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Fix GC-related commentary

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* src/lisp.h: USE_STACK_LISP_OBJECTS is no longer experimental.
Also, remove confusion about scope vs lifetime.
And say that stack-allocated strings should not be given
text properties.
This commit is contained in:
Paul Eggert 2018-06-07 18:53:26 -07:00
parent 9966842a21
commit 7c16392cca
1 changed files with 12 additions and 9 deletions
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21
src/lisp.h
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@ -228,7 +228,7 @@ extern bool suppress_checking EXTERNALLY_VISIBLE;
USE_LSB_TAG not only requires the least 3 bits of pointers returned by
malloc to be 0 but also needs to be able to impose a mult-of-8 alignment
on the few static Lisp_Objects used, all of which are aligned via
on some non-GC Lisp_Objects, all of which are aligned via
'char alignas (GCALIGNMENT) gcaligned;' inside a union. */
enum Lisp_Bits
@ -4680,13 +4680,14 @@ extern void *record_xmalloc (size_t) ATTRIBUTE_ALLOC_SIZE ((1));
#define SAFE_ALLOCA_LISP(buf, nelt) SAFE_ALLOCA_LISP_EXTRA (buf, nelt, 0)
/* If USE_STACK_LISP_OBJECTS, define macros that and functions that allocate
block-scoped conses and strings. These objects are not
managed by the garbage collector, so they are dangerous: passing them
out of their scope (e.g., to user code) results in undefined behavior.
Conversely, they have better performance because GC is not involved.
/* If USE_STACK_LISP_OBJECTS, define macros and functions that
allocate some Lisp objects on the C stack. As the storage is not
managed by the garbage collector, these objects are dangerous:
passing them to user code could result in undefined behavior if the
objects are in use after the C function returns. Conversely, these
objects have better performance because GC is not involved.
This feature is experimental and requires careful debugging.
While debugging you may want to disable allocation on the C stack.
Build with CPPFLAGS='-DUSE_STACK_LISP_OBJECTS=0' to disable it. */
#if (!defined USE_STACK_LISP_OBJECTS \
@ -4751,7 +4752,8 @@ enum
Take its unibyte value from the null-terminated string STR,
an expression that should not have side effects.
STR's value is not necessarily copied. The resulting Lisp string
should not be modified or made visible to user code. */
should not be modified or given text properties or made visible to
user code. */
#define AUTO_STRING(name, str) \
AUTO_STRING_WITH_LEN (name, str, strlen (str))
@ -4760,7 +4762,8 @@ enum
Take its unibyte value from the null-terminated string STR with length LEN.
STR may have side effects and may contain null bytes.
STR's value is not necessarily copied. The resulting Lisp string
should not be modified or made visible to user code. */
should not be modified or given text properties or made visible to
user code. */
#define AUTO_STRING_WITH_LEN(name, str, len) \
Lisp_Object name = \