code review 6817085: cmd/5g: enable xtramodes optimisation

cmd/5g: enable xtramodes optimisation

xtramodes' C_PBIT optimisation transforms:

MOVW          0(R3),R1
ADD           $4,R3,R3

into:

MOVW.P        4(R3),R1

and the AADD optimisation tranforms:

ADD          R0,R1
MOVBU        0(R1),R0

into:

MOVBU        R0<<0(R1),R0

5g does not appear to generate sequences that
can be transformed by xtramodes' AMOVW.

[dfc, 2012-11-05 12:09:05]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[remyoudompheng, 2012-11-05 16:07:04]

igen is supposed to be there to avoid generating sequences that can be optimized
like this. Did you find places where any of these optimizations is triggered?

[dfc, 2012-11-05 16:11:55]

Heaps

The pbit optimisation kicks in for the inc at the bottom of every range loop

AADD is everywhere.

I observe their behaviour by adding a warn above the excise the inspecting
the objdump for the progs referenced.

I have some micro benchmarks which demonstrate it, but they are waiting on
another review.

Dave
On 6 Nov 2012 03:07, <remyoudompheng@gmail.com> wrote:

> igen is supposed to be there to avoid generating sequences that can be
> optimized like this. Did you find places where any of these
> optimizations is triggered?
>
>
http://codereview.appspot.com/**6817085/<http://codereview.appspot.com/6817085/>
>

[remyoudompheng, 2012-11-05 21:02:34]

On 2012/11/5 Dave Cheney <dave@cheney.net> wrote:
> Heaps
>
> The pbit optimisation kicks in for the inc at the bottom of every range loop

Indeed that's a good use case.

> AADD is everywhere.

> I observe their behaviour by adding a warn above the excise the inspecting
> the objdump for the progs referenced.

Usually I do the following:
  go build -gcflags -S -a cmd/go | sed 's,.*/src/,,'
with the appropriate GOARCH and do a diff before and after.

Rémy.

[dfc, 2012-11-05 21:42:11]

Yes, I also use -S once I figure out the prog that is being transformed. I will
post some samples of the AADD transform. 

On 06/11/2012, at 8:02, Rémy Oudompheng <remyoudompheng@gmail.com> wrote:

> On 2012/11/5 Dave Cheney <dave@cheney.net> wrote:
>> Heaps
>> 
>> The pbit optimisation kicks in for the inc at the bottom of every range loop
> 
> Indeed that's a good use case.
> 
>> AADD is everywhere.
> 
>> I observe their behaviour by adding a warn above the excise the inspecting
>> the objdump for the progs referenced.
> 
> Usually I do the following:
>  go build -gcflags -S -a cmd/go | sed 's,.*/src/,,'
> with the appropriate GOARCH and do a diff before and after.
> 
> Rémy.

[remyoudompheng, 2012-11-05 22:13:30]

I think it's an opportunity to make the intention more explicit.

My comments suggestions may not be accurate. Feel free to make them correct.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c
File src/cmd/5g/peep.c (right):

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode265
src/cmd/5g/peep.c:265: Reg*
// uniqp returns a "unique" predecessor to instruction r.
// If the instruction is the first one or has multiple
// predecessors due to jump, R is returned.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode738
src/cmd/5g/peep.c:738: findpre(Reg *r, Adr *v)
// findpre returns the last instruction mentioning v
// before r. It must be a set, and there must be
// a unique path from that instruction to r.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode758
src/cmd/5g/peep.c:758: findinc(Reg *r, Reg *r2, Adr *v)
findinc doesn't find an increment as the name suggests. At least not only
increments by 1.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode847
src/cmd/5g/peep.c:847: int
please adda comment explaining what this function is doing.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode855
src/cmd/5g/peep.c:855: if(debug['h'] && p->as == AMOVB && p->from.type ==
D_OREG)	/* byte load */
I don't understand what the h flag is.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode864
src/cmd/5g/peep.c:864: case AADD:
p1 might have a scond and I don't see a test for this. You don't want to elide a
ADD.S if the carry bit is needed by a subsequent instruction.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode867
src/cmd/5g/peep.c:867: (p->as != AMOVB || (a == &p->from &&
(p1->from.offset&~0xf) == 0))) ||
these if's are absolutely atrocious.

[rsc, 2012-11-06 19:03:30]

i'm happy once remy is

https://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c
File src/cmd/5g/peep.c (right):

https://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode855
src/cmd/5g/peep.c:855: if(debug['h'] && p->as == AMOVB && p->from.type ==
D_OREG)	/* byte load */
On 2012/11/05 22:13:30, remyoudompheng wrote:
> I don't understand what the h flag is.

This is mostly picked up from 5c, and it isn't new code in this CL, so I
wouldn't rearrange it too much. I don't see any meaning assigned to -h in either
case, though, so I'd be inclined to just delete this line.

Early versions of ARM did not have byte load instructions, so it is possible
that the -h flag controlled whether those got used, in the original 5c. But it
is gone even from there.

https://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode864
src/cmd/5g/peep.c:864: case AADD:
On 2012/11/05 22:13:30, remyoudompheng wrote:
> p1 might have a scond and I don't see a test for this. You don't want to elide
a
> ADD.S if the carry bit is needed by a subsequent instruction.

Does the compiler generate ADD.S? 5c never did, so this code does not expect it.
It is not trying to be an optimizer for general ARM code, just an optimizer for
what the compiler generates.

https://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode867
src/cmd/5g/peep.c:867: (p->as != AMOVB || (a == &p->from &&
(p1->from.offset&~0xf) == 0))) ||
On 2012/11/05 22:13:30, remyoudompheng wrote:
> these if's are absolutely atrocious.

Agreed but they match 5c.

[remyoudompheng, 2012-11-07 07:00:44]

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c
File src/cmd/5g/peep.c (right):

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode864
src/cmd/5g/peep.c:864: case AADD:
5g generates ADD.S/ADC for addition of int64's. Although it looks unlikely that
they becomes eligible for this, it might provoke subtle bugs.

[dfc, 2012-11-07 07:23:07]

On 07/11/2012, at 18:00, remyoudompheng@gmail.com wrote:

> 
> http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c
> File src/cmd/5g/peep.c (right):
> 
> http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode864
> src/cmd/5g/peep.c:864: case AADD:
> 5g generates ADD.S/ADC for addition of int64's. Although it looks
> unlikely that they becomes eligible for this, it might provoke subtle
> bugs.

Right. I will address this as I want to revisit kabi's notes in cgen64.c and I
don't want to stumble into this. 


> 
> http://codereview.appspot.com/6817085/

[dfc, 2012-11-07 09:40:31]

Thank you for your comments, I have tried to address as many of them as
possible. Please take another look.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c
File src/cmd/5g/peep.c (right):

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode265
src/cmd/5g/peep.c:265: Reg*
On 2012/11/05 22:13:30, remyoudompheng wrote:
> // uniqp returns a "unique" predecessor to instruction r.
> // If the instruction is the first one or has multiple
> // predecessors due to jump, R is returned.

Done. Thank you for your suggestion. I have modified it to respect the existing
comment style.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode738
src/cmd/5g/peep.c:738: findpre(Reg *r, Adr *v)
On 2012/11/05 22:13:30, remyoudompheng wrote:
> // findpre returns the last instruction mentioning v
> // before r. It must be a set, and there must be
> // a unique path from that instruction to r.

Done.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode758
src/cmd/5g/peep.c:758: findinc(Reg *r, Reg *r2, Adr *v)
On 2012/11/05 22:13:30, remyoudompheng wrote:
> findinc doesn't find an increment as the name suggests. At least not only
> increments by 1.

I've tried to document what I think findinc does, I may not be correct.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode847
src/cmd/5g/peep.c:847: int
On 2012/11/05 22:13:30, remyoudompheng wrote:
> please adda comment explaining what this function is doing.

Done.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode855
src/cmd/5g/peep.c:855: if(debug['h'] && p->as == AMOVB && p->from.type ==
D_OREG)	/* byte load */
Deleted.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode864
src/cmd/5g/peep.c:864: case AADD:
On 2012/11/07 07:00:44, remyoudompheng wrote:
> 5g generates ADD.S/ADC for addition of int64's. Although it looks unlikely
that
> they becomes eligible for this, it might provoke subtle bugs.

I've added a guard to check for this. FWIW, there are 0 occurrences of this in
the stdlib.

http://codereview.appspot.com/6817085/diff/6001/src/cmd/5g/peep.c#newcode867
src/cmd/5g/peep.c:867: (p->as != AMOVB || (a == &p->from &&
(p1->from.offset&~0xf) == 0))) ||
On 2012/11/06 19:03:30, rsc wrote:
> On 2012/11/05 22:13:30, remyoudompheng wrote:
> > these if's are absolutely atrocious.
> 
> Agreed but they match 5c.

I'd prefer to leave them untouched.

[dfc, 2012-11-07 09:40:47]

Hello remyoudompheng@gmail.com, rsc@golang.org (cc:
golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go

[remyoudompheng, 2012-11-10 18:56:08]

LGTM

[dfc, 2012-11-10 20:51:42]

*** Submitted as http://code.google.com/p/go/source/detail?r=422c00e8e022 ***

cmd/5g: enable xtramodes optimisation

xtramodes' C_PBIT optimisation transforms:

MOVW          0(R3),R1
ADD           $4,R3,R3

into:

MOVW.P        4(R3),R1

and the AADD optimisation tranforms:

ADD          R0,R1
MOVBU        0(R1),R0

into:

MOVBU        R0<<0(R1),R0

5g does not appear to generate sequences that
can be transformed by xtramodes' AMOVW.

R=remyoudompheng, rsc
CC=golang-dev
http://codereview.appspot.com/6817085