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Break alias cycles by inserting zero values.

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This commit is contained in:
Dan Gohman
2017-10-26 12:22:16 -07:00
parent 53e93d94a8
commit 683408f30b
1 changed files with 19 additions and 4 deletions
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23
lib/frontend/src/ssa.rs
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@@ -530,13 +530,18 @@ where
// so we don't need to have it as an ebb argument. // so we don't need to have it as an ebb argument.
// We need to replace all the occurences of val with pred_val but since // We need to replace all the occurences of val with pred_val but since
// we can't afford a re-writing pass right now we just declare an alias. // we can't afford a re-writing pass right now we just declare an alias.
func.dfg.remove_ebb_param(temp_arg_val);
// Resolve aliases eagerly so that we can check for cyclic aliasing, // Resolve aliases eagerly so that we can check for cyclic aliasing,
// which can occur in unreachable code. // which can occur in unreachable code.
let resolved = func.dfg.resolve_aliases(pred_val); let mut resolved = func.dfg.resolve_aliases(pred_val);
if temp_arg_val != resolved { if temp_arg_val == resolved {
func.dfg.change_to_alias(temp_arg_val, resolved); // Cycle detected. Break it by creating a zero value.
resolved = emit_zero(
func.dfg.value_type(temp_arg_val),
FuncCursor::new(func).at_first_insertion_point(dest_ebb),
);
} }
func.dfg.remove_ebb_param(temp_arg_val);
func.dfg.change_to_alias(temp_arg_val, resolved);
resolved resolved
} }
ZeroOneOrMore::More() => { ZeroOneOrMore::More() => {
@@ -1183,6 +1188,11 @@ mod tests {
ssa.declare_ebb_predecessor(ebb1, block2, j); ssa.declare_ebb_predecessor(ebb1, block2, j);
} }
ssa.seal_ebb_header_block(ebb1, &mut func); ssa.seal_ebb_header_block(ebb1, &mut func);
let flags = settings::Flags::new(&settings::builder());
match verify_function(&func, &flags) {
Ok(()) => {}
Err(err) => panic!(err.message),
}
} }
#[test] #[test]
@@ -1224,5 +1234,10 @@ mod tests {
} }
ssa.seal_ebb_header_block(ebb1, &mut func); ssa.seal_ebb_header_block(ebb1, &mut func);
ssa.seal_ebb_header_block(ebb2, &mut func); ssa.seal_ebb_header_block(ebb2, &mut func);
let flags = settings::Flags::new(&settings::builder());
match verify_function(&func, &flags) {
Ok(()) => {}
Err(err) => panic!(err.message),
}
} }
} }