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[InstCombine] fold udiv with common shl amount in operands 

(X << Z) / (Y << Z) --> X / Y

https://alive2.llvm.org/ce/z/E5eaxU

This fixes the motivating example from issue #58137,
but it is not the most general transform. We should
probably also convert left-shift in the divisor to
right-shift in the dividend for that, but that exposes
another missed canonicalization for shifts and adds.
This commit is contained in:
Sanjay Patel 2022-10-12 10:58:03 -04:00
parent dbd29ed54b
commit 008a89037a
3 changed files with 47 additions and 29 deletions
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49
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -826,28 +826,43 @@ static Instruction *foldIDivShl(BinaryOperator &I,
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
Type *Ty = I.getType();
Instruction *Ret = nullptr;
Value *X, *Y, *Z;
// With appropriate no-wrap constraints, remove a common factor in the
// dividend and divisor that is disguised as a left-shifted value.
Value *X, *Y, *Z;
if (!match(Op1, m_Shl(m_Value(X), m_Value(Z))) ||
!match(Op0, m_c_Mul(m_Specific(X), m_Value(Y))))
return nullptr;
if (match(Op1, m_Shl(m_Value(X), m_Value(Z))) &&
match(Op0, m_c_Mul(m_Specific(X), m_Value(Y)))) {
// Both operands must have the matching no-wrap for this kind of division.
auto *Mul = cast<OverflowingBinaryOperator>(Op0);
auto *Shl = cast<OverflowingBinaryOperator>(Op1);
bool HasNUW = Mul->hasNoUnsignedWrap() && Shl->hasNoUnsignedWrap();
bool HasNSW = Mul->hasNoSignedWrap() && Shl->hasNoSignedWrap();
// Both operands must have the matching no-wrap for this kind of division.
Instruction *Ret = nullptr;
auto *Mul = cast<OverflowingBinaryOperator>(Op0);
auto *Shl = cast<OverflowingBinaryOperator>(Op1);
bool HasNUW = Mul->hasNoUnsignedWrap() && Shl->hasNoUnsignedWrap();
bool HasNSW = Mul->hasNoSignedWrap() && Shl->hasNoSignedWrap();
// (X * Y) u/ (X << Z) --> Y u>> Z
if (!IsSigned && HasNUW)
Ret = BinaryOperator::CreateLShr(Y, Z);
// (X * Y) u/ (X << Z) --> Y u>> Z
if (!IsSigned && HasNUW)
Ret = BinaryOperator::CreateLShr(Y, Z);
// (X * Y) s/ (X << Z) --> Y s/ (1 << Z)
if (IsSigned && HasNSW && (Op0->hasOneUse() || Op1->hasOneUse())) {
Value *Shl = Builder.CreateShl(ConstantInt::get(Ty, 1), Z);
Ret = BinaryOperator::CreateSDiv(Y, Shl);
}
}
// (X * Y) s/ (X << Z) --> Y s/ (1 << Z)
if (IsSigned && HasNSW && (Op0->hasOneUse() || Op1->hasOneUse())) {
Value *Shl = Builder.CreateShl(ConstantInt::get(Ty, 1), Z);
Ret = BinaryOperator::CreateSDiv(Y, Shl);
// With appropriate no-wrap constraints, remove a common factor in the
// dividend and divisor that is disguised as a left-shift amount.
if (match(Op0, m_Shl(m_Value(X), m_Value(Z))) &&
match(Op1, m_Shl(m_Value(Y), m_Specific(Z)))) {
auto *Shl0 = cast<OverflowingBinaryOperator>(Op0);
auto *Shl1 = cast<OverflowingBinaryOperator>(Op1);
// For unsigned div, we need 'nuw' on both shifts.
// (X << Z) / (Y << Z) --> X / Y
if (!IsSigned && Shl0->hasNoUnsignedWrap() && Shl1->hasNoUnsignedWrap())
Ret = BinaryOperator::CreateUDiv(X, Y);
// TODO: Handle sdiv.
}
if (!Ret)

18
llvm/test/Transforms/InstCombine/div-shift.ll
View File

@ -742,6 +742,8 @@ define i8 @sdiv_lshr_mul_nsw(i8 %x, i8 %y, i8 %z) {
ret i8 %div
}
; TODO: (X << Z) / (Y << Z) --> X / Y
define i8 @sdiv_shl_shl_nsw2_nuw(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @sdiv_shl_shl_nsw2_nuw(
; CHECK-NEXT: [[XZ:%.*]] = shl nsw i8 [[X:%.*]], [[Z:%.*]]
@ -809,11 +811,11 @@ define i8 @sdiv_shl_shl_nuw_nsw2(i8 %x, i8 %y, i8 %z) {
ret i8 %d
}
; (X << Z) / (Y << Z) --> X / Y
define <2 x i8> @udiv_shl_shl_nuw2(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
; CHECK-LABEL: @udiv_shl_shl_nuw2(
; CHECK-NEXT: [[XZ:%.*]] = shl nuw <2 x i8> [[X:%.*]], [[Z:%.*]]
; CHECK-NEXT: [[YZ:%.*]] = shl nuw <2 x i8> [[Y:%.*]], [[Z]]
; CHECK-NEXT: [[D:%.*]] = udiv <2 x i8> [[XZ]], [[YZ]]
; CHECK-NEXT: [[D:%.*]] = udiv <2 x i8> [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: ret <2 x i8> [[D]]
;
%xz = shl nuw <2 x i8> %x, %z
@ -822,13 +824,15 @@ define <2 x i8> @udiv_shl_shl_nuw2(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
ret <2 x i8> %d
}
; extra uses are ok and 'exact' propagates
define i8 @udiv_shl_shl_nuw2_exact_use2(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @udiv_shl_shl_nuw2_exact_use2(
; CHECK-NEXT: [[XZ:%.*]] = shl nuw i8 [[X:%.*]], [[Z:%.*]]
; CHECK-NEXT: call void @use(i8 [[XZ]])
; CHECK-NEXT: [[YZ:%.*]] = shl nuw i8 [[Y:%.*]], [[Z]]
; CHECK-NEXT: call void @use(i8 [[YZ]])
; CHECK-NEXT: [[D:%.*]] = udiv exact i8 [[XZ]], [[YZ]]
; CHECK-NEXT: [[D:%.*]] = udiv exact i8 [[X]], [[Y]]
; CHECK-NEXT: ret i8 [[D]]
;
%xz = shl nuw i8 %x, %z
@ -839,6 +843,8 @@ define i8 @udiv_shl_shl_nuw2_exact_use2(i8 %x, i8 %y, i8 %z) {
ret i8 %d
}
; negative test - wrong wrap
define i8 @udiv_shl_shl_nuw_nsw(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @udiv_shl_shl_nuw_nsw(
; CHECK-NEXT: [[XZ:%.*]] = shl nuw i8 [[X:%.*]], [[Z:%.*]]
@ -852,6 +858,8 @@ define i8 @udiv_shl_shl_nuw_nsw(i8 %x, i8 %y, i8 %z) {
ret i8 %d
}
; negative test - wrong wrap
define i8 @udiv_shl_shl_nsw_nuw(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @udiv_shl_shl_nsw_nuw(
; CHECK-NEXT: [[XZ:%.*]] = shl nsw i8 [[X:%.*]], [[Z:%.*]]
@ -865,6 +873,8 @@ define i8 @udiv_shl_shl_nsw_nuw(i8 %x, i8 %y, i8 %z) {
ret i8 %d
}
; TODO: This could fold.
define i8 @udiv_shl_shl_nuw_nsw2(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @udiv_shl_shl_nuw_nsw2(
; CHECK-NEXT: [[XZ:%.*]] = shl nuw nsw i8 [[X:%.*]], [[Z:%.*]]

9
llvm/test/Transforms/PhaseOrdering/reassociate-instcombine.ll
View File

@ -50,14 +50,7 @@ define i32 @PR58137_sdiv(i32 %a, i32 %b) {
define i32 @PR58137_udiv(i32 %x, i32 %y) {
; CHECK-LABEL: @PR58137_udiv(
; CHECK-NEXT: [[ZX:%.*]] = zext i32 [[X:%.*]] to i64
; CHECK-NEXT: [[ZY:%.*]] = zext i32 [[Y:%.*]] to i64
; CHECK-NEXT: [[M1:%.*]] = shl nuw nsw i64 [[ZX]], 2
; CHECK-NEXT: [[M2:%.*]] = mul i64 [[M1]], [[ZY]]
; CHECK-NEXT: [[M3:%.*]] = shl nuw nsw i64 [[ZY]], 2
; CHECK-NEXT: [[D:%.*]] = udiv i64 [[M2]], [[M3]]
; CHECK-NEXT: [[T:%.*]] = trunc i64 [[D]] to i32
; CHECK-NEXT: ret i32 [[T]]
; CHECK-NEXT: ret i32 [[X:%.*]]
;
%zx = zext i32 %x to i64
%zy = zext i32 %y to i64