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

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

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.

BR,
Kewen