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Revert "[Reassociate] Drop weight reduction to fix issue 91417 (#91469)…
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…" (#94210)

Reverts
3bcccb6
and
9a28272
because #91469 causes a miscompilation
#91469 (comment).
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@dtcxzyw
dtcxzyw authored Jun 3, 2024
1 parent 12949c9 commit 22b63b9
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Showing 3 changed files with 115 additions and 40 deletions.
93 changes: 92 additions & 1 deletion llvm/lib/Transforms/Scalar/Reassociate.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -302,6 +302,97 @@ static BinaryOperator *LowerNegateToMultiply(Instruction *Neg) {
return Res;
}

/// Returns k such that lambda(2^Bitwidth) = 2^k, where lambda is the Carmichael
/// function. This means that x^(2^k) === 1 mod 2^Bitwidth for
/// every odd x, i.e. x^(2^k) = 1 for every odd x in Bitwidth-bit arithmetic.
/// Note that 0 <= k < Bitwidth, and if Bitwidth > 3 then x^(2^k) = 0 for every
/// even x in Bitwidth-bit arithmetic.
static unsigned CarmichaelShift(unsigned Bitwidth) {
if (Bitwidth < 3)
return Bitwidth - 1;
return Bitwidth - 2;
}

/// Add the extra weight 'RHS' to the existing weight 'LHS',
/// reducing the combined weight using any special properties of the operation.
/// The existing weight LHS represents the computation X op X op ... op X where
/// X occurs LHS times. The combined weight represents X op X op ... op X with
/// X occurring LHS + RHS times. If op is "Xor" for example then the combined
/// operation is equivalent to X if LHS + RHS is odd, or 0 if LHS + RHS is even;
/// the routine returns 1 in LHS in the first case, and 0 in LHS in the second.
static void IncorporateWeight(APInt &LHS, const APInt &RHS, unsigned Opcode) {
// If we were working with infinite precision arithmetic then the combined
// weight would be LHS + RHS. But we are using finite precision arithmetic,
// and the APInt sum LHS + RHS may not be correct if it wraps (it is correct
// for nilpotent operations and addition, but not for idempotent operations
// and multiplication), so it is important to correctly reduce the combined
// weight back into range if wrapping would be wrong.

// If RHS is zero then the weight didn't change.
if (RHS.isMinValue())
return;
// If LHS is zero then the combined weight is RHS.
if (LHS.isMinValue()) {
LHS = RHS;
return;
}
// From this point on we know that neither LHS nor RHS is zero.

if (Instruction::isIdempotent(Opcode)) {
// Idempotent means X op X === X, so any non-zero weight is equivalent to a
// weight of 1. Keeping weights at zero or one also means that wrapping is
// not a problem.
assert(LHS == 1 && RHS == 1 && "Weights not reduced!");
return; // Return a weight of 1.
}
if (Instruction::isNilpotent(Opcode)) {
// Nilpotent means X op X === 0, so reduce weights modulo 2.
assert(LHS == 1 && RHS == 1 && "Weights not reduced!");
LHS = 0; // 1 + 1 === 0 modulo 2.
return;
}
if (Opcode == Instruction::Add || Opcode == Instruction::FAdd) {
// TODO: Reduce the weight by exploiting nsw/nuw?
LHS += RHS;
return;
}

assert((Opcode == Instruction::Mul || Opcode == Instruction::FMul) &&
"Unknown associative operation!");
unsigned Bitwidth = LHS.getBitWidth();
// If CM is the Carmichael number then a weight W satisfying W >= CM+Bitwidth
// can be replaced with W-CM. That's because x^W=x^(W-CM) for every Bitwidth
// bit number x, since either x is odd in which case x^CM = 1, or x is even in
// which case both x^W and x^(W - CM) are zero. By subtracting off multiples
// of CM like this weights can always be reduced to the range [0, CM+Bitwidth)
// which by a happy accident means that they can always be represented using
// Bitwidth bits.
// TODO: Reduce the weight by exploiting nsw/nuw? (Could do much better than
// the Carmichael number).
if (Bitwidth > 3) {
/// CM - The value of Carmichael's lambda function.
APInt CM = APInt::getOneBitSet(Bitwidth, CarmichaelShift(Bitwidth));
// Any weight W >= Threshold can be replaced with W - CM.
APInt Threshold = CM + Bitwidth;
assert(LHS.ult(Threshold) && RHS.ult(Threshold) && "Weights not reduced!");
// For Bitwidth 4 or more the following sum does not overflow.
LHS += RHS;
while (LHS.uge(Threshold))
LHS -= CM;
} else {
// To avoid problems with overflow do everything the same as above but using
// a larger type.
unsigned CM = 1U << CarmichaelShift(Bitwidth);
unsigned Threshold = CM + Bitwidth;
assert(LHS.getZExtValue() < Threshold && RHS.getZExtValue() < Threshold &&
"Weights not reduced!");
unsigned Total = LHS.getZExtValue() + RHS.getZExtValue();
while (Total >= Threshold)
Total -= CM;
LHS = Total;
}
}

using RepeatedValue = std::pair<Value*, APInt>;

/// Given an associative binary expression, return the leaf
Expand Down Expand Up @@ -471,7 +562,7 @@ static bool LinearizeExprTree(Instruction *I,
"In leaf map but not visited!");

// Update the number of paths to the leaf.
It->second += Weight;
IncorporateWeight(It->second, Weight, Opcode);

// If we still have uses that are not accounted for by the expression
// then it is not safe to modify the value.
Expand Down
17 changes: 9 additions & 8 deletions llvm/test/Transforms/Reassociate/reassoc_bool_vec.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -56,19 +56,20 @@ define <8 x i1> @vector2(<8 x i1> %a, <8 x i1> %b0, <8 x i1> %b1, <8 x i1> %b2,
; CHECK-NEXT: [[OR5:%.*]] = or <8 x i1> [[B5]], [[A]]
; CHECK-NEXT: [[OR6:%.*]] = or <8 x i1> [[B6]], [[A]]
; CHECK-NEXT: [[OR7:%.*]] = or <8 x i1> [[B7]], [[A]]
; CHECK-NEXT: [[XOR0:%.*]] = xor <8 x i1> [[OR1]], [[OR0]]
; CHECK-NEXT: [[XOR2:%.*]] = xor <8 x i1> [[XOR0]], [[OR2]]
; CHECK-NEXT: [[OR045:%.*]] = xor <8 x i1> [[XOR2]], [[OR3]]
; CHECK-NEXT: [[XOR3:%.*]] = xor <8 x i1> [[OR045]], [[OR4]]
; CHECK-NEXT: [[XOR4:%.*]] = xor <8 x i1> [[XOR3]], [[OR5]]
; CHECK-NEXT: [[XOR5:%.*]] = xor <8 x i1> [[XOR4]], [[OR6]]
; CHECK-NEXT: [[XOR6:%.*]] = xor <8 x i1> [[XOR5]], [[OR7]]
; CHECK-NEXT: [[XOR2:%.*]] = xor <8 x i1> [[OR1]], [[OR0]]
; CHECK-NEXT: [[OR045:%.*]] = xor <8 x i1> [[XOR2]], [[OR2]]
; CHECK-NEXT: [[XOR3:%.*]] = xor <8 x i1> [[OR045]], [[OR3]]
; CHECK-NEXT: [[XOR4:%.*]] = xor <8 x i1> [[XOR3]], [[OR4]]
; CHECK-NEXT: [[XOR5:%.*]] = xor <8 x i1> [[XOR4]], [[OR5]]
; CHECK-NEXT: [[XOR6:%.*]] = xor <8 x i1> [[XOR5]], [[OR6]]
; CHECK-NEXT: [[XOR7:%.*]] = xor <8 x i1> [[XOR6]], [[OR7]]
; CHECK-NEXT: [[OR4560:%.*]] = or <8 x i1> [[OR045]], [[XOR2]]
; CHECK-NEXT: [[OR023:%.*]] = or <8 x i1> [[OR4560]], [[XOR3]]
; CHECK-NEXT: [[OR001:%.*]] = or <8 x i1> [[OR023]], [[XOR4]]
; CHECK-NEXT: [[OR0123:%.*]] = or <8 x i1> [[OR001]], [[XOR5]]
; CHECK-NEXT: [[OR01234567:%.*]] = or <8 x i1> [[OR0123]], [[XOR6]]
; CHECK-NEXT: ret <8 x i1> [[OR01234567]]
; CHECK-NEXT: [[OR1234567:%.*]] = or <8 x i1> [[OR01234567]], [[XOR7]]
; CHECK-NEXT: ret <8 x i1> [[OR1234567]]
;
%or0 = or <8 x i1> %b0, %a
%or1 = or <8 x i1> %b1, %a
Expand Down
45 changes: 14 additions & 31 deletions llvm/test/Transforms/Reassociate/repeats.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ define i8 @nilpotent(i8 %x) {
define i2 @idempotent(i2 %x) {
; CHECK-LABEL: define i2 @idempotent(
; CHECK-SAME: i2 [[X:%.*]]) {
; CHECK-NEXT: ret i2 -1
; CHECK-NEXT: ret i2 [[X]]
;
%tmp1 = and i2 %x, %x
%tmp2 = and i2 %tmp1, %x
Expand Down Expand Up @@ -60,8 +60,7 @@ define i3 @foo3x5(i3 %x) {
; CHECK-SAME: i3 [[X:%.*]]) {
; CHECK-NEXT: [[TMP3:%.*]] = mul i3 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i3 [[TMP3]], [[X]]
; CHECK-NEXT: [[TMP5:%.*]] = mul i3 [[TMP4]], [[TMP3]]
; CHECK-NEXT: ret i3 [[TMP5]]
; CHECK-NEXT: ret i3 [[TMP4]]
;
%tmp1 = mul i3 %x, %x
%tmp2 = mul i3 %tmp1, %x
Expand All @@ -75,8 +74,7 @@ define i3 @foo3x5_nsw(i3 %x) {
; CHECK-LABEL: define i3 @foo3x5_nsw(
; CHECK-SAME: i3 [[X:%.*]]) {
; CHECK-NEXT: [[TMP3:%.*]] = mul i3 [[X]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i3 [[TMP3]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i3 [[TMP2]], [[TMP3]]
; CHECK-NEXT: [[TMP4:%.*]] = mul nsw i3 [[TMP3]], [[X]]
; CHECK-NEXT: ret i3 [[TMP4]]
;
%tmp1 = mul i3 %x, %x
Expand All @@ -91,8 +89,7 @@ define i3 @foo3x6(i3 %x) {
; CHECK-LABEL: define i3 @foo3x6(
; CHECK-SAME: i3 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i3 [[X]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i3 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i3 [[TMP3]], [[TMP3]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i3 [[TMP1]], [[TMP1]]
; CHECK-NEXT: ret i3 [[TMP2]]
;
%tmp1 = mul i3 %x, %x
Expand All @@ -108,9 +105,7 @@ define i3 @foo3x7(i3 %x) {
; CHECK-LABEL: define i3 @foo3x7(
; CHECK-SAME: i3 [[X:%.*]]) {
; CHECK-NEXT: [[TMP5:%.*]] = mul i3 [[X]], [[X]]
; CHECK-NEXT: [[TMP7:%.*]] = mul i3 [[TMP5]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i3 [[TMP7]], [[X]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i3 [[TMP3]], [[TMP7]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i3 [[TMP5]], [[X]]
; CHECK-NEXT: ret i3 [[TMP6]]
;
%tmp1 = mul i3 %x, %x
Expand All @@ -127,8 +122,7 @@ define i4 @foo4x8(i4 %x) {
; CHECK-LABEL: define i4 @foo4x8(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP3]], [[TMP3]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: ret i4 [[TMP4]]
;
%tmp1 = mul i4 %x, %x
Expand All @@ -146,9 +140,8 @@ define i4 @foo4x9(i4 %x) {
; CHECK-LABEL: define i4 @foo4x9(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP2]], [[X]]
; CHECK-NEXT: [[TMP8:%.*]] = mul i4 [[TMP3]], [[TMP2]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP8:%.*]] = mul i4 [[TMP2]], [[TMP1]]
; CHECK-NEXT: ret i4 [[TMP8]]
;
%tmp1 = mul i4 %x, %x
Expand All @@ -167,8 +160,7 @@ define i4 @foo4x10(i4 %x) {
; CHECK-LABEL: define i4 @foo4x10(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP4]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP2]], [[TMP2]]
; CHECK-NEXT: ret i4 [[TMP3]]
;
Expand All @@ -189,8 +181,7 @@ define i4 @foo4x11(i4 %x) {
; CHECK-LABEL: define i4 @foo4x11(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP4]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP2]], [[X]]
; CHECK-NEXT: [[TMP10:%.*]] = mul i4 [[TMP3]], [[TMP2]]
; CHECK-NEXT: ret i4 [[TMP10]]
Expand All @@ -213,9 +204,7 @@ define i4 @foo4x12(i4 %x) {
; CHECK-LABEL: define i4 @foo4x12(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP4]], [[TMP4]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP3]], [[TMP3]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[TMP1]]
; CHECK-NEXT: ret i4 [[TMP2]]
;
%tmp1 = mul i4 %x, %x
Expand All @@ -238,9 +227,7 @@ define i4 @foo4x13(i4 %x) {
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP2]], [[TMP2]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP3]], [[X]]
; CHECK-NEXT: [[TMP12:%.*]] = mul i4 [[TMP4]], [[TMP3]]
; CHECK-NEXT: [[TMP12:%.*]] = mul i4 [[TMP2]], [[TMP1]]
; CHECK-NEXT: ret i4 [[TMP12]]
;
%tmp1 = mul i4 %x, %x
Expand All @@ -263,9 +250,7 @@ define i4 @foo4x14(i4 %x) {
; CHECK-LABEL: define i4 @foo4x14(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP5:%.*]] = mul i4 [[TMP4]], [[TMP4]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i4 [[TMP5]], [[X]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP7:%.*]] = mul i4 [[TMP6]], [[TMP6]]
; CHECK-NEXT: ret i4 [[TMP7]]
;
Expand All @@ -290,9 +275,7 @@ define i4 @foo4x15(i4 %x) {
; CHECK-LABEL: define i4 @foo4x15(
; CHECK-SAME: i4 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = mul i4 [[X]], [[X]]
; CHECK-NEXT: [[TMP4:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i4 [[TMP4]], [[TMP4]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i4 [[TMP3]], [[X]]
; CHECK-NEXT: [[TMP6:%.*]] = mul i4 [[TMP1]], [[X]]
; CHECK-NEXT: [[TMP5:%.*]] = mul i4 [[TMP6]], [[X]]
; CHECK-NEXT: [[TMP14:%.*]] = mul i4 [[TMP5]], [[TMP6]]
; CHECK-NEXT: ret i4 [[TMP14]]
Expand Down

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