Hey, On Thu, 27 Jul 2023, Thomas Koenig via Gcc wrote:
> Intel recommends to have the new registers as caller-saved for > compatibility with current calling conventions. If I understand this > correctly, this is required for exception unwinding, but not if the > function called is __attribute__((nothrow)). That's not the full truth. It's not (only) exception handling but also context switching via setjmp/longjmp and make/get/setcontext. The data structures for that are part of the ABI unfortunately, and can't be assumed to be extensible (as Florian says, for glibc there maybe be hacks (or maybe not) on x86-64. Some other archs implemented extensibility from the outset). So all registers (and register parts!) added after the initial psABI is defined usually _have_ to be call-clobbered. > Since Fortran tends to use a lot of registers for its array descriptors, > and also tends to call nothrow functions (all Fortran functions, and > all Fortran intrinsics, such as sin/cos/etc) a lot, it could profit from > making some of the new registers callee-saved, to save some spills > at function calls. I've recently submitted a patch that adds some attributes that basically say "these-and-those regs aren't clobbered by this function" (I did them for not clobbered xmm8-15). Something similar could be used for the new GPRs as well. Then it would be a matter of ensuring that the interesting functions are marked with that attributes (and then of course do the necessary call-save/restore). Ciao, Michael.
