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Use alignas to fix GCALIGN-related bugs

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Use alignas and unions to specify alignments of objects needing
addresses that are at least a multiple of GCALIGNMENT.  Using
these standard C facilities should be safer than relying on ad hoc
and poorly-understood features like GCC’s __attribute__
((aligned (N))), the root cause for recent porting bugs like
Bug#29040.  The alignas macro was standardized by C11 and Gnulib
supports alignas for pre-C11 platforms.  I have tested this on Sun
Studio 12 sparc (2007) and GCC 4.4.7 x86-64 (2012) as well as on
more recent platforms like GCC 7.2.1 (2017) on Fedora 26 (both
x86-64 and x86).
* lib-src/make-docfile.c (close_emacs_globals): lispsym is now
just an array of struct Lisp_Symbol, since struct Lisp_Symbol is
now properly aligned.  All uses changed.
* src/alloc.c (NEXT_FREE_LISP_STRING): Just use the new u.next
member; this is simpler and safer than casting a pointer that
might not be aligned properly.
(aligned_Lisp_Symbol): Remove.  No longer needed, now that struct
Lisp_Symbol is aligned properly.  All uses replaced with struct
Lisp_Symbol.
* src/lisp.h (GCALIGNED): Remove, as it does not work as expected:
it can cause the natural alignment to be ignored.  All uses
replaced by unions with a ‘char alignas (GCALIGNMENT)’ member as
described below.
(struct Lisp_Symbol, struct Lisp_Cons, struct Lisp_String):
Change definition from ‘struct TAG { MEMBERS };’ to
‘struct TAG { union { struct { MEMBERS } s; char alignas
(GCALIGNMENT) gcaligned; } u; };’.  This guarantees ‘struct TAG’
to have an alignment that at least max (GCALIGNMENT, N) where N is
its old alignment.  All uses like ‘PTR->MEMBER’ changed to
‘PTR->u.s.MEMBER’; these uses were supposed to be mostly private
anyway.  Verify that the resulting ‘struct TAG’ is properly
aligned for Emacs.
(union vectorlike_header): New member ‘gcaligned’ to guarantee
that this type, and its containing types like ‘struct Lisp_Subr’,
‘struct buffer’ and ‘struct thread_state’, are all properly
aligned for Emacs.
(struct Lisp_String): New union member ‘next’, for the benefit
of NEXT_FREE_LISP_STRING.
(union Aligned_Cons, union Aligned_String): Remove.  All uses
replaced by struct Lisp_Cons and struct Lisp_String, since they
are now properly aligned.
(USE_STACK_CONS, USE_STACK_STRING): Simplify now that we can
assume struct Lisp_Cons and struct Lisp_String are properly
aligned.
This commit is contained in:
Paul Eggert 2017-11-13 08:51:41 -08:00
parent 5d68dc9a2f
commit b1573a97e1
16 changed files with 358 additions and 368 deletions
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4
src/bytecode.c
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@ -489,7 +489,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
{
Lisp_Object v1 = vectorp[op], v2;
if (!SYMBOLP (v1)
|| XSYMBOL (v1)->redirect != SYMBOL_PLAINVAL
|| XSYMBOL (v1)->u.s.redirect != SYMBOL_PLAINVAL
|| (v2 = SYMBOL_VAL (XSYMBOL (v1)), EQ (v2, Qunbound)))
v2 = Fsymbol_value (v1);
PUSH (v2);
@ -558,7 +558,7 @@ exec_byte_code (Lisp_Object bytestr, Lisp_Object vector, Lisp_Object maxdepth,
/* Inline the most common case. */
if (SYMBOLP (sym)
&& !EQ (val, Qunbound)
&& !XSYMBOL (sym)->redirect
&& !XSYMBOL (sym)->u.s.redirect
&& !SYMBOL_TRAPPED_WRITE_P (sym))
SET_SYMBOL_VAL (XSYMBOL (sym), val);
else