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[InstSimplify] Fold X * (2^N + 1) >> N -> X when N is half the bitwidth of X #92909

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[InstSimplify] Fold X * (2^N + 1) >> N -> X when N is half the bitwidth of X #92909

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39 changes: 39 additions & 0 deletions llvm/lib/Analysis/InstructionSimplify.cpp
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Expand Up @@ -1479,6 +1479,29 @@ static Value *simplifyLShrInst(Value *Op0, Value *Op1, bool IsExact,
if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWShl(m_Value(X), m_Specific(Op1))))
return X;

// Look for a "splat" mul pattern - it replicates bits across each half
// of a value, so a right shift is just a mask of the low bits:
const APInt *MulC;
const APInt *ShAmt;
if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
match(Op1, m_APInt(ShAmt))) {
unsigned ShAmtC = ShAmt->getZExtValue();
unsigned BitWidth = ShAmt->getBitWidth();
if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
MulC->logBase2() == ShAmtC) {
// FIXME: This condition should be covered by the computeKnownBits, but
// for some reason it is not, so keep this in for now. This has no
// negative effects, but KnownBits should be able to infer a number of
// leading bits based on 2^N + 1 not wrapping, as that means 2^N must not
// wrap either, which means the top N bits of X must be 0.
if (ShAmtC * 2 == BitWidth)
return X;
const KnownBits XKnown = computeKnownBits(X, /* Depth */ 0, Q);
if (XKnown.countMaxActiveBits() <= ShAmtC)
return X;
}
}

// ((X << A) | Y) >> A -> X if effective width of Y is not larger than A.
// We can return X as we do in the above case since OR alters no bits in X.
// SimplifyDemandedBits in InstCombine can do more general optimization for
Expand Down Expand Up @@ -1523,6 +1546,22 @@ static Value *simplifyAShrInst(Value *Op0, Value *Op1, bool IsExact,
if (Q.IIQ.UseInstrInfo && match(Op0, m_NSWShl(m_Value(X), m_Specific(Op1))))
return X;

const APInt *MulC;
const APInt *ShAmt;
if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
match(Op1, m_APInt(ShAmt)) &&
cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap()) {
unsigned ShAmtC = ShAmt->getZExtValue();
unsigned BitWidth = ShAmt->getBitWidth();
if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
MulC->logBase2() == ShAmtC &&
ShAmtC < BitWidth - 1) /* Minus 1 for the sign bit */ {
KnownBits KnownX = computeKnownBits(X, /* Depth */ 0, Q);
if (KnownX.countMaxActiveBits() <= ShAmtC)
return X;
}
}

// Arithmetic shifting an all-sign-bit value is a no-op.
unsigned NumSignBits = ComputeNumSignBits(Op0, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
if (NumSignBits == Op0->getType()->getScalarSizeInBits())
Expand Down
53 changes: 27 additions & 26 deletions llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
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Expand Up @@ -1456,41 +1456,42 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
}

const APInt *MulC;
if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC)))) {
if (match(Op0, m_OneUse(m_NUWMul(m_Value(X), m_APInt(MulC))))) {
if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
MulC->logBase2() == ShAmtC) {
// Look for a "splat" mul pattern - it replicates bits across each half
// of a value, so a right shift is just a mask of the low bits:
// lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
if (ShAmtC * 2 == BitWidth)
return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));

// lshr (mul nuw (X, 2^N + 1)), N -> add nuw (X, lshr(X, N))
if (Op0->hasOneUse()) {
auto *NewAdd = BinaryOperator::CreateNUWAdd(
X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
I.isExact()));
NewAdd->setHasNoSignedWrap(
cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap());
return NewAdd;
}
auto *NewAdd = BinaryOperator::CreateNUWAdd(
X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
I.isExact()));
NewAdd->setHasNoSignedWrap(
cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap());
return NewAdd;
}

// The one-use check is not strictly necessary, but codegen may not be
// able to invert the transform and perf may suffer with an extra mul
// instruction.
if (Op0->hasOneUse()) {
APInt NewMulC = MulC->lshr(ShAmtC);
// if c is divisible by (1 << ShAmtC):
// lshr (mul nuw x, MulC), ShAmtC -> mul nuw nsw x, (MulC >> ShAmtC)
if (MulC->eq(NewMulC.shl(ShAmtC))) {
auto *NewMul =
BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
assert(ShAmtC != 0 &&
"lshr X, 0 should be handled by simplifyLShrInst.");
NewMul->setHasNoSignedWrap(true);
return NewMul;
}
APInt NewMulC = MulC->lshr(ShAmtC);
// if c is divisible by (1 << ShAmtC):
// lshr (mul nuw x, MulC), ShAmtC -> mul nuw nsw x, (MulC >> ShAmtC)
if (MulC->eq(NewMulC.shl(ShAmtC))) {
auto *NewMul =
BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
assert(ShAmtC != 0 &&
"lshr X, 0 should be handled by simplifyLShrInst.");
NewMul->setHasNoSignedWrap(true);
return NewMul;
}
}

// lshr (mul nsw (X, 2^N + 1)), N -> add nsw (X, lshr(X, N))
if (match(Op0, m_OneUse(m_NSWMul(m_Value(X), m_APInt(MulC))))) {
if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
MulC->logBase2() == ShAmtC) {
return BinaryOperator::CreateNSWAdd(
X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
I.isExact()));
}
}

Expand Down
22 changes: 22 additions & 0 deletions llvm/test/Transforms/InstCombine/ashr-lshr.ll
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Expand Up @@ -862,4 +862,26 @@ define i32 @ashr_mul_times_5_div_4_exact_2(i32 %x) {
ret i32 %ashr
}

define i32 @mul_splat_fold_known_active_bits(i32 %x) {
; CHECK-LABEL: @mul_splat_fold_known_active_bits(
; CHECK-NEXT: [[XX:%.*]] = and i32 [[X:%.*]], 360
; CHECK-NEXT: ret i32 [[XX]]
;
%xx = and i32 %x, 360
%m = mul nuw i32 %xx, 65537
%t = ashr i32 %m, 16
ret i32 %t
}

define i32 @mul_splat_fold_no_known_active_bits(i32 %x) {
; CHECK-LABEL: @mul_splat_fold_no_known_active_bits(
; CHECK-NEXT: [[TMP1:%.*]] = ashr i32 [[X:%.*]], 16
; CHECK-NEXT: [[T:%.*]] = add nsw i32 [[TMP1]], [[X]]
; CHECK-NEXT: ret i32 [[T]]
;
%m = mul nsw i32 %x, 65537
%t = ashr i32 %m, 16
ret i32 %t
}

declare void @use(i32)
21 changes: 14 additions & 7 deletions llvm/test/Transforms/InstCombine/lshr.ll
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Expand Up @@ -348,22 +348,31 @@ define <2 x i32> @narrow_lshr_constant(<2 x i8> %x, <2 x i8> %y) {

define i32 @mul_splat_fold(i32 %x) {
; CHECK-LABEL: @mul_splat_fold(
; CHECK-NEXT: [[T:%.*]] = and i32 [[X:%.*]], 65535
; CHECK-NEXT: ret i32 [[T]]
; CHECK-NEXT: ret i32 [[X:%.*]]
;
%m = mul nuw i32 %x, 65537
%t = lshr i32 %m, 16
ret i32 %t
}

define i32 @mul_splat_fold_known_zeros(i32 %x) {
; CHECK-LABEL: @mul_splat_fold_known_zeros(
; CHECK-NEXT: [[XX:%.*]] = and i32 [[X:%.*]], 360
; CHECK-NEXT: ret i32 [[XX]]
;
%xx = and i32 %x, 360
%m = mul nuw i32 %xx, 65537
%t = lshr i32 %m, 16
ret i32 %t
}

; Vector type, extra use, weird types are all ok.

define <3 x i14> @mul_splat_fold_vec(<3 x i14> %x) {
; CHECK-LABEL: @mul_splat_fold_vec(
; CHECK-NEXT: [[M:%.*]] = mul nuw <3 x i14> [[X:%.*]], <i14 129, i14 129, i14 129>
; CHECK-NEXT: call void @usevec(<3 x i14> [[M]])
; CHECK-NEXT: [[T:%.*]] = and <3 x i14> [[X]], <i14 127, i14 127, i14 127>
; CHECK-NEXT: ret <3 x i14> [[T]]
; CHECK-NEXT: ret <3 x i14> [[X]]
;
%m = mul nuw <3 x i14> %x, <i14 129, i14 129, i14 129>
call void @usevec(<3 x i14> %m)
Expand Down Expand Up @@ -628,8 +637,6 @@ define i32 @mul_splat_fold_wrong_lshr_const(i32 %x) {
ret i32 %t
}

; Negative test (but simplifies into a different transform)

define i32 @mul_splat_fold_no_nuw(i32 %x) {
; CHECK-LABEL: @mul_splat_fold_no_nuw(
; CHECK-NEXT: [[TMP1:%.*]] = lshr i32 [[X:%.*]], 16
Expand All @@ -641,7 +648,7 @@ define i32 @mul_splat_fold_no_nuw(i32 %x) {
ret i32 %t
}

; Negative test
; Negative test

define i32 @mul_splat_fold_no_flags(i32 %x) {
; CHECK-LABEL: @mul_splat_fold_no_flags(
Expand Down
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