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authorPaul Eggert <eggert@cs.ucla.edu>2017-11-13 08:51:41 -0800
committerPaul Eggert <eggert@cs.ucla.edu>2017-11-13 10:16:51 -0800
commitb1573a97e17b518723ab3f906eb6d521caed196d (patch)
tree2bcfe8f47a99d7ba9b46e75ed5bffd2ba37970e0 /lib-src
parent5d68dc9a2fd1b9b883db6bc1c226541b50de8bb1 (diff)
downloademacs-b1573a97e17b518723ab3f906eb6d521caed196d.tar.gz
Use alignas to fix GCALIGN-related bugs
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.
Diffstat (limited to 'lib-src')
-rw-r--r--lib-src/make-docfile.c4
1 files changed, 1 insertions, 3 deletions
diff --git a/lib-src/make-docfile.c b/lib-src/make-docfile.c
index ff84df94a69..9e4755b63ac 100644
--- a/lib-src/make-docfile.c
+++ b/lib-src/make-docfile.c
@@ -667,9 +667,7 @@ close_emacs_globals (ptrdiff_t num_symbols)
"#ifndef DEFINE_SYMBOLS\n"
"extern\n"
"#endif\n"
- "struct {\n"
- " struct GCALIGNED Lisp_Symbol s;\n"
- "} lispsym[%td];\n"),
+ "struct Lisp_Symbol lispsym[%td];\n"),
num_symbols);
}