[PATCH] Disable loop2_invariant for -Os

[PATCH] Disable loop2_invariant for -Os

[Zhenqiang Chen]

Hi,

In general, invariant motion itself can not reduce code size. But it will
change the liverange of the invariant, which might lead to more spilling.
The patch disables loop2_invariant when optimizing for size.

I measured the code size benefit for four targets based on CSiBE benchmark:

ARM: 0.33%
MIPS: 1.15%
PPC: 0.24%
X86: 0.45%

Is it OK for trunk?

Thanks!
-Zhenqiang

ChangeLog:
2012-06-27  Zhenqiang Chen <zhenqiang.chen@arm.com>

	* loop-init.c (gate_rtl_move_loop_invariants): Disable
loop2_invariant
	when optimizing function for size.

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;
+
   return flag_move_loop_invariants;
 }

RE: [PATCH] Disable loop2_invariant for -Os

[Zhenqiang Chen]

>>
>> 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.
>
>Still there is code that tries to deal with -Os.  Simply disabling the pass makes
>that logic pointless.

If -fira-loop-pressure is not enabled, function estimate_reg_pressure_cost (cfgloopanal.c) is used to estimate the cost. At the beginning of the function, it checks

/* If we have enough registers, we should use them and not restrict
     the transformations unnecessarily.  */
  if (regs_needed + target_res_regs <= available_regs)
    return 0;

Here are the CSiBE benchmark logs before "if (...)" for ARM/MIPS/PPC/X86.

     available_regs target_res_regs regs_needed
ARM : 9                 3              2
MIPS: 10/26             3              2
PPC : 18/29             3              2
X86 : 6/15              3              2

regs_needed++ after invariant motion. The size_cost of the first several invariant (available_regs - target_res_regs(3) - regs_needed(2)) motions are always 0. So I prefer to disable the pass if -fira-loop-pressure is not enabled.

>Thus, please try to fix the code that is there to deal with -Os (a target may opt to
>enable -fira-loop-pressure by default for -Os).

Yes. Targets need tune to enable -fira-loop-pressure.

For -fira-loop-pressure, CSiBE logs show MIPS and PPC have a little improvement and X86 has a little regression compared with -fira-loop-pressure is not enabled.
If fira-loop-pressure is enabled, the cost check bases on

  if ((int) new_regs[pressure_class]
      + (int) regs_needed[pressure_class]
      + LOOP_DATA (curr_loop)->max_reg_pressure[pressure_class]
      + IRA_LOOP_RESERVED_REGS
      > ira_available_class_regs[pressure_class])

But a reg is available does not mean it can be used in any instruction. e.g. For ARM Cortex-M0, only few instructions can use r8-r15. (r8-r11, r13-r15 are already excluded in the available_regs). Logs show the result is much better if r12 is also excluded.

Thanks!
-Zhenqiang

Re: [PATCH] Disable loop2_invariant for -Os

[Richard Guenther]

On Tue, Jul 3, 2012 at 10:29 AM, Zhenqiang Chen <zhenqiang.chen@arm.com> wrote:
>>-----Original Message-----
>>From: Richard Guenther [mailto:richard.guenther@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.chen@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.

Still there is code that tries to deal with -Os.  Simply disabling the pass
makes that logic pointless.

Thus, please try to fix the code that is there to deal with -Os (a target may
opt to enable -fira-loop-pressure by default for -Os).

Thanks,
Richard.

> Thanks!
> -Zhenqiang
>
>
>

RE: [PATCH] Disable loop2_invariant for -Os

[Zhenqiang Chen]

>-----Original Message-----
>From: Richard Guenther [mailto:richard.guenther@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.chen@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

Re: [PATCH] Disable loop2_invariant for -Os

[Richard Guenther]

On Thu, Jun 28, 2012 at 10:33 AM, Zhenqiang Chen <zhenqiang.chen@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.

Richard.

>     }
>
> ChangeLog:
> 2012-06-28  Zhenqiang Chen <zhenqiang.chen@arm.com>
>
>        * loop-invariant.c (move_loop_invariants): Skip
>        move_single_loop_invariants when optimizing loop for size
>
>
>
>

RE: [PATCH] Disable loop2_invariant for -Os

[Zhenqiang Chen]

>-----Original Message-----
>From: Steven Bosscher [mailto:stevenb.gcc@gmail.com]
>Sent: 2012年6月27日 16:54
>To: Zhenqiang Chen
>Cc: gcc-patches@gcc.gnu.org
>Subject: Re: [PATCH] Disable loop2_invariant for -Os
>
>On Wed, Jun 27, 2012 at 10:40 AM, Zhenqiang Chen
><zhenqiang.chen@arm.com> wrote:
>> Hi,
>>
>> In general, invariant motion itself can not reduce code size. But it
>> will change the liverange of the invariant, which might lead to more
spilling.
>
>This may be true for ARM but it's not true in general. Sometimes
loop-invariant

Benchmark tests show it also benefits MIPS, PPC and X86 for code size.

>address arithmetic, that is not exposed in GIMPLE, is profitable to hoist
out of
>the loop. See e.g. PR41026 (for which I still have a patch in the queue).

>If this goes in anyway, please mention PR39837 in your ChangeLog entry.

It can not handle the case.

Thanks!
-Zhenqiang

RE: [PATCH] Disable loop2_invariant for -Os

[Zhenqiang Chen]

>> 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);
     }

ChangeLog:
2012-06-28  Zhenqiang Chen <zhenqiang.chen@arm.com>

	* loop-invariant.c (move_loop_invariants): Skip
	move_single_loop_invariants when optimizing loop for size

Re: [PATCH] Disable loop2_invariant for -Os

[Steven Bosscher]

On Wed, Jun 27, 2012 at 10:40 AM, Zhenqiang Chen <zhenqiang.chen@arm.com> wrote:
> Hi,
>
> In general, invariant motion itself can not reduce code size. But it will
> change the liverange of the invariant, which might lead to more spilling.

This may be true for ARM but it's not true in general. Sometimes
loop-invariant address arithmetic, that is not exposed in GIMPLE, is
profitable to hoist out of the loop. See e.g. PR41026 (for which I
still have a patch in the queue).

If this goes in anyway, please mention PR39837 in your ChangeLog entry.

Ciao!
Steven

Re: [PATCH] Disable loop2_invariant for -Os

[Richard Guenther]

On Wed, Jun 27, 2012 at 10:40 AM, Zhenqiang Chen <zhenqiang.chen@arm.com> wrote:
> Hi,
>
> In general, invariant motion itself can not reduce code size.

It can expose CSE opportunities across loops though.

> But it will
> change the liverange of the invariant, which might lead to more spilling.

"might" - indeed.  I wonder what the trade-off is here ... but given that you
leave tree loop invariant motion enabled it might not make much of a difference.

Still as this is mostly a spilling issue it looks odd to do that generally.  In
fact you could improve things by only disabling motion when that increases
register lifetime - it can after all reduce overall register lifetime:

for (;;)
  inv = inv1 + inv2;
  ... use inv;

to

inv = inv1 + inv2;
for (;;)
  ... use inv;

has register lifetime reduced.

Or at least like I suggest below.

> The patch disables loop2_invariant when optimizing for size.
>
> I measured the code size benefit for four targets based on CSiBE benchmark:
>
> ARM: 0.33%
> MIPS: 1.15%
> PPC: 0.24%
> X86: 0.45%
>
> Is it OK for trunk?
>
> Thanks!
> -Zhenqiang
>
> ChangeLog:
> 2012-06-27  Zhenqiang Chen <zhenqiang.chen@arm.com>
>
>        * loop-init.c (gate_rtl_move_loop_invariants): Disable
> loop2_invariant
>        when optimizing function for size.
>
> 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?

Thanks,
Richard.

>   return flag_move_loop_invariants;
>  }
>
>
>