On 3/25/25 3:34 AM, Jakub Jelinek wrote:
Hi!
As discussed here and in bugzilla, [[clang::musttail]] attribute in clang
not just strongly asks for tail call or error, but changes behavior.
To quote:
https://clang.llvm.org/docs/AttributeReference.html#musttail
"The lifetimes of all local variables and function parameters end immediately
before the call to the function. This means that it is undefined behaviour
to pass a pointer or reference to a local variable to the called function,
which is not the case without the attribute. Clang will emit a warning in
common cases where this happens."
The GCC behavior was just to error if we can't prove the musttail callee
could not have dereferenced escaped pointers to local vars or parameters
of the caller. That is still the case for variables with non-trivial
destruction (even in clang), like vars with C++ non-trivial destructors or
variables with cleanup attribute.
The following patch changes the behavior to match that of clang, for all of
[[clang::musttail]], [[gnu::musttail]] and __attribute__((musttail)).
clang 20 actually added warning for some cases of it in
https://github.com/llvm/llvm-project/pull/109255
but it is under -Wreturn-stack-address warning.
Now, gcc doesn't have that warning, but -Wreturn-local-addr instead, and
IMHO it is better to have this under new warnings, because this isn't about
returning local address, but about passing it to a musttail call, or maybe
escaping to a musttail call. And perhaps users will appreciate they can
control it separately as well.
The patch introduces 2 new warnings.
-Wmusttail-local-addr
which is turn on by default and warns for the always dumb cases of passing
an address of a local variable or parameter to musttail call's argument.
I don't think this is a significantly different case from
-Wreturn-local-addr; in both cases we are passing a local address out at
the same time as the stack frame goes away, the only difference is
whether it's passed by return or argument. I don't think that
difference justifies using a different flag.
If others agree with you I don't mind going along, just wanted to make
my case.
And then
-Wmaybe-musttail-local-addr
which is only diagnosed if -Wmusttail-local-addr was not diagnosed and
diagnoses at most one (so that we don't emit 100s of warnings for one call
if 100s of vars can escape) case where an address of a local var could have
escaped to the musttail call. This is less severe, the code doesn't have
to be obviously wrong, so the warning is only enabled in -Wextra.
I agree that this should be a different flag.
Jason
