Re: [PATCH] Optimize macro: make it more predictable

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Jul 1, 2021 at 3:13 PM Martin Liška <mliska@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
>