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bug#8623: 23.3.50; (woman "git-remote") crashes


From: Sven Joachim
Subject: bug#8623: 23.3.50; (woman "git-remote") crashes
Date: Fri, 06 May 2011 14:36:04 +0200
User-agent: Gnus/5.110017 (No Gnus v0.17) Emacs/23.3.50 (gnu/linux)

On 2011-05-06 12:50 +0200, Sven Joachim wrote:

> On 2011-05-06 12:15 +0200, Eli Zaretskii wrote:
>
>>> From: Sven Joachim <address@hidden>
>>> Cc: address@hidden
>>> Date: Thu, 05 May 2011 23:12:18 +0200
>>> 
>>> $ gcc --version | head -n1
>>> gcc (Debian 4.6.0-6) 4.6.1 20110428 (prerelease)
>>> 
>>> > Also, can you build Emacs without optimizations, and try reproducing
>>> > in the unoptimized binary?
>>> 
>>> Alas, the unoptimized binary does not crash.
>>
>> Darn!  Debugging optimized code is pretty much hopeless with current
>> versions of GCC.
>>
>> Paul, could you please take a look at this?  Could this be due to the
>> same problems with GCC 4.6 optimizations you recently fixed on the
>> trunk?  If so, we may need similar changes on the release branch.
>
> I suspect so, becauseā€¦
>
>> Sven, could you also try with the trunk build?
>
> The trunk is fine.  I'm already bisecting the problem, but have to leave
> in 1.5 hours and will then be unavailable until Sunday.  If I find out
> the commit which fixed the bug, I'll let you know.

The following commit (merged into the trunk as revision 104021)
contained the fix:

,----
|     revno: 103939.1.42
|     committer: Paul Eggert <address@hidden>
|     branch nick: atest
|     timestamp: Mon 2011-04-25 00:14:46 -0700
|     message:
|       lisp.h: Fix a problem with aliasing and vector headers.
|       
|       GCC 4.6.0 optimizes based on type-based alias analysis.  For
|       example, if b is of type struct buffer * and v of type struct
|       Lisp_Vector *, then gcc -O2 was incorrectly assuming that &b->size
|       != &v->size, and therefore "v->size = 1; b->size = 2; return
|       v->size;" must therefore return 1.  This assumption is incorrect
|       for Emacs, since it type-puns struct Lisp_Vector * with many other
|       types.  To fix this problem, this patch adds a new type struct
|       vector_header that documents the constraints on layout of vectors
|       and pseudovectors, and helps optimizing compilers not get fooled
|       by Emacs's type punning.  It also adds the macros XSETTYPED_PVECTYPE
|       XSETTYPED_PSEUDOVECTOR, TYPED_PSEUDOVECTORP, for similar reasons.
|       * lisp.h (XVECTOR_SIZE): New convenience macro.  All previous uses of
|       XVECTOR (foo)->size replaced to use this macro, to avoid the hassle
|       of writing XVECTOR (foo)->header.size.
|       (XVECTOR_HEADER_SIZE): New macro, for use in XSETPSEUDOVECTOR.
|       (XSETTYPED_PVECTYPE): New macro, specifying the name of the size
|       member.
|       (XSETPVECTYPE): Rewrite in terms of new macro.
|       (XSETPVECTYPESIZE): New macro, specifying both type and size.
|       This is a bit clearer, and further avoids the possibility of
|       undesirable aliasing.
|       (XSETTYPED_PSEUDOVECTOR): New macro, specifying the size.
|       (XSETPSEUDOVECTOR): Rewrite in terms of XSETTYPED_PSEUDOVECTOR
|       and XVECTOR_HEADER_SIZE.
|       (XSETSUBR): Rewrite in terms of XSETTYPED_PSEUDOVECTOR and XSIZE,
|       since Lisp_Subr is a special case (no "next" field).
|       (ASIZE): Rewrite in terms of XVECTOR_SIZE.
|       (struct vector_header): New type.
|       (TYPED_PSEUDOVECTORP): New macro, also specifying the C type of the
|       object, to help avoid aliasing.
|       (PSEUDOVECTORP): Rewrite in terms of TYPED_PSEUDOVECTORP.
|       (SUBRP): Likewise, since Lisp_Subr is a special case.
|       * lisp.h (struct Lisp_Vector, struct Lisp_Char_Table):
|       (struct Lisp_Sub_Char_Table, struct Lisp_Bool_Vector):
|       (struct Lisp_Hash_Table): Combine first two members into a single
|       struct vector_header member.  All uses of "size" and "next" members
|       changed to be "header.size" and "header.next".
|       * buffer.h (struct buffer): Likewise.
|       * font.h (struct font_spec, struct font_entity, struct font): Likewise.
|       * frame.h (struct frame): Likewise.
|       * process.h (struct Lisp_Process): Likewise.
|       * termhooks.h (struct terminal): Likewise.
|       * window.c (struct save_window_data, struct saved_window): Likewise.
|       * window.h (struct window): Likewise.
|       * alloc.c (allocate_buffer, Fmake_bool_vector, allocate_pseudovector):
|       Use XSETPVECTYPESIZE, not XSETPVECTYPE, to avoid aliasing problems.
|       * buffer.c (init_buffer_once): Likewise.
|       * lread.c (defsubr): Use XSETTYPED_PVECTYPE, since Lisp_Subr is a
|       special case.
|       * process.c (Fformat_network_address): Use local var for size,
|       for brevity.
| 
`----

This commit touches 39 files.  How to backport it to the emacs-23 branch
is left as an exercise for the reader.

Bye,
    Sven





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