>-----Original Message-----
>From: Richard Guenther [mailto:richard.guent...@gmail.com]
>Sent: 2012年6月28日 17:24
>To: Zhenqiang Chen
>Cc: gcc-patches@gcc.gnu.org
>Subject: Re: [PATCH] Disable loop2_invariant for -Os
>
>On Thu, Jun 28, 2012 at 10:33 AM, Zhenqiang Chen <zhenqiang.c...@arm.com>
>wrote:
>>>> diff --git a/gcc/loop-init.c b/gcc/loop-init.c index
>>>> 03f8f61..5d8cf73
>>>> 100644
>>>> --- a/gcc/loop-init.c
>>>> +++ b/gcc/loop-init.c
>>>> @@ -273,6 +273,12 @@ struct rtl_opt_pass pass_rtl_loop_done =
>>>> static bool
>>>> gate_rtl_move_loop_invariants (void)
>>>> {
>>>> + /* In general, invariant motion can not reduce code size. But it
>>>> + will
>>>> + change the liverange of the invariant, which increases the
>>>> + register
>>>> + pressure and might lead to more spilling. */
>>>> + if (optimize_function_for_size_p (cfun))
>>>> + return false;
>>>> +
>>>
>>>Can you do this per loop instead? Using optimize_loop_nest_for_size_p?
>>
>> Update it according to the comments.
>>
>> Thanks!
>> -Zhenqiang
>>
>> diff --git a/gcc/loop-invariant.c b/gcc/loop-invariant.c index
>> f8405dd..b0e84a7 100644
>> --- a/gcc/loop-invariant.c
>> +++ b/gcc/loop-invariant.c
>> @@ -1931,7 +1931,8 @@ move_loop_invariants (void)
>> curr_loop = loop;
>> /* move_single_loop_invariants for very large loops
>> is time consuming and might need a lot of memory. */
>> - if (loop->num_nodes <= (unsigned)
>> LOOP_INVARIANT_MAX_BBS_IN_LOOP)
>> + if (loop->num_nodes <= (unsigned)
>> + LOOP_INVARIANT_MAX_BBS_IN_LOOP
>> + && ! optimize_loop_nest_for_size_p (loop))
>> move_single_loop_invariants (loop);
>
>Wait - move_single_loop_invariants itself already uses
>optimize_loop_for_speed_p.
>And looking down it seems to have support for tracking spill cost (eventually
>only
>with -fira-loop-pressure) - please work out why this support is not working for
>you.
1) If -fira_loop_pressure is enabled, it reduces ~24% invariant motions in my
tests. But it does not help on total code size. Seams there is issue to update
the "regs_needed" after moving an invariant out of the loop (My benchmark logs
show ~73% cases have more than one invariants moved).
During tracing, I found that move an integer constant out of the loop does not
increase regs_needed. Function "get_pressure_class_and_nregs (rtx insn, int
*nregs)" computes the "regs_needed".
*nregs
= ira_reg_class_max_nregs[pressure_class][GET_MODE (SET_SRC (set))];
In ARM, the insn to set an integer is like
(set (reg:SI 183)
(const_int 32 [0x20])) inv1.c:64 182 {*thumb1_movsi_insn}
(nil))
GET_MODE (SET_SRC (set)) is VOIDMode and
ira_reg_class_max_nregs[pressure_class][VOIDMode] is 0. In one of my test
cases, it moves 4 integer constants out of the loop, which leads to spilling.
According to the algorithm in "calculate_loop_reg_pressure", moving an
invariant out of the loop should impact on the register pressure. So I try to
add the following code
if (! (*nregs))
*nregs = ira_reg_class_max_nregs[pressure_class][GET_MODE (reg)];
Logs show it reduces another 32% invariant motions. But the code size is still
far from disabling the pass. Logs show -fira_loop_pressure impact other passes
in addition to loop2_invariant (The result of "-fira_loop_pressure
-fno-move-loop-invariants" is different from the result of
"-fno-move-loop-invariants").
2) By default -fira_loop_pressure is not enabled for -Os, the logic to compute
"regs_used" seams not sound. The following codes is from function
"find_invariants_to_move"
{
unsigned int n_regs = DF_REG_SIZE (df);
regs_used = 2;
for (i = 0; i < n_regs; i++)
{
if (!DF_REGNO_FIRST_DEF (i) && DF_REGNO_LAST_USE (i))
{
/* This is a value that is used but not changed inside loop. */
regs_used++;
}
}
}
* There is no loop related inform in the code.
* Benchmark logs show the condition (!DF_REGNO_FIRST_DEF (i) &&
DF_REGNO_LAST_USE (i)) is never true.
Thanks!
-Zhenqiang
