Skip to content

[visibility] Remove our special handling for bypass paths when only one edge was kept #4505

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
copybara-service wants to merge 1 commit into
base: main
Choose a base branch
from
+31 −7
Draft

[visibility] Remove our special handling for bypass paths when only one edge was kept #4505

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
7 changes: 0 additions & 7 deletions xls/passes/visibility_expr_builder.cc
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -411,13 +411,6 @@ absl::StatusOr<Node*> VisibilityBuilder::BuildVisibilityIRExpr(
XLS_ASSIGN_OR_RETURN(
Literal * always_visible,
func->MakeNode<Literal>(SourceInfo(), Value(UBits(1, 1))));
if (conditional_edges.size() == 1) {
XLS_ASSIGN_OR_RETURN(
Node * user_uses_node,
BuildVisibilityExpr(conditional_edges.begin()->operand,
conditional_edges.begin()->node, node, func));
return user_uses_node ? user_uses_node : always_visible;
}
absl::flat_hash_map<Node*, Node*> node_to_visibility_ir_cache;
absl::flat_hash_map<std::tuple<Op, Node*, Node*>, Node*> binary_op_cache;
return BuildVisibilityIRExprFromEdges(func, node, node, conditional_edges,
Expand Down
31 changes: 31 additions & 0 deletions xls/passes/visibility_expr_builder_test.cc
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -260,5 +260,36 @@ TEST_F(VisibilityExprBuilderTest, NotAFunctionOfSelf) {
EXPECT_THAT(is_x_used.first, m::Ne(m::Param("y"), m::Literal(7)));
}

TEST_F(VisibilityExprBuilderTest, VisibilityExpressionWithOneKeptEdge) {
auto p = CreatePackage();
FunctionBuilder fb(TestName(), p.get());
Type* u1 = p->GetBitsType(1);
BValue x = fb.Param("x", u1);
BValue y = fb.Param("y", u1);

// z == select(y, {false, x}) == and(y, x)
BValue literal_false = fb.Literal(UBits(0, 1));
BValue z = fb.Select(y, {literal_false, x});

BValue ret = fb.Xor(x, z);
XLS_ASSERT_OK_AND_ASSIGN(Function * f, fb.BuildWithReturnValue(ret));
std::pair<Node*, VisibilityEstimator::AreaDelay> is_x_used;
XLS_ASSERT_OK_AND_ASSIGN(is_x_used,
BuildDefaultVisibilityExpr(f, x.node(), {}));

// The visibility of `x` should be `Literal(1)`, since:
// XOR(x, z) == XOR(x, AND(y, x)),
// and our analysis can't determine the visibility of `x` through `XOR(x, z)`.
//
// In case we later improve the analysis, the correct visibility expression
// for `x` is `y == 0`. We can see this with a casewise analysis:
// If y == 0, then z == AND(y, x) == 0, so XOR(x, z) == XOR(x, 0) == x.
// If y == 1, then z == AND(y, x) == x, so XOR(x, z) == XOR(x, x) == 0.
// So the output is x if y == 0, and 0 if y == 1. Equivalently:
// ret == x & !y
// As such, `x` is actually visible iff `y == 0`.
EXPECT_THAT(is_x_used.first, m::Literal(1));
}

} // namespace
} // namespace xls
Loading