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* lisp.h: Add comments about struct vectorlike_header.

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This commit is contained in:
Paul Eggert 2011-04-26 12:14:07 -07:00
parent 69e9b5a37c
commit aa0b008761
1 changed files with 19 additions and 4 deletions
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23
src/lisp.h
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@ -606,6 +606,8 @@ extern Lisp_Object make_number (EMACS_INT);
((v)->size_member |= PSEUDOVECTOR_FLAG | (code))
#define XSETPVECTYPESIZE(v, code, sizeval) \
((v)->header.size = PSEUDOVECTOR_FLAG | (code) | (sizeval))
/* The cast to struct vectorlike_header * avoids aliasing issues. */
#define XSETPSEUDOVECTOR(a, b, code) \
XSETTYPED_PSEUDOVECTOR(a, b, \
((struct vectorlike_header *) XPNTR (a))->size, \
@ -614,11 +616,13 @@ extern Lisp_Object make_number (EMACS_INT);
(XSETVECTOR (a, b), \
eassert ((size & (PSEUDOVECTOR_FLAG | PVEC_TYPE_MASK)) \
== (PSEUDOVECTOR_FLAG | (code))))
#define XSETWINDOW_CONFIGURATION(a, b) \
(XSETPSEUDOVECTOR (a, b, PVEC_WINDOW_CONFIGURATION))
#define XSETPROCESS(a, b) (XSETPSEUDOVECTOR (a, b, PVEC_PROCESS))
#define XSETWINDOW(a, b) (XSETPSEUDOVECTOR (a, b, PVEC_WINDOW))
#define XSETTERMINAL(a, b) (XSETPSEUDOVECTOR (a, b, PVEC_TERMINAL))
/* XSETSUBR is special since Lisp_Subr lacks struct vectorlike_header. */
#define XSETSUBR(a, b) \
XSETTYPED_PSEUDOVECTOR (a, b, XSUBR (a)->size, PVEC_SUBR)
#define XSETCOMPILED(a, b) (XSETPSEUDOVECTOR (a, b, PVEC_COMPILED))
@ -787,12 +791,21 @@ struct Lisp_String
unsigned char *data;
};
/* Header of vector-like objects. This type documents the constraints on
layout of vectors and pseudovectors, and helps optimizing compilers not get
fooled by Emacs's type punning. */
/* Header of vector-like objects. This documents the layout constraints on
vectors and pseudovectors other than struct Lisp_Subr. It also prevents
compilers from being fooled by Emacs's type punning: the XSETPSEUDOVECTOR
and PSEUDOVECTORP macros cast their pointers to struct vectorlike_header *,
because when two such pointers potentially alias, a compiler won't
incorrectly reorder loads and stores to their size fields. See
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8546>. */
struct vectorlike_header
{
EMACS_UINT size;
/* Pointer to the next vector-like object. It is generally a buffer or a
Lisp_Vector alias, so for convenience it is a union instead of a
pointer: this way, one can write P->next.vector instead of ((struct
Lisp_Vector *) P->next). */
union {
struct buffer *buffer;
struct Lisp_Vector *vector;
@ -1647,7 +1660,8 @@ typedef struct {
#define BUFFER_OBJFWDP(x) (XFWDTYPE (x) == Lisp_Fwd_Buffer_Obj)
#define KBOARD_OBJFWDP(x) (XFWDTYPE (x) == Lisp_Fwd_Kboard_Obj)
/* True if object X is a pseudovector whose code is CODE. */
/* True if object X is a pseudovector whose code is CODE. The cast to struct
vectorlike_header * avoids aliasing issues. */
#define PSEUDOVECTORP(x, code) \
TYPED_PSEUDOVECTORP(x, vectorlike_header, code)
@ -1664,6 +1678,7 @@ typedef struct {
#define PROCESSP(x) PSEUDOVECTORP (x, PVEC_PROCESS)
#define WINDOWP(x) PSEUDOVECTORP (x, PVEC_WINDOW)
#define TERMINALP(x) PSEUDOVECTORP (x, PVEC_TERMINAL)
/* SUBRP is special since Lisp_Subr lacks struct vectorlike_header. */
#define SUBRP(x) TYPED_PSEUDOVECTORP (x, Lisp_Subr, PVEC_SUBR)
#define COMPILEDP(x) PSEUDOVECTORP (x, PVEC_COMPILED)
#define BUFFERP(x) PSEUDOVECTORP (x, PVEC_BUFFER)