Re: The performance data for two different implementation of new security feature -ftrivial-auto-var-init

[Qing Zhao <qing.zhao@oracle.com>]

Hi, Richard,

I have adjusted SRA phase to split calls to DEFERRED_INIT per you suggestion.

And now the routine “bump_map” in 511.povray is like following:
...

 # DEBUG BEGIN_STMT
  xcoor = 0.0;
  ycoor = 0.0;
  # DEBUG BEGIN_STMT
  index = .DEFERRED_INIT (index, 2);
  index2 = .DEFERRED_INIT (index2, 2);
  index3 = .DEFERRED_INIT (index3, 2);
  # DEBUG BEGIN_STMT
  colour1 = .DEFERRED_INIT (colour1, 2);
  colour2 = .DEFERRED_INIT (colour2, 2);
  colour3 = .DEFERRED_INIT (colour3, 2);
  # DEBUG BEGIN_STMT
  p1$0_181 = .DEFERRED_INIT (p1$0_195(D), 2);
  # DEBUG p1$0 => p1$0_181
  p1$1_184 = .DEFERRED_INIT (p1$1_182(D), 2);
  # DEBUG p1$1 => p1$1_184
  p1$2_172 = .DEFERRED_INIT (p1$2_185(D), 2);
  # DEBUG p1$2 => p1$2_172
  p2$0_177 = .DEFERRED_INIT (p2$0_173(D), 2);
  # DEBUG p2$0 => p2$0_177
  p2$1_135 = .DEFERRED_INIT (p2$1_178(D), 2);
  # DEBUG p2$1 => p2$1_135
  p2$2_137 = .DEFERRED_INIT (p2$2_136(D), 2);
  # DEBUG p2$2 => p2$2_137
  p3$0_377 = .DEFERRED_INIT (p3$0_376(D), 2);
  # DEBUG p3$0 => p3$0_377
  p3$1_379 = .DEFERRED_INIT (p3$1_378(D), 2);
  # DEBUG p3$1 => p3$1_379
  p3$2_381 = .DEFERRED_INIT (p3$2_380(D), 2);
  # DEBUG p3$2 => p3$2_381


In the above, p1, p2, and p3 are all splitted to calls to DEFERRED_INIT of the components of p1, p2 and p3. 

With this change, the stack usage numbers with -fstack-usage for approach A, old approach D and new D with the splitting in SRA are:

  Approach A	Approach D-old	Approach D-new

	272			624			368

From the above, we can see that splitting the call to DEFERRED_INIT in SRA can reduce the stack usage increase dramatically. 

However, looks like that the stack size for D is still bigger than A. 

I checked the IR again, and found that the alias analysis might be responsible for this (by compare the image.cpp.026t.ealias for both A and D):

(Due to the call to:

  colour1 = .DEFERRED_INIT (colour1, 2);
)

******Approach A:

Points_to analysis:

Constraints:
…
colour1 = &NULL
…
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
...
callarg(53) = &colour1
...
_53 = colour1

Points_to sets:
…
colour1 = { NULL ESCAPED NONLOCAL } same as _53
...
CALLUSED(48) = { NULL ESCAPED NONLOCAL index colour1 }
CALLCLOBBERED(49) = { NULL ESCAPED NONLOCAL index colour1 } same as CALLUSED(48)
...
callarg(53) = { NULL ESCAPED NONLOCAL colour1 }

******Apprach D:

Points_to analysis:

Constraints:
…
callarg(19) = colour1
callarg(19) = &NONLOCAL
colour1 = callarg(19) + UNKNOWN
colour1 = &NONLOCAL
…
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
colour1 = &NONLOCAL
…
callarg(74) = &colour1
callarg(74) = callarg(74) + UNKNOWN
callarg(74) = *callarg(74) + UNKNOWN
…
_53 = colour1
_54 = _53
_55 = _54 + UNKNOWN
_55 = &NONLOCAL
_56 = colour1
_57 = _56
_58 = _57 + UNKNOWN
_58 = &NONLOCAL
_59 = _55 + UNKNOWN
_59 = _58 + UNKNOWN
_60 = colour1
_61 = _60
_62 = _61 + UNKNOWN
_62 = &NONLOCAL
_63 = _59 + UNKNOWN
_63 = _62 + UNKNOWN
_64 = _63 + UNKNOWN
..
Points_to set:
…
colour1 = { ESCAPED NONLOCAL } same as callarg(19)
…
CALLUSED(69) = { ESCAPED NONLOCAL index colour1 }
CALLCLOBBERED(70) = { ESCAPED NONLOCAL index colour1 } same as CALLUSED(69)
callarg(71) = { ESCAPED NONLOCAL }
callarg(72) = { ESCAPED NONLOCAL }
callarg(73) = { ESCAPED NONLOCAL }
callarg(74) = { ESCAPED NONLOCAL colour1 }

My question:

Is it possible to adjust alias analysis to resolve this issue?

thanks.

Qing

> On Jan 18, 2021, at 10:12 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
>>>>> I checked the routine “poverties::bump_map” in 511.povray_r since it
>>>>> has a lot stack increase 
>>>>> due to implementation D, by examine the IR immediate before RTL
>>>>> expansion phase.  
>>>>> (image.cpp.244t.optimized), I found that we have the following
>>>>> additional statements for the array elements:
>>>>> 
>>>>> void  pov::bump_map (double * EPoint, struct TNORMAL * Tnormal, double
>>>>> * normal)
>>>>> {
>>>>> …
>>>>> double p3[3];
>>>>> double p2[3];
>>>>> double p1[3];
>>>>> float colour3[5];
>>>>> float colour2[5];
>>>>> float colour1[5];
>>>>> …
>>>>> # DEBUG BEGIN_STMT
>>>>> colour1 = .DEFERRED_INIT (colour1, 2);
>>>>> colour2 = .DEFERRED_INIT (colour2, 2);
>>>>> colour3 = .DEFERRED_INIT (colour3, 2);
>>>>> # DEBUG BEGIN_STMT
>>>>> MEM <double> [(double[3] *)&p1] = p1$0_144(D);
>>>>> MEM <double> [(double[3] *)&p1 + 8B] = p1$1_135(D);
>>>>> MEM <double> [(double[3] *)&p1 + 16B] = p1$2_138(D);
>>>>> p1 = .DEFERRED_INIT (p1, 2);
>>>>> # DEBUG D#12 => MEM <double> [(double[3] *)&p1]
>>>>> # DEBUG p1$0 => D#12
>>>>> # DEBUG D#11 => MEM <double> [(double[3] *)&p1 + 8B]
>>>>> # DEBUG p1$1 => D#11
>>>>> # DEBUG D#10 => MEM <double> [(double[3] *)&p1 + 16B]
>>>>> # DEBUG p1$2 => D#10
>>>>> MEM <double> [(double[3] *)&p2] = p2$0_109(D);
>>>>> MEM <double> [(double[3] *)&p2 + 8B] = p2$1_111(D);
>>>>> MEM <double> [(double[3] *)&p2 + 16B] = p2$2_254(D);
>>>>> p2 = .DEFERRED_INIT (p2, 2);
>>>>> # DEBUG D#9 => MEM <double> [(double[3] *)&p2]
>>>>> # DEBUG p2$0 => D#9
>>>>> # DEBUG D#8 => MEM <double> [(double[3] *)&p2 + 8B]
>>>>> # DEBUG p2$1 => D#8
>>>>> # DEBUG D#7 => MEM <double> [(double[3] *)&p2 + 16B]
>>>>> # DEBUG p2$2 => D#7
>>>>> MEM <double> [(double[3] *)&p3] = p3$0_256(D);
>>>>> MEM <double> [(double[3] *)&p3 + 8B] = p3$1_258(D);
>>>>> MEM <double> [(double[3] *)&p3 + 16B] = p3$2_260(D);
>>>>> p3 = .DEFERRED_INIT (p3, 2);
>>>>> ….
>>>>> }
>>>>> 
>>>>> I guess that the above “MEM <double>….. = …” are the ones that make the
>>>>> differences. Which phase introduced them?
>>>> 
>>>> Looks like SRA. But you can just dump all and grep for the first occurrence. 
>>> 
>>> Yes, looks like that SRA is the one:
>>> 
>>> image.cpp.035t.esra:  MEM <double> [(double[3] *)&p1] = p1$0_195(D);
>>> image.cpp.035t.esra:  MEM <double> [(double[3] *)&p1 + 8B] = p1$1_182(D);
>>> image.cpp.035t.esra:  MEM <double> [(double[3] *)&p1 + 16B] = p1$2_185(D);
>> 
>> I realise no-one was suggesting otherwise, but FWIW: SRA could easily
>> be extended to handle .DEFERRED_INIT if that's the main source of
>> excess stack usage.  A single .DEFERRED_INIT of an aggregate can
>> be split into .DEFERRED_INITs of individual components.
> 
> Thanks a lot for the suggestion,
> I will study the code of SRA to see how to do this and then see whether this can resolve the issue.