It's probably a small thing that I'm missing, but I have a small problem where the code below causes undesired padding to be added between two symbols in the object file. This code writes 5 words to the object file:

    symbol = symbol_name(sname, SCstatic, type_fake(TYnptr));
    symbol->Sdt = dt;
    symbol->Sseg = objc_getsegment(SEGprotocol);
    outdata(symbol);

If you call it 2 times, it generates 5*2 + 3 padding bytes between the two (added dashes to show padding):

[25] 002d2 0034 000da6    0 0f94   2 10000000 00 00 __protocol __OBJC
 0000:   0  0  0  0  6  3  0  0  0  0  0  0  0  0  0  0   ................
 0010:   0  0  0  0 -0--0--0--0--0--0--0--0--0--0--0--0   ................
 0020:   0  0  0  0 36  3  0  0  0  0  0  0  0  0  0  0   ....6...........
 0030:   0  0  0  0                     ....

This padding is causing a crash when starting the Objective-C runtime because it expects the two to be contiguous.

What needs to be changed to remove this padding?

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/

Symbols are aligned when written out to the data segment.

To defeat the alignment, symbols that should be adjacent should be merged into one symbol.

Michel Fortin wrote:
> It's probably a small thing that I'm missing, but I have a small problem where the code below causes undesired padding to be added between two symbols in the object file. This code writes 5 words to the object file:
>
>     symbol = symbol_name(sname, SCstatic, type_fake(TYnptr));
>     symbol->Sdt = dt;
>     symbol->Sseg = objc_getsegment(SEGprotocol);
>     outdata(symbol);
>
> If you call it 2 times, it generates 5*2 + 3 padding bytes between the two (added dashes to show padding):
>
> [25] 002d2 0034 000da6    0 0f94   2 10000000 00 00 __protocol __OBJC
>  0000:   0  0  0  0  6  3  0  0  0  0  0  0  0  0  0  0   ................
>  0010:   0  0  0  0 -0--0--0--0--0--0--0--0--0--0--0--0   ................
>  0020:   0  0  0  0 36  3  0  0  0  0  0  0  0  0  0  0   ....6...........
>  0030:   0  0  0  0                     ....
>
> This padding is causing a crash when starting the Objective-C runtime because it expects the two to be contiguous.
>
> What needs to be changed to remove this padding?
>
> 

I hoped it wouldn't have to come to that. I'm a little surprised that symbols have to be 16-byte aligned.

But thanks for confirming what I'll have to do.


Le 2010-11-07 ? 21:05, Walter Bright a ?crit :

> Symbols are aligned when written out to the data segment.
> 
> To defeat the alignment, symbols that should be adjacent should be merged into one symbol.
> 
> Michel Fortin wrote:
>> It's probably a small thing that I'm missing, but I have a small problem where the code below causes undesired padding to be added between two symbols in the object file. This code writes 5 words to the object file:
>> 
>>    symbol = symbol_name(sname, SCstatic, type_fake(TYnptr));
>>    symbol->Sdt = dt;
>>    symbol->Sseg = objc_getsegment(SEGprotocol);
>>    outdata(symbol);
>> 
>> If you call it 2 times, it generates 5*2 + 3 padding bytes between the two (added dashes to show padding):
>> 
>> [25] 002d2 0034 000da6    0 0f94   2 10000000 00 00 __protocol __OBJC
>> 0000:   0  0  0  0  6  3  0  0  0  0  0  0  0  0  0  0   ................
>> 0010:   0  0  0  0 -0--0--0--0--0--0--0--0--0--0--0--0   ................
>> 0020:   0  0  0  0 36  3  0  0  0  0  0  0  0  0  0  0   ....6...........
>> 0030:   0  0  0  0                     ....
>> This padding is causing a crash when starting the Objective-C runtime because it expects the two to be contiguous.
>> 
>> What needs to be changed to remove this padding?
>> 
>> 
> _______________________________________________
> dmd-internals mailing list
> dmd-internals at puremagic.com
> http://lists.puremagic.com/mailman/listinfo/dmd-internals

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/

Turns out it doesn't work because those symbols refer to each other, so by coalescing them in one symbol I'm creating a self-referenced symbol and the backend doesn't like that.

I resorted to adding a 'noalign' flag to outdata() in the backend. It's a small change and it works, but it feels like a hack. Ideally there should be a way to specify the alignment per segment (there might already be one, but I can't figure it out). Since the Objective-C stuff belongs to separate segments it'd make things really easy.

I know there is the 'align' parameter in mach_getsegment(), but it doesn't seem to have any effect other than changing alignment value for the section in the object file.


Le 2010-11-07 ? 21:05, Walter Bright a ?crit :

> Symbols are aligned when written out to the data segment.
> 
> To defeat the alignment, symbols that should be adjacent should be merged into one symbol.
> 
> Michel Fortin wrote:
>> It's probably a small thing that I'm missing, but I have a small problem where the code below causes undesired padding to be added between two symbols in the object file. This code writes 5 words to the object file:
>> 
>>    symbol = symbol_name(sname, SCstatic, type_fake(TYnptr));
>>    symbol->Sdt = dt;
>>    symbol->Sseg = objc_getsegment(SEGprotocol);
>>    outdata(symbol);
>> 
>> If you call it 2 times, it generates 5*2 + 3 padding bytes between the two (added dashes to show padding):
>> 
>> [25] 002d2 0034 000da6    0 0f94   2 10000000 00 00 __protocol __OBJC
>> 0000:   0  0  0  0  6  3  0  0  0  0  0  0  0  0  0  0   ................
>> 0010:   0  0  0  0 -0--0--0--0--0--0--0--0--0--0--0--0   ................
>> 0020:   0  0  0  0 36  3  0  0  0  0  0  0  0  0  0  0   ....6...........
>> 0030:   0  0  0  0                     ....
>> This padding is causing a crash when starting the Objective-C runtime because it expects the two to be contiguous.
>> 
>> What needs to be changed to remove this padding?
>> 
>> 
> _______________________________________________
> dmd-internals mailing list
> dmd-internals at puremagic.com
> http://lists.puremagic.com/mailman/listinfo/dmd-internals

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/

Michel Fortin wrote:
> Turns out it doesn't work because those symbols refer to each other, so by coalescing them in one symbol I'm creating a self-referenced symbol and the backend doesn't like that.
>
> I resorted to adding a 'noalign' flag to outdata() in the backend. It's a small change and it works, but it feels like a hack. Ideally there should be a way to specify the alignment per segment (there might already be one, but I can't figure it out). Since the Objective-C stuff belongs to separate segments it'd make things really easy.
>
> I know there is the 'align' parameter in mach_getsegment(), but it doesn't seem to have any effect other than changing alignment value for the section in the object file.
> 

Do you mean alignment per segment, or per symbol?

>
> Le 2010-11-07 ? 21:05, Walter Bright a ?crit :
>
> 
>> Symbols are aligned when written out to the data segment.
>>
>> To defeat the alignment, symbols that should be adjacent should be merged into one symbol.
>>
>> Michel Fortin wrote:
>> 
>>> It's probably a small thing that I'm missing, but I have a small problem where the code below causes undesired padding to be added between two symbols in the object file. This code writes 5 words to the object file:
>>>
>>>    symbol = symbol_name(sname, SCstatic, type_fake(TYnptr));
>>>    symbol->Sdt = dt;
>>>    symbol->Sseg = objc_getsegment(SEGprotocol);
>>>    outdata(symbol);
>>>
>>> If you call it 2 times, it generates 5*2 + 3 padding bytes between the two (added dashes to show padding):
>>>
>>> [25] 002d2 0034 000da6    0 0f94   2 10000000 00 00 __protocol __OBJC
>>> 0000:   0  0  0  0  6  3  0  0  0  0  0  0  0  0  0  0   ................
>>> 0010:   0  0  0  0 -0--0--0--0--0--0--0--0--0--0--0--0   ................
>>> 0020:   0  0  0  0 36  3  0  0  0  0  0  0  0  0  0  0   ....6...........
>>> 0030:   0  0  0  0                     ....
>>> This padding is causing a crash when starting the Objective-C runtime because it expects the two to be contiguous.
>>>
>>> What needs to be changed to remove this padding?
>>>
>>> 
>>> 
>> _______________________________________________
>> dmd-internals mailing list
>> dmd-internals at puremagic.com
>> http://lists.puremagic.com/mailman/listinfo/dmd-internals
>> 
>
> 

Le 2010-11-08 ? 16:01, Walter Bright a ?crit :

> Michel Fortin wrote:
>> Turns out it doesn't work because those symbols refer to each other, so by coalescing them in one symbol I'm creating a self-referenced symbol and the backend doesn't like that.
>> 
>> I resorted to adding a 'noalign' flag to outdata() in the backend. It's a small change and it works, but it feels like a hack. Ideally there should be a way to specify the alignment per segment (there might already be one, but I can't figure it out). Since the Objective-C stuff belongs to separate segments it'd make things really easy.
>> 
>> I know there is the 'align' parameter in mach_getsegment(), but it doesn't seem to have any effect other than changing alignment value for the section in the object file.
> 
> Do you mean alignment per segment, or per symbol?

I mean the alignment of the symbols in a specific segment. In this case I need symbols inside the __protocol segment of the __OBJC section to have a 4-byte alignment. I'd have expected the align parameter of mach_getsegment() to do that, but setting it to 2 doesn't do anything.

Since all the symbols are 20 bytes I can just emit them with no alignment at all and I get the same result, that's what my small hack does.

If you clone the Git repository you'll see it's the last commit (01c7708c82).

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/

Michel Fortin wrote:
> L
> I mean the alignment of the symbols in a specific segment. In this case I need symbols inside the __protocol segment of the __OBJC section to have a 4-byte alignment. I'd have expected the align parameter of mach_getsegment() to do that, but setting it to 2 doesn't do anything.
> 

The segment alignment is for the whole segment, not for individual data items in it.

> Since all the symbols are 20 bytes I can just emit them with no alignment at all and I get the same result, that's what my small hack does.
>
> If you clone the Git repository you'll see it's the last commit (01c7708c82).
>
> 

Le 2010-11-08 ? 18:14, Walter Bright a ?crit :

> Michel Fortin wrote:
>> L
>> I mean the alignment of the symbols in a specific segment. In this case I need symbols inside the __protocol segment of the __OBJC section to have a 4-byte alignment. I'd have expected the align parameter of mach_getsegment() to do that, but setting it to 2 doesn't do anything.
> 
> The segment alignment is for the whole segment, not for individual data items in it.

Indeed, that makes sense.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/