Re: [google] support for building Linux kernel with FDO (issue4523061)

[Xinliang David Li <davidxl at google dot com>]

On Fri, May 13, 2011 at 5:54 AM, Paolo Bonzini <bonzini@gnu.org> wrote:
> On 05/13/2011 03:03 AM, Rong Xu wrote:
>>
>> ? ? ? ?* gcc/coverage.c ? ? ? ?(revision 173717): set a flag if building
>> for Linux kernel.
>> ? ? ? ?* gcc/tree-profile.c ? ?(revision 173717): don't emit TLS
>> declarations for Linux kernel builds.
>
> I think this should be done without touching at all the profiling machinery
> in GCC.
>
> 1) add a new TLS model -ftls-model=none and make the kernel uses it. The
> model would simply force targetm.have_tls to false.
>

This is a good idea.


> 2) as Richi mentioned, gcov-io and libgcov changes then can move to the
> kernel, and GCC needs no change at all here.
>

In fact -- reuse gcc code profiling machinery for FDO is the KEY
objective in this effort --- the effort tries to minimize gcc changes
by refactoring gcc code and isolating/abstracting away part of gcc
implementation that is user space program specific without using
runtime hooks.  Aside from the kernel FDO change -- the refactoring
itself actually makes the libgcov code more readable -- the existing
implementation has many huge functions etc.

Kernel source has their implementation of coverage testing -- but it
makes lots of data structure assumptions and hard coded -- it can
easily out of sync with gcc and is considered  unmaintainable.

Rong will have more explanation on the design.

Thanks,

David


> BTW, these parts of LIPO:
>
>> + ? ? ?if (!is_kernel_build)
>> + ? ? ? ?DECL_TLS_MODEL (dc_gcov_type_ptr_var) =
>> + ? ? ? ? decl_default_tls_model (dc_gcov_type_ptr_var);
>>
>> ? ? ? dc_void_ptr_var =
>> ? ? ? ?build_decl (UNKNOWN_LOCATION, VAR_DECL,
>> @@ -1488,8 +1493,9 @@
>> ? ? ? ? ? ? ? ? ? ?ptr_void);
>> ? ? ? DECL_ARTIFICIAL (dc_void_ptr_var) = 1;
>> ? ? ? DECL_EXTERNAL (dc_void_ptr_var) = 1;
>> - ? ? ?DECL_TLS_MODEL (dc_void_ptr_var) =
>> - ? ? ? decl_default_tls_model (dc_void_ptr_var);
>> + ? ? ?if (!is_kernel_build)
>> + ? ? ? ?DECL_TLS_MODEL (dc_void_ptr_var) =
>> + ? ? ? ? decl_default_tls_model (dc_void_ptr_var);
>
> Probably are missing a !targetm.have_tls.
>
> Paolo
>