Re: [patch]: Implement PR104327 for avr

[Georg-Johann Lay <avr@gjlay.de>]

Am 25.05.23 um 08:35 schrieb Richard Biener:
> On Wed, May 24, 2023 at 5:44 PM Georg-Johann Lay <avr@gjlay.de> wrote:
>> Am 24.05.23 um 11:38 schrieb Richard Biener:
>>> On Tue, May 23, 2023 at 2:56 PM Georg-Johann Lay <avr@gjlay.de> wrote:
>>>>
>>>> PR target/104327 not only affects s390 but also avr:
>>>> The avr backend pre-sets some options depending on optimization level.
>>>> The inliner then thinks that always_inline functions are not eligible
>>>> for inlining and terminates with an error.
>>>>
>>>> Proposing the following patch that implements TARGET_CAN_INLINE_P.
>>>>
>>>> Ok to apply?
>>>>
>>>> Johann
>>>>
>>>> target/104327: Allow more inlining between different optimization levels.
>>>>
>>>> avr-common.cc introduces the following options that are set depending
>>>> on optimization level: -mgas-isr-prologues, -mmain-is-OS-task and
>>>> -fsplit-wide-types-early.  The inliner thinks that different options
>>>> disallow cross-optimization inlining, so provide can_inline_p.
>>>>
>>>> gcc/
>>>>           PR target/104327
>>>>           * config/avr/avr.cc (avr_can_inline_p): New static function.
>>>>           (TARGET_CAN_INLINE_P): Define to that function.
>>>> diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
>>>> index 9fa50ca230d..55b48f63865 100644
>>>> --- a/gcc/config/avr/avr.cc
>>>> +++ b/gcc/config/avr/avr.cc
>>>> @@ -1018,6 +1018,22 @@ avr_no_gccisr_function_p (tree func)
>>>>       return avr_lookup_function_attribute1 (func, "no_gccisr");
>>>>     }
>>>>
>>>> +
>>>> +/* Implement `TARGET_CAN_INLINE_P'.  */
>>>> +/* Some options like -mgas_isr_prologues depend on optimization level,
>>>> +   and the inliner might think that due to different options, inlining
>>>> +   is not permitted; see PR104327.  */
>>>> +
>>>> +static bool
>>>> +avr_can_inline_p (tree /* caller */, tree callee)
>>>> +{
>>>> +  // For now, dont't allow to inline ISRs.  If the user actually wants
>>>> +  // to inline ISR code, they have to turn the body of the ISR into an
>>>> +  // ordinary function.
>>>> +
>>>> +  return ! avr_interrupt_function_p (callee);
>>>
>>> I'm not sure if AVR has ISA extensions but the above will likely break
>>> things like
>>>
>>> void __attribute__((target("-mX"))) foo () { asm ("isa X opcode");
>>> stmt-that-generates-X-ISA; }
>>
>> This yields
>>
>> warning: target attribute is not supported on this machine [-Wattributes]
> 
> Ah, that's an interesting fact.  So that indeed leaves
> __attribute__((optimize(...)))
> influencing the set of active target attributes via the generic option target
> hooks like in your case the different defaults.
> 
>> avr has -mmcu=<arch> target options, but switching them in mid-air
>> won't work because the file prologue might already be different
>> and incompatible across different architectures.  And I never
>> saw any user requesting such a thing, and I can't imagine
>> any reasonable use case...  If the warning is not strong enough,
>> may be it can be turned into an error, but -Wattributes is not
>> specific enough for that.
> 
> Note the target attribute is then simply ignored.
> 
>>> void bar ()
>>> {
>>>     if (cpu-has-X)
>>>       foo ();
>>> }
>>>
>>> if always-inlines are the concern you can use
>>>
>>>     bool always_inline
>>>       = (DECL_DISREGARD_INLINE_LIMITS (callee)
>>>          && lookup_attribute ("always_inline",
>>>                               DECL_ATTRIBUTES (callee)));
>>>     /* Do what the user says.  */
>>>     if (always_inline)
>>>       return true;
>>>
>>>     return default_target_can_inline_p (caller, callee);
>>
>> The default implementation of can_inline_p worked fine for avr.
>> As far as I understand, the new behavior is due to clean-up
>> of global states for options?
> 
> I think the last change was r8-2658-g9b25e12d2d940a which
> for targets without target attribute support made it more likely
> to run into the default hook actually comparing the options.
> Previously the "default" was oddly special-cased but you
> could have still run into compares with two different set of
> defaults when there's another "default" default.  Say, compile
> with -O2 and have one optimize(0) and one optimize(Os)
> function it would compare the optimize(0) and optimize(Os)
> set if they were distinct from the -O2 set.  That probably never
> happened for AVR.
> 
>> So I need to take into account inlining costs and decide on that
>> whether it's preferred to inline a function or not?
> 
> No, the hook isn't about cost, it's about full incompatibility.  So
> if the different -m options that could be in effect for AVR in
> a single TU for different functions never should prevent inlining
> then simply make the hook return true.  If there's a specific
> option (that can differ from what specified on the compiler
> command line!) that should, then you should compare the
> setting of that option from the DECL_FUNCTION_SPECIFIC_TARGET
> of the caller and the callee.
> 
> But as far as I can see simply returning true should be correct
> for AVR, or like your patch handle interrupts differently (though
> the -Winline diagnostic will tell the user there's a mismatch in
> target options which might be confusing).

Ok, simply "true" sounds reasonable.  Is that change ok then?

Johann


> Richard.
> 
>> Johann
>>
>>>> +}
>>>> +
>>>>     /* Implement `TARGET_SET_CURRENT_FUNCTION'.  */
>>>>     /* Sanity cheching for above function attributes.  */
>>>>
>>>> @@ -14713,6 +14729,9 @@ avr_float_lib_compare_returns_bool (machine_mode
>>>> mode, enum rtx_code)
>>>>     #undef  TARGET_MD_ASM_ADJUST
>>>>     #define TARGET_MD_ASM_ADJUST avr_md_asm_adjust
>>>>
>>>> +#undef  TARGET_CAN_INLINE_P
>>>> +#define TARGET_CAN_INLINE_P avr_can_inline_p
>>>> +
>>>>     struct gcc_target targetm = TARGET_INITIALIZER;