On Thu, Mar 20, 2025 at 09:19:02AM -0700, Andi Kleen wrote:
> The inlining was just one of the issue, there are some related to
> different semantics of escaped locals. gcc always errors out while
> LLVM declares it undefined.
>
> But maybe we can fix that one too.
I have 3 patches to be tested, the inline one, fnsplit one and ipa-icf one.
For the escaped locals, I guess we need to decide if [[clang::musttail]]
will be something different from [[gnu::musttail]] in GCC or not (and if
yes, what __attribute__((musttail)) will be).
The current difference in behavior is on
int foo (void);
void bar (int *);
int
baz (void)
{
int a;
bar (&a);
[[clang::musttail]] return foo ();
}
Clang makes the attribute not just a request to tail call it or fail,
but also changes behavior and says if the code ever accesses a from the
above during foo () (which without the attribute is completely valid), then
it is UB.
So, clang can compile this into a tail call, while GCC takes the safer
approach and says it is invalid. Users can of course adjust their code to
make it explicitly end lifetime before the tailcall, like with
int
qux (void)
{
{
int a;
bar (&a);
}
[[clang::musttail]] return foo ();
}
and then gcc should tail call it too. Strangely, it there is a var with
non-trivial destructor which ends lifetime after the call, clang rejects it
like gcc:
struct S { S (); ~S (); int s; };
int
corge (void)
{
S a;
bar (&a.s);
[[clang::musttail]] return foo ();
}
Jakub
