On Sat, 31 Dec 2022, Iain Sandoe wrote:

> builtins.def unconditionally defines these builtins to be DEF_EXT_LIB_BUILTIN
> which expands to the libcall, this is currently hard-wired to FALLBACK_P = true.
> 
> but, AFAIU the builtins.def descriptions:
> 
>  FALLBACK_P should be false if the libc (or libm, I suppose, if that’s different)
>  does not have the function, perhaps that’s an underlying bug or at least an
>  oversight?
> 
>  (or, of course, I misunderstood the intent of that param)

FALLBACK_P true means that it's the user's responsibility, if calling 
__builtin_X, to make sure the library function X is also available in 
cases where the call doesn't get expected inline - that is, that the API 
for that __builtin_X function is that it may call an underlying library 
function X, which is expected to exist and have a compatible interface.

Information about whether a function is present in libc / libm is 
generally only relevant when __builtin_X might expand to call Y instead of 
X; then GCC needs to know whether Y is available.

> - at present, it seems that this crt might be the least invasive 
> solution (since ‘significand*()’ are not obsolete AFAIU, we still need 
> to provide those implementations, regardless of any subsitution of 
> scalbn*() in Modula-2).

The significand functions can be considered obsolete and were never in any 
standard (thus glibc does not provide a version for _Float128, for 
example).

-- 
Joseph S. Myers
joseph@codesourcery.com