On Sun, 12 Dec 2021, 17:34 Segher Boessenkool, <segher@xxxxxxxxxxxxxxxxxxx> wrote: > On Fri, Dec 10, 2021 at 03:40:30PM -0700, Jeff Law wrote: > > On 12/9/2021 6:06 PM, Marc Feeley wrote: > > > In any case, as I said previously, I don’t want to check if gcc did > > > not optimize the tail calls… it is too late to do anything other than > > > essentially tell the programmer “sorry I couldn’t compile that > > > program correctly”. > > We don't generally like that kind of diagnostic particularly from the > > code generator. If it's valid C code, then we need to generate correct > > target code. > > Yes. But we *do* have sorry()s for some other cases where it is in > principle possible to generate correct code, but we just don't have that > implemented. > > > >So what I need is some way to test at compile time if for the target > > >architecture and compile options there is a guarantee that tail calls > will > > >be optimized (if the C code obeys the usual set of constraints). > > >Something like > > > > > > #if __has_attribute(tail_call_supported_on_this_architecture) > > > /* use tail calls for jumping around... */ > > > #else > > > /* use trampoline fallback... */ > > > #endif > > Which is the fundamental problem. GCC simply doesn't have the concept > > that "this must be implemented as a tail call". > > All architecture support tail calls, and all architectures do not > support them everywhere: for some code GCC will not do a tail call on > any architecture. > > An attribute that means "sorry() if you cannot do a tail call here" (or > "on any call to this function") can be implemented, and will work fine > in practice for all users, in my estimation. > That seems roughly analogous to always_inline.
