[BZ #10686] Reserve new TLS field for x86 and x86_64

[Andreas Jaeger]

Ian Taylor asked in BZ #10686 for a private TLS field:

"In http://gcc.gnu.org/wiki/SplitStacks I describe an enhancement to gcc 
to support discontiguous stacks.  This is useful for permitting a large 
number of threads to flexibly share stack space.  It is possible to have 
many more threads executing when they do not all have to simultaneously 
have the maximum required stack space.  This is naturally most useful 
when using a 32-bit address space.
I'm working on adding this to gcc.

I think the best approach to make this efficient on x86 and x86_64 will 
be to store the current stack limit, less some appropriate offset, in 
the TCB header.
  Therefore, I would like to request a new field in the TCB header, at a 
fixed offset, for x86 and x86_64.  The field should have type void * or 
uintptr_t."

On advise of Richard Henderson he's using the last field of the tm fields.

Here's a patch to document this.

Ok to commit?

Andreas

2013-05-14  Andreas Jaeger  <aj@suse.de>

	[BZ #10686]
	* sysdeps/x86_64/tls.h (struct tcbhead_t): Add __private_ss field.
	* sysdeps/i386/tls.h (struct tcbhead_t): Likewise.

diff --git a/nptl/sysdeps/i386/tls.h b/nptl/sysdeps/i386/tls.h
index 1405f60..3d18b1d 100644
--- a/nptl/sysdeps/i386/tls.h
+++ b/nptl/sysdeps/i386/tls.h
@@ -59,7 +59,9 @@ typedef struct
    int __unused1;
  #endif
    /* Reservation of some values for the TM ABI.  */
-  void *__private_tm[5];
+  void *__private_tm[4];
+  /* GCC split stack support.  */
+  void *__private_ss;
  } tcbhead_t;

  # define TLS_MULTIPLE_THREADS_IN_TCB 1
diff --git a/nptl/sysdeps/x86_64/tls.h b/nptl/sysdeps/x86_64/tls.h
index 3d67dbc..61df1af 100644
--- a/nptl/sysdeps/x86_64/tls.h
+++ b/nptl/sysdeps/x86_64/tls.h
@@ -69,7 +69,9 @@ typedef struct
  # endif
    int rtld_must_xmm_save;
    /* Reservation of some values for the TM ABI.  */
-  void *__private_tm[5];
+  void *__private_tm[4];
+  /* GCC split stack support.  */
+  void *__private_ss;
    long int __unused2;
    /* Have space for the post-AVX register size.  */
    __128bits rtld_savespace_sse[8][4] __attribute__ ((aligned (32)));

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  Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
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