On Thu, Jul 1, 2021 at 3:13 PM Martin Liška <mli...@suse.cz> wrote:
>
> On 10/23/20 1:47 PM, Martin Liška wrote:
> > Hey.
>
> Hello.
>
> I deferred the patch for GCC 12. Since the time, I messed up with options
> I feel more familiar with the option handling. So ...
>
> >
> > This is a follow-up of the discussion that happened in thread about
> > no_stack_protector
> > attribute: https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545916.html
> >
> > The current optimize attribute works in the following way:
> > - 1) we take current global_options as base
> > - 2) maybe_default_options is called for the currently selected
> > optimization level, which
> > means all rules in default_options_table are executed
> > - 3) attribute values are applied (via decode_options)
> >
> > So the step 2) is problematic: in case of -O2 -fno-omit-frame-pointer and
> > __attribute__((optimize("-fno-stack-protector")))
> > ends basically with -O2 -fno-stack-protector because
> > -fno-omit-frame-pointer is default:
> > /* -O1 and -Og optimizations. */
> > { OPT_LEVELS_1_PLUS, OPT_fomit_frame_pointer, NULL, 1 },
> >
> > My patch handled and the current optimize attribute really behaves that
> > same as appending attribute value
> > to the command line. So far so good. We should also reflect that in
> > documentation entry which is quite
> > vague right now:
>
> ^^^ all these are still valid arguments, plus I'm adding a new test-case that
> tests that.
There is also handle_common_deferred_options that's not called so any
option processed there should
probably be excempt from being set/unset in the optimize attribute?
> >
> > """
> > The optimize attribute is used to specify that a function is to be compiled
> > with different optimization options than specified on the command line.
> > """
>
> I addressed that with documentation changes, should be more clear to users.
> Moreover, I noticed that we declare 'optimize' attribute
> as something not for a production use:
>
> "The optimize attribute should be used for debugging purposes only. It is not
> suitable in production code."
>
> Are we sure about the statement? I know that e.g. glibc uses that.
Well, given we're changing behavior now that warning looks valid ;)
I'll also note that
"The optimize attribute arguments of a function behave
as if they were added to the command line options."
is still likely untrue, the global state init is complicated ;)
> >
> > and we may want to handle -Ox in the attribute in a special way. I guess
> > many macro/pragma users expect that
> >
> > -O2 -ftree-vectorize and __attribute__((optimize(1))) will end with -O1 and
> > not
> > with -ftree-vectorize -O1 ?
As implemented your patch seems to turn it into -ftree-vectorize -O1.
IIRC multiple optimize attributes apply
ontop of each other, and it makes sense to me that optimize (2),
optimize ("tree-vectorize") behaves the same
as optimize (2, "tree-vectorize"). I'm not sure this is still the
case after your patch? Also consider
#pragma GCC optimize ("tree-vectorize")
void foo () { ...}
#pragma GCC optimize ("tree-loop-distribution")
void bar () {... }
I'd expect bar to have both vectorization and loop distribution
enabled? (note I didn't use push/pop here)
> The situation with 'target' attribute is different. When parsing the
> attribute, we intentionally drop all existing target flags:
>
> $ cat -n gcc/config/i386/i386-options.c
> ...
> 1245 if (opt == IX86_FUNCTION_SPECIFIC_ARCH)
> 1246 {
> 1247 /* If arch= is set, clear all bits in
> x_ix86_isa_flags,
> 1248 except for ISA_64BIT, ABI_64, ABI_X32, and
> CODE16
> 1249 and all bits in x_ix86_isa_flags2. */
> 1250 opts->x_ix86_isa_flags &= (OPTION_MASK_ISA_64BIT
> 1251 | OPTION_MASK_ABI_64
> 1252 | OPTION_MASK_ABI_X32
> 1253 | OPTION_MASK_CODE16);
> 1254 opts->x_ix86_isa_flags_explicit &=
> (OPTION_MASK_ISA_64BIT
> 1255 |
> OPTION_MASK_ABI_64
> 1256 |
> OPTION_MASK_ABI_X32
> 1257 |
> OPTION_MASK_CODE16);
> 1258 opts->x_ix86_isa_flags2 = 0;
> 1259 opts->x_ix86_isa_flags2_explicit = 0;
> 1260 }
>
> That seems logical because target attribute is used for e.g. ifunc
> multi-versioning and one needs
> to be sure all existing ISA flags are dropped. However, I noticed clang
> behaves differently:
>
> $ cat hreset.c
> #pragma GCC target "arch=geode"
> #include <immintrin.h>
> void foo(unsigned int eax)
> {
> _hreset (eax);
> }
>
> $ clang hreset.c -mhreset -c -O2 -m32
> $ gcc hreset.c -mhreset -c -O2 -m32
> In file included from
> /home/marxin/bin/gcc/lib64/gcc/x86_64-pc-linux-gnu/12.0.0/include/x86gprintrin.h:97,
> from
> /home/marxin/bin/gcc/lib64/gcc/x86_64-pc-linux-gnu/12.0.0/include/immintrin.h:27,
> from hreset.c:2:
> hreset.c: In function ‘foo’:
> /home/marxin/bin/gcc/lib64/gcc/x86_64-pc-linux-gnu/12.0.0/include/hresetintrin.h:39:1:
> error: inlining failed in call to ‘always_inline’ ‘_hreset’: target specific
> option mismatch
> 39 | _hreset (unsigned int __EAX)
> | ^~~~~~~
> hreset.c:5:3: note: called from here
> 5 | _hreset (eax);
> | ^~~~~~~~~~~~~
>
> Anyway, I think the current target attribute handling should be preserved.
I think this and the -O1 argument above suggests that there should be
a way to distinguish
two modes - add to the active set of options and starting from scratch.
Maybe it's over-designing things but do we want to preserve the
existing behavior
and instead add optimize ("+ftree-vectorize") and target ("+avx2") as
a way to amend
the state?
OTOH as we're missing global_options re-init even with your patch we won't get
the defaults correct (aka what toplev::main does with init_options_struct and
the corresponding langhook). Likewise if lang_hooks.init_options performs any
defaulting a later flag overrides and we override that with optimize() that
doesn't work - I'm thinking of things like flag_complex_method and -fcx-* flags.
So -O2 -fcx-fortran-rules on the command-line and optimize
("no-cx-fortran-rules")
to cancel the -fcx-fortran-rules switch wouldn't work?
Thanks,
Richard.
> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>
> Ready to be installed?
> Thanks,
> Martin
>
> >
> > I'm also planning to take a look at the target macro/attribute, I expect
> > similar problems:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97469
> >
> > Thoughts?
> > Thanks,
> > Martin
>
