Hi Andrew,
On a Microsoft Windows target the following (placed inside a function of
course) will only work correctly if volatile is specified in the basic asm
block (or if the attached patch was applied to gcc):
asm ("1:" "\n"
"\t" ".seh_handler __C_specific_handler, @except" "\n"
"\t" ".seh_handlerdata" "\n"
"\t" ".long 1" "\n"
"\t" ".rva 1b, 2f, 3f, 4f" "\n"
"\t" ".seh_code");
{
// std::printf("Guarded\n");
RaiseException(EXCEPTION_BREAKPOINT, 0, 0, nullptr);
}
asm ("nop" "\n"
"\t" "2: nop" "\n"
"\t" "jmp 5f" "\n"
"\t" "3:" "\n"
"\t" "push rbp" "\n"
"\t" "mov rbp, rsp"
"\t" "push r15" "\n"
"\t" "mov r15, rcx" "\n");
[] [[gnu::noinline, gnu::used]] () -> long {
return EXCEPTION_EXECUTE_HANDLER;
}();
asm ("pop r15" "\n"
"\t" "pop rbp" "\n"
"\t" "ret" "\n"
"\t" "nop" "\n"
"4:");
{
std::printf("Exception\n");
}
asm ("5:");
In any case I doubt marking it as volatile in the parser hurts either,
since this is the behaviour it's supposed to have
best regards,
Julian
On Wed, Jun 28, 2023 at 12:24 AM Andrew Pinski <pins...@gmail.com> wrote:
> On Tue, Jun 27, 2023 at 9:15 AM Julian Waters <tanksherma...@gmail.com>
> wrote:
> >
> > Hi Andrew,
> >
> > That can't be right, on my system a test of asm vs asm volatile with -O3
> and -flto=auto yields very different results, with only the latter being
> correct. The patch fixed it and caused gcc to emit correct assembly
>
> Can you provide a few testcases? Because the gimplifier should always
> happen.
>
> Thanks,
> Andrew Pinski
>
> >
> > best regards,
> > Julian
> >
> > On Wed, Jun 28, 2023 at 12:08 AM Andrew Pinski <pins...@gmail.com>
> wrote:
> >>
> >> On Tue, Jun 27, 2023 at 9:03 AM Julian Waters via Gcc <gcc@gcc.gnu.org>
> wrote:
> >> >
> >> > gcc's documentatation mentions that all basic asm blocks are always
> volatile,
> >> > yet the parser fails to account for this by only ever setting
> >> > volatile_p to true
> >> > if the volatile qualifier is found. This patch fixes this by adding a
> >> > special case check for extended_p before finish_asm_statement is
> called
> >>
> >> The patch which are you doing will not change the behavior of GCC as
> >> GCC already treats them as volatile later on.
> >> non-extended inline-asm has no outputs so the following code in the
> >> gimplifier will kick in and turn the gimple statement into volatile:
> >> gimple_asm_set_volatile (stmt, ASM_VOLATILE_P (expr) || noutputs
> == 0);
> >>
> >> (note I am about to push a patch which changes the condition slightly
> >> to have `asm goto` as volatile).
> >>
> >> Thanks,
> >> Andrew
> >>
> >> >
> >> > From 3094be39e3e65a6a638f05fafd858b89fefde6b5 Mon Sep 17 00:00:00 2001
> >> > From: TheShermanTanker <tanksherma...@gmail.com>
> >> > Date: Tue, 27 Jun 2023 23:56:38 +0800
> >> > Subject: [PATCH] asm not using extended syntax should always be
> volatile
> >> >
> >> > ---
> >> > gcc/cp/parser.cc | 3 +++
> >> > 1 file changed, 3 insertions(+)
> >> >
> >> > diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> >> > index a6341b9..ef3d06a 100644
> >> > --- a/gcc/cp/parser.cc
> >> > +++ b/gcc/cp/parser.cc
> >> > @@ -22355,6 +22355,9 @@ cp_parser_asm_definition (cp_parser* parser)
> >> > /* Create the ASM_EXPR. */
> >> > if (parser->in_function_body)
> >> > {
> >> > + if (!extended_p) {
> >> > + volatile_p = true;
> >> > + }
> >> > asm_stmt = finish_asm_stmt (asm_loc, volatile_p, string, outputs,
> >> > inputs, clobbers, labels, inline_p);
> >> > /* If the extended syntax was not used, mark the ASM_EXPR. */
> >> > --
> >> > 2.35.1.windows.2
>
