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[opt] Optimize one-bit logical shifts. #3638

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copybara-service merged 2 commits into from
Jan 5, 2026
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Merged

[opt] Optimize one-bit logical shifts. #3638

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164bacf
[opt] Optimize one-bit logical shifts.
cdleary Dec 31, 2025
3417825
Move the logical shift opt down towards related opt.
cdleary Jan 2, 2026
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16 changes: 16 additions & 0 deletions xls/passes/arith_simplification_pass.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1231,6 +1231,22 @@ absl::StatusOr<bool> MatchArithPatterns(int64_t opt_level, Node* n,
return true;
}

// Logical shift of a 1-bit value:
// shll(y:bits[1], amt) == y & (amt == 0)
// shrl(y:bits[1], amt) == y & (amt == 0)
//
// A 1-bit logical shift by a nonzero amount produces zero, and a shift by
// zero is a no-op.
if ((n->op() == Op::kShll || n->op() == Op::kShrl) &&
n->operand(0)->BitCountOrDie() == 1) {
VLOG(2) << "FOUND: logical shift of 1-bit value";
XLS_ASSIGN_OR_RETURN(Node * amt_is_zero,
CompareLiteral(n->operand(1), 0, Op::kEq));
std::vector<Node*> args = {n->operand(0), amt_is_zero};
XLS_RETURN_IF_ERROR(n->ReplaceUsesWithNew<NaryOp>(args, Op::kAnd).status());
return true;
}

// An arithmetic shift-right of a 1-bit value is a no-op.
if (n->op() == Op::kShra && n->operand(0)->BitCountOrDie() == 1) {
VLOG(2) << "FOUND: arithmetic shift-right of 1-bit value";
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26 changes: 26 additions & 0 deletions xls/passes/arith_simplification_pass_test.cc
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Expand Up @@ -2170,6 +2170,32 @@ TEST_F(ArithSimplificationPassTest, GuardedShiftOperationLowLimit) {
EXPECT_THAT(f->return_value(), m::Shll(m::Param("x"), m::Select()));
}

TEST_F(ArithSimplificationPassTest, LogicalShiftLeftOfOneBitUnknownAmount) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue amt = fb.Param("amt", p->GetBitsType(10));
BValue y = fb.Param("y", p->GetBitsType(1));
fb.Shll(y, amt);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.Build());
ScopedVerifyEquivalence sve(f, kProverTimeout);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Param("y"), m::Eq(m::Param("amt"), m::Literal(0))));
}

TEST_F(ArithSimplificationPassTest, LogicalShiftRightOfOneBitUnknownAmount) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
BValue amt = fb.Param("amt", p->GetBitsType(10));
BValue y = fb.Param("y", p->GetBitsType(1));
fb.Shrl(y, amt);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.Build());
ScopedVerifyEquivalence sve(f, kProverTimeout);
ASSERT_THAT(Run(p.get()), IsOkAndHolds(true));
EXPECT_THAT(f->return_value(),
m::And(m::Param("y"), m::Eq(m::Param("amt"), m::Literal(0))));
}

TEST_F(ArithSimplificationPassTest, UMulCompare) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
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