Re: [patch]: Implement PR104327 for avr

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

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).

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;