Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

After fixing the assembler code and other issues, e.g., interruption
handlers were not restoring all registers, I observed an speed up of ~12%
with -O2 when running a simple webserver example. That's very impresive!

Thanks for you help, Matias.

2018-01-10 18:19 GMT+01:00 Matias Vara :

>
> 2018-01-10 18:09 GMT+01:00 Karoly Balogh (Charlie/SGR) <
> char...@scenergy.dfmk.hu>:
>
>> Hi,
>>
>> On Wed, 10 Jan 2018, Matias Vara wrote:
>>
>> > BTW, this only applies to inline assembler functions rigth? In the case
>> > of normal procedures that contains a block asm end; there is no problem,
>> > Am I right?
>>
>> No, it applies to *ALL* assembler code. Also inline blocks. The only
>> difference is, for blocks you can hint the compiler which registers you
>> use, so the compiler can also tailor code which improves the interaction
>> with your assembler code:
>>
>> https://www.freepascal.org/docs-html/ref/refse87.html
>>
>> This is not true for pure assembler functions tho', for performance
>> reasons among others.
>>
>> Charlie
>>
>
> This is very interesting.
>
> Thanks a lot.
>
>
>
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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

2018-01-10 18:09 GMT+01:00 Karoly Balogh (Charlie/SGR) <
char...@scenergy.dfmk.hu>:

> Hi,
>
> On Wed, 10 Jan 2018, Matias Vara wrote:
>
> > BTW, this only applies to inline assembler functions rigth? In the case
> > of normal procedures that contains a block asm end; there is no problem,
> > Am I right?
>
> No, it applies to *ALL* assembler code. Also inline blocks. The only
> difference is, for blocks you can hint the compiler which registers you
> use, so the compiler can also tailor code which improves the interaction
> with your assembler code:
>
> https://www.freepascal.org/docs-html/ref/refse87.html
>
> This is not true for pure assembler functions tho', for performance
> reasons among others.
>
> Charlie
>

This is very interesting.

Thanks a lot.



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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

Hello,

2018-01-10 18:03 GMT+01:00 Karoly Balogh (Charlie/SGR) <
char...@scenergy.dfmk.hu>:

> Hi,
>
> On Wed, 10 Jan 2018, Matias Vara wrote:
>
> > Thank you very much Karol, I completly missed this point during the
> > development of my kernel (Or maybe I had in mind sometime ago but I
> > forgot it)It was only when I decided to play with -02 that all these
> > issues arrised.
>
> BTW, your code (as shown by the parameter passing in your assembler dumps)
> seems to use the Microsoft x64 calling convention, which declares RSI
> value as callee-saved, and then your code overwrites that without saving.
>
> So the fact that your code overwrites it, could be the culprit. (And then
> the OUTSD instruction also increases RSI by 4, so even if that MOV
> wouldn't be there, you'd still need to save it.)
>
> Also note that the 64bit x86-64 ABI is very different between Linux and
> Windows, for example.
>
>
Thanks, I am going to review all the assembler code.


> BTW, developing a kernel in Pascal is cool... ;)
>

It is indeed :)

Matias


>
> Charlie
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Re: [fpc-pascal] issue when enabling -O2

[Karoly Balogh (Charlie/SGR)]

Hi,

On Wed, 10 Jan 2018, Matias Vara wrote:

> BTW, this only applies to inline assembler functions rigth? In the case
> of normal procedures that contains a block asm end; there is no problem,
> Am I right?

No, it applies to *ALL* assembler code. Also inline blocks. The only
difference is, for blocks you can hint the compiler which registers you
use, so the compiler can also tailor code which improves the interaction
with your assembler code:

https://www.freepascal.org/docs-html/ref/refse87.html

This is not true for pure assembler functions tho', for performance
reasons among others.

Charlie
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Re: [fpc-pascal] issue when enabling -O2

[Karoly Balogh (Charlie/SGR)]

Hi,

On Wed, 10 Jan 2018, Matias Vara wrote:

> Thank you very much Karol, I completly missed this point during the
> development of my kernel (Or maybe I had in mind sometime ago but I
> forgot it)It was only when I decided to play with -02 that all these
> issues arrised. 

BTW, your code (as shown by the parameter passing in your assembler dumps)
seems to use the Microsoft x64 calling convention, which declares RSI
value as callee-saved, and then your code overwrites that without saving.

So the fact that your code overwrites it, could be the culprit. (And then
the OUTSD instruction also increases RSI by 4, so even if that MOV
wouldn't be there, you'd still need to save it.)

Also note that the 64bit x86-64 ABI is very different between Linux and
Windows, for example.

BTW, developing a kernel in Pascal is cool... ;)

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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

BTW, this only applies to inline assembler functions rigth? In the case of
normal procedures that contains a block asm end; there is no problem, Am I
right?

Matias

2018-01-10 17:51 GMT+01:00 Matias Vara :

> Thank you very much Karol, I completly missed this point during the
> development of my kernel (Or maybe I had in mind sometime ago but I forgot
> it)
> It was only when I decided to play with -02 that all these issues arrised.
>
> Matias
>
> 2018-01-10 17:34 GMT+01:00 Karoly Balogh (Charlie/SGR) <
> char...@scenergy.dfmk.hu>:
>
>> Hi,
>>
>> On Wed, 10 Jan 2018, Matias Vara wrote:
>>
>> > Hi Karol and thanks you very much! I got confused with the function
>> > names, I feel very sorry.As I said in my previous email, I fixed by
>> > rewriting the assembler function. However, I don't why it worked.
>>
>> By accident. Simply the register/stack/memory layout being different upon
>> entry, and it worked by pure luck.
>>
>> > So I understand you correctly, if I use assembler in my procedures there
>> > could be a risk that I trash a variable that the compiler is using.
>>
>> Yes. All operating systems and CPU architectures define a so called ABI or
>> calling convention, which functions must respect. This details which
>> registers are the parameters to be passed on, and which registers are free
>> to destroy in a function and which ones *MUST* be saved/preserved (usually
>> on the stack). There's no way around this. The compiler will expect that
>> your assembly subfunctions play by the rules.
>>
>> See here, for example, for x86:
>> https://en.wikipedia.org/wiki/X86_calling_conventions
>>
>> > Is  the compiler warning me about this?
>>
>> No. Assembly is quite a minefield in this regard. If you use assembler,
>> the compiler assumes you know what you're doing, and doesn't analyze the
>> assembler function. You have to respect the ABI of your CPU *AND* target
>> platform by hand, and save the nonvolatile registers.
>>
>> (This is BTW, not Free Pascal specific. Delphi does the same, or more or
>> less any other language which supports inline assembly.)
>>
>> Charlie
>>
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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

Thank you very much Karol, I completly missed this point during the
development of my kernel (Or maybe I had in mind sometime ago but I forgot
it)
It was only when I decided to play with -02 that all these issues arrised.

Matias

2018-01-10 17:34 GMT+01:00 Karoly Balogh (Charlie/SGR) <
char...@scenergy.dfmk.hu>:

> Hi,
>
> On Wed, 10 Jan 2018, Matias Vara wrote:
>
> > Hi Karol and thanks you very much! I got confused with the function
> > names, I feel very sorry.As I said in my previous email, I fixed by
> > rewriting the assembler function. However, I don't why it worked.
>
> By accident. Simply the register/stack/memory layout being different upon
> entry, and it worked by pure luck.
>
> > So I understand you correctly, if I use assembler in my procedures there
> > could be a risk that I trash a variable that the compiler is using.
>
> Yes. All operating systems and CPU architectures define a so called ABI or
> calling convention, which functions must respect. This details which
> registers are the parameters to be passed on, and which registers are free
> to destroy in a function and which ones *MUST* be saved/preserved (usually
> on the stack). There's no way around this. The compiler will expect that
> your assembly subfunctions play by the rules.
>
> See here, for example, for x86:
> https://en.wikipedia.org/wiki/X86_calling_conventions
>
> > Is  the compiler warning me about this?
>
> No. Assembly is quite a minefield in this regard. If you use assembler,
> the compiler assumes you know what you're doing, and doesn't analyze the
> assembler function. You have to respect the ABI of your CPU *AND* target
> platform by hand, and save the nonvolatile registers.
>
> (This is BTW, not Free Pascal specific. Delphi does the same, or more or
> less any other language which supports inline assembly.)
>
> Charlie
>
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Re: [fpc-pascal] issue when enabling -O2

[Karoly Balogh (Charlie/SGR)]

Hi,

On Wed, 10 Jan 2018, Matias Vara wrote:

> Hi Karol and thanks you very much! I got confused with the function
> names, I feel very sorry.As I said in my previous email, I fixed by
> rewriting the assembler function. However, I don't why it worked. 

By accident. Simply the register/stack/memory layout being different upon
entry, and it worked by pure luck.

> So I understand you correctly, if I use assembler in my procedures there
> could be a risk that I trash a variable that the compiler is using.

Yes. All operating systems and CPU architectures define a so called ABI or
calling convention, which functions must respect. This details which
registers are the parameters to be passed on, and which registers are free
to destroy in a function and which ones *MUST* be saved/preserved (usually
on the stack). There's no way around this. The compiler will expect that
your assembly subfunctions play by the rules.

See here, for example, for x86:
https://en.wikipedia.org/wiki/X86_calling_conventions

> Is  the compiler warning me about this? 

No. Assembly is quite a minefield in this regard. If you use assembler,
the compiler assumes you know what you're doing, and doesn't analyze the
assembler function. You have to respect the ABI of your CPU *AND* target
platform by hand, and save the nonvolatile registers.

(This is BTW, not Free Pascal specific. Delphi does the same, or more or
less any other language which supports inline assembly.)

Charlie
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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

Hi Karol and thanks you very much! I got confused with the function names,
I feel very sorry.
As I said in my previous email, I fixed by rewriting the assembler
function. However, I don't why it worked.
So I understand you correctly, if I use assembler in my procedures there
could be a risk that I trash a variable that the compiler is using. Is the
compiler warning me about this?

Matias

2018-01-10 17:14 GMT+01:00 Karoly Balogh (Charlie/SGR) <
char...@scenergy.dfmk.hu>:

> Hi,
>
> On Wed, 10 Jan 2018, Matias Vara wrote:
>
> > I am getting an exception when I enable the -O2 optimization. More
> > precisaily, the line that stars with write_portd is corrupting the
> > data section. This is the pascal code:
> >
> > function PciReadDword(const bus, device, func, regnum: UInt32): UInt32;
> > var
> >   Send: DWORD;
> > begin
> >   Send := $8000 or (bus shl 16) or (device shl 11) or (func shl 8)
> or (regnum shl 2);
> >   write_portd(@Send, PCI_CONF_PORT_INDEX);
> >   read_portd(@Send, PCI_CONF_PORT_DATA);
> >   Result := Send;
> > end;
> >
> > which generates (without -02):
> >
> > .section .text.n_arch_$$_pcireaddword$longword$longword$longword$
> longword$$longword,"x"
> > .balign 16,0x90
> > .globl ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$
> LONGWORD
> > ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD:
> >
> > (***shipp***)
> >
> > and with -O2:
> >
> > .section .text.n_arch_$$_pciwriteword$word$word$word$word$word,"x"
> > .balign 16,0x90
> > .globl ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD
> > ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD:
> >
> > The first thing that I realize was the the optimized version is not
> > generating the correct source when is exiting since it should return
> > "Send", but am I right? The assembler code of write_portd remains the
> > same, Am I missing something?
>
> The -O2 version of the function you sent is from a different one, it's
> from a Write function, not a Read... So no wonder it doesn't return
> anything... :)
>
> BTW, -O2 uses register variables, while -O- doesn't. If your assembler
> trashes one of them without preserving one, it can lead to crashes...
> But there could be many other reasons.
>
> Without seeing the actual assembler functions it's hard to tell.
>
> Charlie
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Re: [fpc-pascal] issue when enabling -O2

[Karoly Balogh (Charlie/SGR)]

Hi,

On Wed, 10 Jan 2018, Matias Vara wrote:

> I am getting an exception when I enable the -O2 optimization. More
> precisaily, the line that stars with write_portd is corrupting the
> data section. This is the pascal code: 
>
> function PciReadDword(const bus, device, func, regnum: UInt32): UInt32;
> var
>   Send: DWORD;
> begin
>   Send := $8000 or (bus shl 16) or (device shl 11) or (func shl 8) or 
> (regnum shl 2);
>   write_portd(@Send, PCI_CONF_PORT_INDEX);
>   read_portd(@Send, PCI_CONF_PORT_DATA);
>   Result := Send;
> end;  
>
> which generates (without -02):
>
> .section 
> .text.n_arch_$$_pcireaddword$longword$longword$longword$longword$$longword,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD
> ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD:
>
> (***shipp***)
>
> and with -O2:
>
> .section .text.n_arch_$$_pciwriteword$word$word$word$word$word,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD
> ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD:
>
> The first thing that I realize was the the optimized version is not
> generating the correct source when is exiting since it should return
> "Send", but am I right? The assembler code of write_portd remains the
> same, Am I missing something? 

The -O2 version of the function you sent is from a different one, it's
from a Write function, not a Read... So no wonder it doesn't return
anything... :)

BTW, -O2 uses register variables, while -O- doesn't. If your assembler
trashes one of them without preserving one, it can lead to crashes...
But there could be many other reasons.

Without seeing the actual assembler functions it's hard to tell.

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Re: [fpc-pascal] issue when enabling -O2

[Matias Vara]

I think the problem was the way write_portd() is implemented:

procedure write_portd(const Data: Pointer; const Port: Word); {$IFDEF
ASMINLINE} inline; {$ENDIF}
asm // RCX: data, RDX: port
  {$IFDEF LINUX} mov dx, port {$ENDIF}
mov rsi, data // DX=port
outsd
end;

If I replace with something that does not use the outsd instruction, it
works fine.

Matias

2018-01-10 15:55 GMT+01:00 Matias Vara :

> Hello everyone,
>
> I am getting an exception when I enable the -O2 optimization. More
> precisaily, the line that stars with write_portd is corrupting the data
> section. This is the pascal code:
>
> function PciReadDword(const bus, device, func, regnum: UInt32): UInt32;
> var
>   Send: DWORD;
> begin
>   Send := $8000 or (bus shl 16) or (device shl 11) or (func shl 8) or
> (regnum shl 2);
>   write_portd(@Send, PCI_CONF_PORT_INDEX);
>   read_portd(@Send, PCI_CONF_PORT_DATA);
>   Result := Send;
> end;
>
> *which generates (without -02):*
>
> .section .text.n_arch_$$_pcireaddword$longword$longword$longword$
> longword$$longword,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD
> ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD:
> .Lc207:
> .seh_proc ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$
> LONGWORD
> .Ll464:
> # [992] begin
> pushq %rbp
> .seh_pushreg %rbp
> .Lc209:
> .Lc210:
> movq %rsp,%rbp
> .Lc211:
> leaq -80(%rsp),%rsp
> .seh_stackalloc 80
> .seh_endprologue
> # Var bus located at rbp-8, size=OS_32
> # Var device located at rbp-16, size=OS_32
> # Var func located at rbp-24, size=OS_32
> # Var regnum located at rbp-32, size=OS_32
> # Var $result located at rbp-40, size=OS_32
> # Var Send located at rbp-48, size=OS_32
> movl %ecx,-8(%rbp)
> movl %edx,-16(%rbp)
> movl %r8d,-24(%rbp)
> movl %r9d,-32(%rbp)
> .Ll465:
> # [993] Send := $8000 or (bus shl 16) or (device shl 11) or (func shl
> 8) or (regnum shl 2);
> movl -8(%rbp),%eax
> shll $16,%eax
> orl $2147483648,%eax
> movl -16(%rbp),%edx
> shll $11,%edx
> orl %eax,%edx
> movl -24(%rbp),%eax
> shll $8,%eax
> orl %edx,%eax
> movl -32(%rbp),%edx
> shll $2,%edx
> orl %eax,%edx
> movl %edx,-48(%rbp)
> .Ll466:
> # [995] write_portd(@Send, PCI_CONF_PORT_INDEX);
> leaq -48(%rbp),%rcx
> movl $3320,%edx
> call ARCH_$$_WRITE_PORTD$POINTER$WORD
> .Ll467:
> # [996] read_portd(@Send, PCI_CONF_PORT_DATA);
> leaq -48(%rbp),%rcx
> movl $3324,%edx
> call ARCH_$$_READ_PORTD$POINTER$WORD
> .Ll468:
> # [997] Result := Send;
> movl -48(%rbp),%eax
> movl %eax,-40(%rbp)
> .Ll469:
> # [998] end;
> movl -40(%rbp),%eax
> nop
> leaq (%rbp),%rsp
> popq %rbp
> ret
> .seh_endproc
> .Lc208:
> .Lt28:
> .Ll470:
>
> *and with -O2:*
>
> .section .text.n_arch_$$_pciwriteword$word$word$word$word$word,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD
> ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD:
> .Lc148:
> # Temps allocated between rbp-16 and rbp-8
> .seh_proc ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD
> .Ll471:
> # [1014] begin
> pushq %rbp
> .seh_pushreg %rbp
> .Lc150:
> .Lc151:
> movq %rsp,%rbp
> .Lc152:
> leaq -48(%rsp),%rsp
> .seh_stackalloc 48
> # Var bus located in register ax
> # Var device located in register dx
> # Var func located in register r8w
> # Var regnum located in register r9w
> # Var value located in register cx
> movq %rbx,-16(%rbp)
> .seh_savereg %rbx, 32
> .seh_endprologue
> # Var Send located at rbp-8, size=OS_32
> movw %cx,%ax
> movw 48(%rbp),%bx
> # PeepHole Optimization,var11
> .Ll472:
> # [1015] Send := $8000 or (bus shl 16) or (device shl 11) or (func shl
> 8) or (regnum and $fc);
> andl $65535,%eax
> shll $16,%eax
> orl $2147483648,%eax
> # PeepHole Optimization,var11
> andl $65535,%edx
> shll $11,%edx
> orl %eax,%edx
> # PeepHole Optimization,var11
> andl $65535,%r8d
> shll $8,%r8d
> orl %edx,%r8d
> # PeepHole Optimization,var1
> # PeepHole Optimization,var11
> andl $252,%r9d
> orl %r8d,%r9d
> movl %r9d,-8(%rbp)
> .Ll473:
> # [1016] write_portd(@Send, PCI_CONF_PORT_INDEX);
> leaq -8(%rbp),%rcx
> movl $3320,%edx
> call ARCH_$$_WRITE_PORTD$POINTER$WORD
> .Ll474:
> # [1017] write_portw(value, PCI_CONF_PORT_DATA);
> movw %bx,%cx
> # Var value located in register cx
> # PeepHole Optimization,var11
> andl $65535,%ecx
> movl $3324,%edx
> call ARCH_$$_WRITE_PORTW$WORD$WORD
> .Ll475:
> # [1018] end;
> movq -16(%rbp),%rbx
> leaq (%rbp),%rsp
> popq %rbp
> ret
> .seh_endproc
>
> The first thing that I realize was the the optimized version is not
> generating the correct source when is exiting since it should return
> "Send", but am I right? The assembler code of write_portd remains the same,
> Am I missing something?
>
> Regards, Matias.
>
>
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