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[opt] Reduction under mask -- sign_ext elimination #3643

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copybara-service merged 1 commit into from
Jan 13, 2026
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[opt] Reduction under mask -- sign_ext elimination #3643

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73 changes: 73 additions & 0 deletions xls/passes/basic_simplification_pass.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -253,6 +253,79 @@ absl::StatusOr<bool> MatchPatterns(Node* n) {
return true;
}

// Reduction of a value masked by sign_ext(bits[1]) (eliminate mask plumbing).
//
// Note on abbreviation in the pattern below:
//
// - `w` is the bit-width of the masked datapath value `x` (and thus the
// width of the `and(x, mask)`).
// - `b` is the 1-bit predicate (bits[1]) being sign-extended to a `w`-bit
// mask via `sign_ext(b, w)` (all-zeros when b=0, all-ones when b=1).
//
// For w > 1, sign_ext(b:bits[1], w) is either 0_w or all_ones_w, so:
//
// or_reduce(and(x, sign_ext(b,w))) == and(b, or_reduce(x))
// and_reduce(and(x, sign_ext(b,w))) == and(b, and_reduce(x))
//
// And similarly for the inverted mask not(sign_ext(b,w)).
//
// This is always semantics-preserving and strictly reduces mask plumbing for
// w > 1.
if (n->OpIn({Op::kOrReduce, Op::kAndReduce}) &&
n->operand(0)->op() == Op::kAnd && n->operand(0)->operand_count() == 2) {
Node* and_op = n->operand(0);
const int64_t w = and_op->BitCountOrDie();
if (w > 1) {
auto match_sign_ext_mask = [&](Node* node, Node** b,
bool* inverted) -> bool {
*b = nullptr;
*inverted = false;
Node* maybe_sign_ext = node;
if (node->op() == Op::kNot && node->operand(0)->op() == Op::kSignExt) {
*inverted = true;
maybe_sign_ext = node->operand(0);
}
if (maybe_sign_ext->op() != Op::kSignExt) {
return false;
}
if (maybe_sign_ext->BitCountOrDie() != w) {
return false;
}
Node* sign_ext_operand = maybe_sign_ext->As<ExtendOp>()->operand(0);
if (sign_ext_operand->BitCountOrDie() != 1) {
return false;
}
*b = sign_ext_operand;
return true;
};

Node* b = nullptr;
bool inverted = false;
Node* x = nullptr;
if (match_sign_ext_mask(and_op->operand(0), &b, &inverted)) {
x = and_op->operand(1);
} else if (match_sign_ext_mask(and_op->operand(1), &b, &inverted)) {
x = and_op->operand(0);
}
if (b != nullptr && x != nullptr && x->BitCountOrDie() == w) {
FunctionBase* f = n->function_base();
Node* cond = b;
if (inverted) {
XLS_ASSIGN_OR_RETURN(cond, f->MakeNode<UnOp>(n->loc(), b, Op::kNot));
}
XLS_ASSIGN_OR_RETURN(Node * reduced_x, f->MakeNode<BitwiseReductionOp>(
n->loc(), x, n->op()));
XLS_ASSIGN_OR_RETURN(
Node * gated,
f->MakeNode<NaryOp>(n->loc(), std::vector<Node*>{cond, reduced_x},
Op::kAnd));
VLOG(2) << "FOUND: reduction of sign_ext(mask)-masked value";
XLS_RETURN_IF_ERROR(n->ReplaceUsesWith(gated));
return true;
}
}
}

// Replaces uses of n with a new node by eliminating operands for which the
// "predicate" holds. If the predicate holds for all operands, the
// NaryOpNullaryResult is used as a replacement.
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95 changes: 95 additions & 0 deletions xls/passes/basic_simplification_pass_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -426,6 +426,101 @@ TEST_F(BasicSimplificationPassTest, EmptyXorReduce) {
EXPECT_THAT(f->return_value(), m::Literal(0, 1));
}

TEST_F(BasicSimplificationPassTest, OrReduceMaskedBySignExtPredicate) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(8));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/8);
BValue masked = fb.And(x, se);
BValue result = fb.OrReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Param("b"), m::OrReduce(m::Param("x"))));
}

TEST_F(BasicSimplificationPassTest,
OrReduceMaskedBySignExtPredicateCommutedAnd) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(8));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/8);
BValue masked = fb.And(se, x);
BValue result = fb.OrReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Param("b"), m::OrReduce(m::Param("x"))));
}

TEST_F(BasicSimplificationPassTest, AndReduceMaskedBySignExtPredicate) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(8));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/8);
BValue masked = fb.And(x, se);
BValue result = fb.AndReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Param("b"), m::AndReduce(m::Param("x"))));
}

TEST_F(BasicSimplificationPassTest, OrReduceMaskedByNotSignExtPredicate) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(8));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/8);
BValue inv = fb.Not(se);
BValue masked = fb.And(x, inv);
BValue result = fb.OrReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Not(m::Param("b")), m::OrReduce(m::Param("x"))));
}

TEST_F(BasicSimplificationPassTest, AndReduceMaskedByNotSignExtPredicate) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(8));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/8);
BValue inv = fb.Not(se);
BValue masked = fb.And(inv, x);
BValue result = fb.AndReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Not(m::Param("b")), m::AndReduce(m::Param("x"))));
}

TEST_F(BasicSimplificationPassTest, NoRewriteForWEquals1) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue x = fb.Param("x", p->GetBitsType(1));
BValue b = fb.Param("b", p->GetBitsType(1));
BValue se = fb.SignExtend(b, /*new_bit_count=*/1);
BValue masked = fb.And(x, se);
BValue result = fb.OrReduce(masked);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(result));
ScopedVerifyEquivalence sve(f);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
// or_reduce(bits[1]) is simplified to its operand; the w==1
// reduction-under-sign_ext(mask) simplification is intentionally not applied.
EXPECT_THAT(f->return_value(),
m::And(m::Param("x"), m::SignExt(m::Param("b"))));
}

TEST_F(BasicSimplificationPassTest, AndWithDuplicateOperands) {
auto p = CreatePackage();
XLS_ASSERT_OK_AND_ASSIGN(Function * f, ParseFunction(R"(
Expand Down