Re: [PATCH] predict: Adjust optimize_function_for_size_p [PR105818]

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2022/6/15 14:20, Kewen.Lin wrote:
> Hi Honza,
> 
> Thanks for the comments!  Some replies are inlined below.
> 
> on 2022/6/14 19:37, Jan Hubicka wrote:
>>> Hi,
>>>
>>> Function optimize_function_for_size_p returns OPTIMIZE_SIZE_NO
>>> if func->decl is not null but no cgraph node is available for it.
>>> As PR105818 shows, this could give unexpected result.  For the
>>> case in PR105818, when parsing bar decl in function foo, the cfun
>>> is a function structure for foo, for which there is none cgraph
>>> node, so it returns OPTIMIZE_SIZE_NO.  But it's incorrect since
>>> the context is to optimize for size, the flag optimize_size is
>>> true.
>>>
>>> The patch is to make optimize_function_for_size_p to check
>>> optimize_size as what it does when func->decl is unavailable.
>>>
>>> One regression failure got exposed on aarch64-linux-gnu:
>>>
>>> PASS->FAIL: gcc.dg/guality/pr54693-2.c   -Os \
>>> 	    -DPREVENT_OPTIMIZATION  line 21 x == 10 - i
>>>
>>> The difference comes from the macro LOGICAL_OP_NON_SHORT_CIRCUIT
>>> used in function fold_range_test during c parsing, it uses
>>> optimize_function_for_speed_p which is equal to the invertion
>>> of optimize_function_for_size_p.  At that time cfun->decl is valid
>>> but no cgraph node for it, w/o this patch function
>>> optimize_function_for_speed_p returns true eventually, while it
>>> returns false with this patch.  Since the command line option -Os
>>> is specified, there is no reason to interpret it as "for speed".
>>> I think this failure is expected and adjust the test case
>>> accordingly.
>>>
>>> Is it ok for trunk?
>>>
>>> BR,
>>> Kewen
>>> -----
>>>
>>> 	PR target/105818
>>>
>>> gcc/ChangeLog:
>>>
>>> 	* predict.cc (optimize_function_for_size_p): Check optimize_size when
>>> 	func->decl is valid but its cgraph node is unavailable.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* gcc.target/powerpc/pr105818.c: New test.
>>> 	* gcc.dg/guality/pr54693-2.c: Adjust for aarch64.
>>> ---
>>>  gcc/predict.cc                              | 2 +-
>>>  gcc/testsuite/gcc.dg/guality/pr54693-2.c    | 2 +-
>>>  gcc/testsuite/gcc.target/powerpc/pr105818.c | 9 +++++++++
>>>  3 files changed, 11 insertions(+), 2 deletions(-)
>>>  create mode 100644 gcc/testsuite/gcc.target/powerpc/pr105818.c
>>>
>>> diff --git a/gcc/predict.cc b/gcc/predict.cc
>>> index 5734e4c8516..6c60a973236 100644
>>> --- a/gcc/predict.cc
>>> +++ b/gcc/predict.cc
>>> @@ -268,7 +268,7 @@ optimize_function_for_size_p (struct function *fun)
>>>    cgraph_node *n = cgraph_node::get (fun->decl);
>>>    if (n)
>>>      return n->optimize_for_size_p ();
>>> -  return OPTIMIZE_SIZE_NO;
>>> +  return optimize_size ? OPTIMIZE_SIZE_MAX : OPTIMIZE_SIZE_NO;
>>
>> We could also do (opt_for_fn (cfun->decl, optimize_size) that is
>> probably better since one can change optimize_size with optimization
>> attribute.
> 
> Good point, agree!
> 
>> However I think in most cases we check for optimize_size early I think
>> we are doing something wrong, since at that time htere is no profile
>> available.  Why exactly PR105818 hits the flag change issue?
> 
> For PR105818, the reason why the flag changs is that:
> 
> Firstly, the inconsistent flag is OPTION_MASK_SAVE_TOC_INDIRECT bit
> of rs6000_isa_flags_explicit, it's set as below:
> 
> /* 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;
> 
> Initially, rs6000 initialize target_option_default_node with
> OPTION_MASK_SAVE_TOC_INDIRECT unset, at that time cfun is NULL
> and optimize_size is true.
> 
> Later, when c parser handling function foo, it builds target
> option node as target_option_default_node in function
> handle_optimize_attribute, it does global option saving and
> verifying there as well, at that time the cfun is NULL, no
> issue is found.  And function store_parm_decls allocates
> struct_function for foo then, cfun becomes function struct
> for foo, when c parser continues to handle the decl bar in
> foo, function handle_optimize_attribute works as before,
> tries to restore the target options at the end, it calls
> targetm.target_option.restore (rs6000_function_specific_restore)
> which calls function rs6000_option_override_internal again,
> at this time the cfun is not NULL while there is no cgraph
> node for its decl, optimize_function_for_speed_p returns true
> and gets the OPTION_MASK_SAVE_TOC_INDIRECT bit of flag
> rs6000_isa_flags set unexpectedly.  It becomes inconsistent
> as the one saved previously.
> 
> IMHO, both contexts of global and function decl foo here hold
> optimize_size, function optimize_function_for_speed_p should
> not return true anyway.
> 
> btw, the aarch64 failed case also gets the unexpected
> result for optimize_function_for_speed_p during c parsing
> (fold_range_test <- ... <- c_parser_condition).
> 
> IIUC, in parsing time we don't have the profile information
> available.
> 

Hi Honza,

Does the above explanation sound reasonable to you?

BR,
Kewen