = IC.computeOverflowForUnsignedAdd(LHSCache, RHSCache, &I);
if (OR == OverflowResult::NeverOverflows) {
auto *BMinusOne =
ICB.CreateAdd(B, Constant::getAllOnesValue(I.getType()));
return ICB.CreateUDiv(ICB.CreateAdd(A, BMinusOne), B);
}
}

return nullptr;
}

// Transform:
// (add A, (shl (neg B), Y))
// -> (sub A, (shl B, Y))
Expand Down Expand Up @@ -1785,30 +1853,8 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
I, Builder.CreateIntrinsic(Intrinsic::ctpop, {I.getType()},
{Builder.CreateOr(A, B)}));

// Fold the log2_ceil idiom:
// zext(ctpop(A) >u/!= 1) + (ctlz(A, true) ^ (BW - 1))
// -->
// BW - ctlz(A - 1, false)
const APInt *XorC;
ICmpInst::Predicate Pred;
if (match(&I,
m_c_Add(
m_ZExt(m_ICmp(Pred, m_Intrinsic<Intrinsic::ctpop>(m_Value(A)),
m_One())),
m_OneUse(m_ZExtOrSelf(m_OneUse(m_Xor(
m_OneUse(m_TruncOrSelf(m_OneUse(
m_Intrinsic<Intrinsic::ctlz>(m_Deferred(A), m_One())))),
m_APInt(XorC))))))) &&
(Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_NE) &&
*XorC == A->getType()->getScalarSizeInBits() - 1) {
Value *Sub = Builder.CreateAdd(A, Constant::getAllOnesValue(A->getType()));
Value *Ctlz = Builder.CreateIntrinsic(Intrinsic::ctlz, {A->getType()},
{Sub, Builder.getFalse()});
Value *Ret = Builder.CreateSub(
ConstantInt::get(A->getType(), A->getType()->getScalarSizeInBits()),
Ctlz, "", /*HasNUW*/ true, /*HasNSW*/ true);
return replaceInstUsesWith(I, Builder.CreateZExtOrTrunc(Ret, I.getType()));
}
if (Value *V = foldCeilIdioms(I, *this))
return replaceInstUsesWith(I, V);

if (Instruction *Res = foldSquareSumInt(I))
return Res;
Expand Down
213 changes: 213 additions & 0 deletions llvm/test/Transforms/InstCombine/fold-ceil-div-idiom.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,213 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

define i8 @ceil_div_idiom(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[WO:%.*]] = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 [[X]], i8 [[Y]])
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i8, i1 } [[WO]], 1
; CHECK-NEXT: [[OV_NOT:%.*]] = xor i1 [[OV]], true
; CHECK-NEXT: call void @llvm.assume(i1 [[OV_NOT]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[DIV:%.*]] = udiv i8 [[SUB]], [[Y]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%wo = call {i8, i1} @llvm.uadd.with.overflow(i8 %x, i8 %y)
%ov = extractvalue {i8, i1} %wo, 1
%ov.not = xor i1 %ov, true
call void @llvm.assume(i1 %ov.not)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%div = udiv i8 %sub, %y
%add = add i8 %div, %bias
ret i8 %add
}

define i8 @ceil_div_idiom_2(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_2(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[OV_NOT:%.*]] = add nuw i8 [[X]], [[Y]]
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i8 [[OV_NOT]] to i1
; CHECK-NEXT: call void @llvm.assume(i1 [[TRUNC]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[DIV:%.*]] = udiv i8 [[SUB]], [[Y]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%ov.not = add nuw i8 %x, %y
%trunc = trunc i8 %ov.not to i1
call void @llvm.assume(i1 %trunc)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%div = udiv i8 %sub, %y
%add = add i8 %div, %bias
ret i8 %add
}

define i8 @ceil_div_idiom_with_lshr(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_with_lshr(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[WO:%.*]] = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 [[X]], i8 [[Y]])
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i8, i1 } [[WO]], 1
; CHECK-NEXT: [[OV_NOT:%.*]] = xor i1 [[OV]], true
; CHECK-NEXT: call void @llvm.assume(i1 [[OV_NOT]])
; CHECK-NEXT: [[CTPOPULATION:%.*]] = call range(i8 0, 9) i8 @llvm.ctpop.i8(i8 [[Y]])
; CHECK-NEXT: [[IS_POW_2:%.*]] = icmp eq i8 [[CTPOPULATION]], 1
; CHECK-NEXT: call void @llvm.assume(i1 [[IS_POW_2]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[CTLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[N:%.*]] = xor i8 [[CTLZ]], 7
; CHECK-NEXT: [[DIV:%.*]] = lshr i8 [[SUB]], [[N]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%wo = call {i8, i1} @llvm.uadd.with.overflow(i8 %x, i8 %y)
%ov = extractvalue {i8, i1} %wo, 1
%ov.not = xor i1 %ov, true
call void @llvm.assume(i1 %ov.not)

%ctpopulation = call i8 @llvm.ctpop.i8(i8 %y)
%is_pow_2 = icmp eq i8 %ctpopulation, 1
call void @llvm.assume(i1 %is_pow_2)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%ctlz = tail call i8 @llvm.ctlz.i8(i8 %y, i1 true)
%n = sub i8 7, %ctlz
%div = lshr i8 %sub, %n
%add = add i8 %div, %bias
ret i8 %add
}

define i8 @ceil_div_idiom_add_may_overflow(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_add_may_overflow(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[DIV:%.*]] = udiv i8 [[SUB]], [[Y]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%div = udiv i8 %sub, %y
%add = add i8 %div, %bias
ret i8 %add
}

define i8 @ceil_div_idiom_multiuse_bias(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_multiuse_bias(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[WO:%.*]] = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 [[X]], i8 [[Y]])
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i8, i1 } [[WO]], 1
; CHECK-NEXT: [[OV_NOT:%.*]] = xor i1 [[OV]], true
; CHECK-NEXT: call void @llvm.assume(i1 [[OV_NOT]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[DIV:%.*]] = udiv i8 [[SUB]], [[Y]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: call void @use(i8 [[BIAS]])
; CHECK-NEXT: ret i8 [[ADD]]
;
%wo = call {i8, i1} @llvm.uadd.with.overflow(i8 %x, i8 %y)
%ov = extractvalue {i8, i1} %wo, 1
%ov.not = xor i1 %ov, true
call void @llvm.assume(i1 %ov.not)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%div = udiv i8 %sub, %y
%add = add i8 %div, %bias
call void @use(i8 %bias)
ret i8 %add
}

define i8 @ceil_div_idiom_with_lshr_not_power_2(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_with_lshr_not_power_2(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[WO:%.*]] = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 [[X]], i8 [[Y]])
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i8, i1 } [[WO]], 1
; CHECK-NEXT: [[OV_NOT:%.*]] = xor i1 [[OV]], true
; CHECK-NEXT: call void @llvm.assume(i1 [[OV_NOT]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[CTLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[N:%.*]] = xor i8 [[CTLZ]], 7
; CHECK-NEXT: [[DIV:%.*]] = lshr i8 [[SUB]], [[N]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%wo = call {i8, i1} @llvm.uadd.with.overflow(i8 %x, i8 %y)
%ov = extractvalue {i8, i1} %wo, 1
%ov.not = xor i1 %ov, true
call void @llvm.assume(i1 %ov.not)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%ctlz = tail call i8 @llvm.ctlz.i8(i8 %y, i1 true)
%n = sub i8 7, %ctlz
%div = lshr i8 %sub, %n
%add = add i8 %div, %bias
ret i8 %add
}

define i8 @ceil_div_idiom_with_lshr_wrong_bw(i8 %x, i8 %y) {
; CHECK-LABEL: define i8 @ceil_div_idiom_with_lshr_wrong_bw(
; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
; CHECK-NEXT: [[WO:%.*]] = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 [[X]], i8 [[Y]])
; CHECK-NEXT: [[OV:%.*]] = extractvalue { i8, i1 } [[WO]], 1
; CHECK-NEXT: [[OV_NOT:%.*]] = xor i1 [[OV]], true
; CHECK-NEXT: call void @llvm.assume(i1 [[OV_NOT]])
; CHECK-NEXT: [[CTPOPULATION:%.*]] = call range(i8 0, 9) i8 @llvm.ctpop.i8(i8 [[Y]])
; CHECK-NEXT: [[IS_POW_2:%.*]] = icmp eq i8 [[CTPOPULATION]], 1
; CHECK-NEXT: call void @llvm.assume(i1 [[IS_POW_2]])
; CHECK-NEXT: [[NONZERO:%.*]] = icmp ne i8 [[X]], 0
; CHECK-NEXT: [[BIAS:%.*]] = zext i1 [[NONZERO]] to i8
; CHECK-NEXT: [[SUB:%.*]] = sub i8 [[X]], [[BIAS]]
; CHECK-NEXT: [[CTLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[N:%.*]] = sub nuw nsw i8 8, [[CTLZ]]
; CHECK-NEXT: [[DIV:%.*]] = lshr i8 [[SUB]], [[N]]
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[DIV]], [[BIAS]]
; CHECK-NEXT: ret i8 [[ADD]]
;
%wo = call {i8, i1} @llvm.uadd.with.overflow(i8 %x, i8 %y)
%ov = extractvalue {i8, i1} %wo, 1
%ov.not = xor i1 %ov, true
call void @llvm.assume(i1 %ov.not)

%ctpopulation = call i8 @llvm.ctpop.i8(i8 %y)
%is_pow_2 = icmp eq i8 %ctpopulation, 1
call void @llvm.assume(i1 %is_pow_2)

%nonzero = icmp ne i8 %x, 0
%bias = zext i1 %nonzero to i8
%sub = sub i8 %x, %bias
%ctlz = tail call i8 @llvm.ctlz.i8(i8 %y, i1 true)
%n = sub i8 8, %ctlz
%div = lshr i8 %sub, %n
%add = add i8 %div, %bias
ret i8 %add
}

declare { i8, i1 } @llvm.uadd.with.overflow.i8(i8, i8)
declare i8 @llvm.ctpop.i8(i8)
declare void @llvm.assume(i1)
declare void @use(i8)
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