diff options
Diffstat (limited to 'src/alloc.c')
| -rw-r--r-- | src/alloc.c | 55 |
1 files changed, 29 insertions, 26 deletions
diff --git a/src/alloc.c b/src/alloc.c index d5a6d9167ea..602282e5704 100644 --- a/src/alloc.c +++ b/src/alloc.c @@ -112,9 +112,9 @@ along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>. */ adds sizeof (size_t) to SIZE for internal overhead, and then rounds up to a multiple of MALLOC_ALIGNMENT. Emacs can improve performance a bit on GNU platforms by arranging for the resulting - size to be a power of two. This heuristic is good for glibc 2.0 - (1997) through at least glibc 2.31 (2020), and does not affect - correctness on other platforms. */ + size to be a power of two. This heuristic is good for glibc 2.26 + (2017) and later, and does not affect correctness on other + platforms. */ #define MALLOC_SIZE_NEAR(n) \ (ROUNDUP (max (n, sizeof (size_t)), MALLOC_ALIGNMENT) - sizeof (size_t)) @@ -655,28 +655,30 @@ buffer_memory_full (ptrdiff_t nbytes) #define COMMON_MULTIPLE(a, b) \ ((a) % (b) == 0 ? (a) : (b) % (a) == 0 ? (b) : (a) * (b)) -/* LISP_ALIGNMENT is the alignment of Lisp objects. It must be at - least GCALIGNMENT so that pointers can be tagged. It also must be - at least as strict as the alignment of all the C types used to - implement Lisp objects; since pseudovectors can contain any C type, - this is max_align_t. On recent GNU/Linux x86 and x86-64 this can - often waste up to 8 bytes, since alignof (max_align_t) is 16 but - typical vectors need only an alignment of 8. Although shrinking - the alignment to 8 would save memory, it cost a 20% hit to Emacs - CPU performance on Fedora 28 x86-64 when compiled with gcc -m32. */ -enum { LISP_ALIGNMENT = alignof (union { max_align_t x; - GCALIGNED_UNION_MEMBER }) }; -verify (LISP_ALIGNMENT % GCALIGNMENT == 0); +/* A lower bound on the alignment of malloc. For better performance + this bound should be tighter. For glibc 2.26 and later a tighter + bound is known. */ +#if 2 < __GLIBC__ + (26 <= __GLIBC_MINOR__) +enum { MALLOC_ALIGNMENT_BOUND = MALLOC_ALIGNMENT }; +#else +/* A bound known to work for all Emacs porting targets. Tightening + this looser bound by using max_align_t instead of long long int + would break buggy malloc implementations like MinGW circa 2020. */ +enum { MALLOC_ALIGNMENT_BOUND = alignof (long long int) }; +#endif + +/* A lower bound on the alignment of Lisp objects. All Lisp objects + must have an address that is a multiple of LISP_ALIGNMENT; + otherwise maybe_lisp_pointer can issue false negatives, causing crashes. + It's good to make this bound tight: if Lisp objects are always + aligned more strictly than LISP_ALIGNMENT, maybe_lisp_pointer will + issue more false positives, hurting performance. */ +enum { LISP_ALIGNMENT = max (max (GCALIGNMENT, MALLOC_ALIGNMENT_BOUND), + alignof (union emacs_align_type)) }; /* True if malloc (N) is known to return storage suitably aligned for - Lisp objects whenever N is a multiple of LISP_ALIGNMENT. In - practice this is true whenever alignof (max_align_t) is also a - multiple of LISP_ALIGNMENT. This works even for x86, where some - platform combinations (e.g., GCC 7 and later, glibc 2.25 and - earlier) have bugs where alignof (max_align_t) is 16 even though - the malloc alignment is only 8, and where Emacs still works because - it never does anything that requires an alignment of 16. */ -enum { MALLOC_IS_LISP_ALIGNED = alignof (max_align_t) % LISP_ALIGNMENT == 0 }; + Lisp objects whenever N is a multiple of LISP_ALIGNMENT. */ +enum { MALLOC_IS_LISP_ALIGNED = MALLOC_ALIGNMENT_BOUND % LISP_ALIGNMENT == 0 }; /* If compiled with XMALLOC_BLOCK_INPUT_CHECK, define a symbol BLOCK_INPUT_IN_MEMORY_ALLOCATORS that is visible to the debugger. @@ -4885,9 +4887,10 @@ test_setjmp (void) as a stack scan limit. */ typedef union { - /* Align the stack top properly. Even if !HAVE___BUILTIN_UNWIND_INIT, - jmp_buf may not be aligned enough on darwin-ppc64. */ - max_align_t o; + /* Make sure stack_top and m_stack_bottom are properly aligned as GC + expects. */ + Lisp_Object o; + void *p; #ifndef HAVE___BUILTIN_UNWIND_INIT sys_jmp_buf j; char c; |