code review 7563045: cmd/ld: steps toward 386 host linking

cmd/ld: steps toward 386 host linking

- Introduce MaxAlign constant and use in data layout
and ELF section header.

- Allow up to 16-byte alignment for large objects
(will help Keith's hash changes).

- Emit ELF symbol for .rathole (global /dev/null used by 8c).

- Invoke gcc with -m32/-m64 as appropriate.

- Don't invoke gcc if writing the .o file failed.

[rsc, 2013-03-07 18:01:59]

Hello golang-dev@googlegroups.com,

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

[iant, 2013-03-07 19:27:23]

https://codereview.appspot.com/7563045/diff/6008/src/cmd/8l/l.h
File src/cmd/8l/l.h (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/8l/l.h#newcode45
src/cmd/8l/l.h:45: MaxAlign = 16,	// max data alignment
Why use 16 for 8l and 8 for 6l?  Shouldn't they be the same?

Note that there are now AVX instructions (vmovaps and friends) that require
alignment to a 32-byte boundary.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c
File src/cmd/ld/data.c (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode886
src/cmd/ld/data.c:886: if(t >= 16 && MaxAlign >= 16)
Add a line for 32 to handle vmovaps.  It will only affect arrays.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode893
src/cmd/ld/data.c:893: return rnd(datsize, 4);
Why put a 2 byte value at a 4-byte alignment?

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode1015
src/cmd/ld/data.c:1015: datsize = rnd(datsize, MaxAlign);
Don't you need to check s->align here?  You need to honor the ELF section
requested alignment even if it is larger than MaxAlign.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/elf.c
File src/cmd/ld/elf.c (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/elf.c#newcode765
src/cmd/ld/elf.c:765: sh->addralign = MaxAlign;
We should really be recording the maximum alignment that we see in the section
and storing it in a new field in Section and using it here.

[rsc, 2013-03-07 20:53:54]

PTAL

https://codereview.appspot.com/7563045/diff/6008/src/cmd/8l/l.h
File src/cmd/8l/l.h (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/8l/l.h#newcode45
src/cmd/8l/l.h:45: MaxAlign = 16,	// max data alignment
On 2013/03/07 19:27:24, iant wrote:
> Why use 16 for 8l and 8 for 6l?  Shouldn't they be the same?
> 
> Note that there are now AVX instructions (vmovaps and friends) that require
> alignment to a 32-byte boundary.

Done.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c
File src/cmd/ld/data.c (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode886
src/cmd/ld/data.c:886: if(t >= 16 && MaxAlign >= 16)
On 2013/03/07 19:27:24, iant wrote:
> Add a line for 32 to handle vmovaps.  It will only affect arrays.

Done.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode893
src/cmd/ld/data.c:893: return rnd(datsize, 4);
On 2013/03/07 19:27:24, iant wrote:
> Why put a 2 byte value at a 4-byte alignment?

I have no idea. That's what alignsymsize used to do. Removed.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/data.c#newcode1015
src/cmd/ld/data.c:1015: datsize = rnd(datsize, MaxAlign);
On 2013/03/07 19:27:24, iant wrote:
> Don't you need to check s->align here?  You need to honor the ELF section
> requested alignment even if it is larger than MaxAlign.

Done.

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/elf.c
File src/cmd/ld/elf.c (right):

https://codereview.appspot.com/7563045/diff/6008/src/cmd/ld/elf.c#newcode765
src/cmd/ld/elf.c:765: sh->addralign = MaxAlign;
On 2013/03/07 19:27:24, iant wrote:
> We should really be recording the maximum alignment that we see in the section
> and storing it in a new field in Section and using it here.

Done.

[iant, 2013-03-07 22:03:34]

LGTM

[rsc, 2013-03-08 03:57:27]

*** Submitted as https://code.google.com/p/go/source/detail?r=bb71b657b5bc ***

cmd/ld: steps toward 386 host linking

- Introduce MaxAlign constant and use in data layout
and ELF section header.

- Allow up to 16-byte alignment for large objects
(will help Keith's hash changes).

- Emit ELF symbol for .rathole (global /dev/null used by 8c).

- Invoke gcc with -m32/-m64 as appropriate.

- Don't invoke gcc if writing the .o file failed.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7563045