From 7c16392ccae50fe09ca3cbb11cc2cd59e5c376cc Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Thu, 7 Jun 2018 18:53:26 -0700 Subject: [PATCH] Fix GC-related commentary * 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. --- src/lisp.h | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) diff --git a/src/lisp.h b/src/lisp.h index ee2e72d32b7..c5af4fa6c75 100644 --- a/src/lisp.h +++ b/src/lisp.h @@ -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 = \ -- 2.39.5