i386: Guard noreturn no-callee-saved-registers optimization with -mnoreturn-no-callee-saved-registers [PR38534]
The following patch hides the noreturn no_callee_saved_registers (except bp)
optimization with a not enabled by default option.
The reason is that most noreturn functions should be called just once in a
program (unless they are recursive or invoke longjmp or similar, for exceptions
we already punt), so it isn't that essential to save a few instructions in their
prologue, but more importantly because it interferes with debugging.
And unlike most other optimizations, doesn't actually make it harder to debug
the given function, which can be solved by recompiling the given function if
it is too hard to debug, but makes it harder to debug the callers of that
noreturn function. Those can be from a different translation unit, different
binary or even different package, so if e.g. glibc abort needs to use all
of the callee saved registers (%rbx, %rbp, %r12, %r13, %r14, %r15), debugging
any programs which abort will be harder because any DWARF expressions which
use those registers will be optimized out, not just in the immediate caller,
but in other callers as well until some frame restores a particular register
from some stack slot.
2024-03-08 Jakub Jelinek <jakub@redhat.com>
PR target/38534
* config/i386/i386.opt (mnoreturn-no-callee-saved-registers): New
option.
* config/i386/i386-options.cc (ix86_set_func_type): Don't use
TYPE_NO_CALLEE_SAVED_REGISTERS_EXCEPT_BP unless
ix86_noreturn_no_callee_saved_registers is enabled.
* doc/invoke.texi (-mnoreturn-no-callee-saved-registers): Document.
projects / thirdparty / gcc.git / commit
