> In the patches Jeff is going to post, we have (at least for > -fasynchronous-unwind-tables which is on by default on e.g. x86) > precise unwind info even with the new stack check mode. > ira.c currently has: > /* We need the frame pointer to catch stack overflow exceptions if > the stack pointer is moving (as for the alloca case just above). > */ > || (STACK_CHECK_MOVING_SP > > && flag_stack_check > && flag_exceptions > && cfun->can_throw_non_call_exceptions)
There is the same test in ix86_finalize_stack_realign_flags. > For alloca we have a frame pointer for other reasons, the question is > if we really need this hunk even if we provided proper unwind info > even for the Ada -fstack-check mode. Or, if we provide proper unwind info > for -fasynchronous-unwind-tables, if the above could not be also > && !flag_asynchronous_unwind_tables. Eric, what exactly is the reason > for the above, is it just lack of proper CFI notes, or something different? The reason was the assertion present in ix86_adjust_stack_and_probe that the CFA was not the stack pointer and which was removed in r230245: https://gcc.gnu.org/ml/gcc-patches/2015-11/msg01499.html I agree that it's probably obsolete by now. > Also, on i?86 orq $0, (%rsp) or orl $0, (%esp) is used to probe stack, > while it is shorter, is it actually faster or as slow as movq $0, (%rsp) > or movl $0, (%esp) ? It's the idiom used on Windows since day #1, see libgcc/config/i386/cygwin.S. -- Eric Botcazou
