Fallout: save/restore target options in handle_optimize_attribute

Martin Liška mliska@suse.cz
Tue Jun 1 13:20:49 GMT 2021


On 6/1/21 3:11 PM, Richard Biener wrote:
> On Tue, Jun 1, 2021 at 1:17 PM Martin Liška <mliska@suse.cz> wrote:
>>
>> On 5/28/21 2:46 PM, Richard Biener wrote:
>>> On Fri, May 28, 2021 at 11:48 AM Martin Liška <mliska@suse.cz> wrote:
>>>>
>>>> Hi.
>>>>
>>>> There's a fallout after my revision ebd5e86c0f41dc1d692f9b2b68a510b1f6835a3e. I would like to analyze
>>>> all case and discuss possible solution. To be honest it's a can of worms and reverting the commit
>>>> is an option on the table.
>>>>
>>>> So the cases:
>>>>
>>>> 1) PR100759 - ppc64le
>>>>
>>>> $ cat pr.C
>>>> #pragma GCC optimize 0
>>>> void main();
>>>>
>>>> $ ./xgcc -B. -Os pr.C
>>>> pr.C:2:11: internal compiler error: ‘global_options’ are modified in local context
>>>>        2 | void main();
>>>>
>>>> What happens: we change from -Os to -O0 and rs6000_isa_flags differ in cl_optimization_compare.
>>>> Problem is that OPTION_MASK_SAVE_TOC_INDIRECT is set based on optimize flag:
>>>>
>>>>      /* If we can shrink-wrap the TOC register save separately, then use
>>>>         -msave-toc-indirect unless explicitly disabled.  */
>>>>      if ((rs6000_isa_flags_explicit & OPTION_MASK_SAVE_TOC_INDIRECT) == 0
>>>>          && flag_shrink_wrap_separate
>>>>          && optimize_function_for_speed_p (cfun))
>>>>        rs6000_isa_flags |= OPTION_MASK_SAVE_TOC_INDIRECT;
>>>
>>> So that means that
>>>
>>>         /* Restore current options.  */
>>>         cl_optimization_restore (&global_options, &global_options_set,
>>>                                  &cur_opts);
>>>         cl_target_option_restore (&global_options, &global_options_set,
>>>                                   TREE_TARGET_OPTION (prev_target_node));
>>>
>>> does not result in the same outcome as the original command-line processing?
>>>
>>> Given both restore processes could interact (not sure if that's the issue here)
>>> shouldn't we just have a single restore operation and a single target
>>> hook instead of both targetm.override_options_after_change and
>>> targetm.target_option.restore?
>>
>> That's not this case. But it can be a unification approach for the future.
>>
>>>
>>> Likewise we should probably _always_ set both, DECL_FUNCTION_SPECIFIC_OPT
>>> and _TARGET as a step towards unifying them.
>>
>> Yes, that's basically what's happening at various places.
>>
>>>
>>> That said, for the above case a more detailed run-down as to how things go wrong
>>> would be nice to see.
>>
>> Anyway, detail analysis of this issue is:
>>
>> 1) one provides -Os on the command-line, thus global_options.x_optimize_size == 1
>> 2) then we reach #pragma GCC optimize 0, at this point parse_optimize_options is called
>>      and thus global_options are modified (global_options.x_optimize_size)
>>      That's reflected in optimization_current_node, which is now different from optimization_default_node.
>> 3) targetm.override_options_after_change is not called, so global_options.x_rs6000_isa_flags
>>      is not changed to 1.
>> 4) for all subsequent functions, handle_optimize_attribute is called as we are in a 'pragma optimize'
>> 5) here the sanity checking code saves saved_global_options, parsing happens and cl_*_restore is done
>> 6) as cl_target_option_restore calls targetm.override_options_after_change, the global_options.x_rs6000_isa_flags
>>      has OPTION_MASK_SAVE_TOC_INDIRECT set
>> 7) and the cl_optimization_compare complains
>>
>> I have a patch that reflects that. In fact, we global options state is correct for each function.
>> Apart from that, PR100759 mentions a test-case that fails due to a missing cl_target_option_restore
>> for 'pragma pop'.
>>
>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests. And it survives tests on ppc64-linux-gnu.
>>
>> Ready to be installed?
> 
> It sounds like a clear progression so OK.

Good, I'm going to install it.

> 
> I still don't get
> 
> +      /* When #pragma GCC optimize pragma is used, it modifies global_options
> +        without calling targetm.override_options_after_change.  That can leave
> +        target flags inconsistent for comparison.  */
> 
> fully, esp. as to why we cannot fix pragma handling and thus why the
> "inconsistent"
> state is actually OK.

Well, the sanity check is designed simply as it saved global_options, then
parse_optimize_options happens and cl_*_restore is done. After that we want
to be sure the global_options is equal to the saved one.

And here comes the problem. We saved global_options modified after '#pragma GCC optimize 0'.

Martin

> 
> Richard.
> 
>> Thanks,
>> Martin
>>
>>>
>>>> Suggested solution is doing:
>>>>
>>>>      if ((rs6000_isa_flags_explicit & OPTION_MASK_SAVE_TOC_INDIRECT) == 0
>>>>          && flag_shrink_wrap_separate
>>>>        rs6000_isa_flags |= OPTION_MASK_SAVE_TOC_INDIRECT;
>>>>
>>>> and add '&& optimize_function_for_speed_p (cfun)' to the place where the option mask is used.
>>>>
>>>> 2) Joseph's case:
>>>>
>>>> $ cat ~/Programming/testcases/opts-bug.i
>>>> extern unsigned long int x;
>>>> extern float f (float);
>>>> extern __typeof (f) f_power8;
>>>> extern __typeof (f) f_power9;
>>>> extern __typeof (f) f __attribute__ ((ifunc ("f_ifunc")));
>>>> static __attribute__ ((optimize ("-fno-stack-protector"))) __typeof (f) *
>>>> f_ifunc (void)
>>>> {
>>>>      __typeof (f) *res = x ? f_power9 : f_power8;
>>>>      return res;
>>>> }
>>>>
>>>> $ ./xgcc -B. ~/Programming/testcases/opts-bug.i -c -S -O2 -mlong-double-128 -mabi=ibmlongdouble
>>>> /home/marxin/Programming/testcases/opts-bug.i:8:1: error: ‘-mabi=ibmlongdouble’ requires ‘-mlong-double-128’
>>>>
>>>> This is caused by a weird option override:
>>>>
>>>>      else if (rs6000_long_double_type_size == 128)
>>>>        rs6000_long_double_type_size = FLOAT_PRECISION_TFmode; (it's 127)
>>>>
>>>> later when rs6000_option_override_internal is called for saved target flags (127), it complains.
>>>> Possible fix:
>>>>
>>>>      else if (rs6000_long_double_type_size == 128
>>>>              || rs6000_long_double_type_size == FLOAT_PRECISION_TFmode)
>>>>
>>>> 3) ARM issue reported here:
>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98636#c20
>>>>
>>>>      arm_fp16_inst = bitmap_bit_p (arm_active_target.isa, isa_bit_fp16);
>>>>      if (arm_fp16_inst)
>>>>        {
>>>>          if (arm_fp16_format == ARM_FP16_FORMAT_ALTERNATIVE)
>>>>           error ("selected fp16 options are incompatible");
>>>>          arm_fp16_format = ARM_FP16_FORMAT_IEEE;
>>>>        }
>>>>
>>>> there's likely missing else branch which would reset when arm_fp16_inst is null.
>>>> Anyway, can be moved again to the ignored list
>>>>
>>>> 4) Jeff reported the following for v850-elf:
>>>>
>>>> $ cat ~/Programming/testcases/j.c
>>>> typedef __SIZE_TYPE__ size_t;
>>>>
>>>> extern inline __attribute__ ((__always_inline__, __gnu_inline__, __artificial__, __nothrow__, __leaf__)) void *
>>>> memcpy (void *__restrict __dest, const void *__restrict __src, size_t __len)
>>>> {
>>>>      return __builtin___memcpy_chk (__dest, __src, __len, __builtin_object_size (__dest, 0));
>>>> }
>>>>
>>>> __attribute__((optimize ("Ofast"))) void
>>>> bar (void *d, void *s, size_t l)
>>>> {
>>>>      memcpy (d, s, l);
>>>> }
>>>>
>>>> $ ./xgcc -B. ~/Programming/testcases/j.c  -S
>>>> /home/marxin/Programming/testcases/j.c: In function ‘bar’:
>>>> /home/marxin/Programming/testcases/j.c:4:1: error: inlining failed in call to ‘always_inline’ ‘memcpy’: target specific option mismatch
>>>>        4 | memcpy (void *__restrict __dest, const void *__restrict __src, size_t __len)
>>>>          | ^~~~~~
>>>> /home/marxin/Programming/testcases/j.c:12:3: note: called from here
>>>>       12 |   memcpy (d, s, l);
>>>>          |   ^~~~~~~~~~~~~~~~
>>>>
>>>> This one is pretty clear. The target does:
>>>>
>>>>        { OPT_LEVELS_1_PLUS, OPT_mprolog_function, NULL, 1 },
>>>>
>>>> So it sets a target option based on optimize level.
>>>> This one will need:
>>>>
>>>> diff --git a/gcc/config/v850/v850.c b/gcc/config/v850/v850.c
>>>> index e0e5005d865..49f91f12766 100644
>>>> --- a/gcc/config/v850/v850.c
>>>> +++ b/gcc/config/v850/v850.c
>>>> @@ -3140,6 +3140,11 @@ v850_option_override (void)
>>>>       /* The RH850 ABI does not (currently) support the use of the CALLT instruction.  */
>>>>       if (! TARGET_GCC_ABI)
>>>>         target_flags |= MASK_DISABLE_CALLT;
>>>> +
>>>> +  /* Save the initial options in case the user does function specific
>>>> +     options.  */
>>>> +  target_option_default_node = target_option_current_node
>>>> +    = build_target_option_node (&global_options, &global_options_set);
>>>>     }
>>>>
>>>> plus a custom can_inline_p target hook where the MASK_PROLOG_FUNCTION is ignored because
>>>> caller does not have it set, while callee has.
>>>>
>>>> What target maintainers thing about it?
>>>>
>>>> Martin
>>



More information about the Gcc-patches mailing list