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Improve FOR_EACH_TAIL (bug#81115)

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* src/lisp.h (struct for_each_tail_internal): Reduce to two words.
(FOR_EACH_TAIL_BASIC): Add compiler hint to indicate that tail is most
likely Qnil after the loop and a non-nil non-cons is unlikely.
(FOR_EACH_TAIL_STEP_CYCLEP): Rewrite.
This commit is contained in:
Pip Cet 2026-05-25 09:10:23 +00:00
parent de67c677fe
commit 0bef3c0e87
1 changed files with 33 additions and 16 deletions
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49
src/lisp.h
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@ -5833,7 +5833,8 @@ enum { SMALL_LIST_LEN_MAX = 127 };
set TAIL to the current cons. If the loop exits normally,
set TAIL to the terminating non-cons, typically nil. The loop body
should not modify the list’s top level structure other than by
perhaps deleting the current cons. */
perhaps deleting the current cons. In practice, this macro rarely
quits. */
#define FOR_EACH_TAIL(tail) \
FOR_EACH_TAIL_INTERNAL (tail, circular_list (tail), true)
@ -5847,16 +5848,17 @@ enum { SMALL_LIST_LEN_MAX = 127 };
/* Iterator intended for use only within FOR_EACH_TAIL_INTERNAL. */
struct for_each_tail_internal
{
/* The object we're comparing to. */
Lisp_Object tortoise;
intptr_t max, n;
unsigned short int q;
/* How many iterations of the algorithm have happened. */
ptrdiff_t l;
};
/* Like FOR_EACH_TAIL (LIST), except evaluate CYCLE if a cycle is
found, and check for quit if CHECK_QUIT. This is an internal macro
intended for use only by the above macros.
Use Brent’s teleporting tortoise-hare algorithm. See:
Modifies Brent’s teleporting tortoise-hare algorithm. See:
Brent RP. BIT. 1980;20(2):176-184. doi:10.1007/BF01933190
https://maths-people.anu.edu.au/~brent/pd/rpb051i.pdf
@ -5871,20 +5873,35 @@ struct for_each_tail_internal
FOR_EACH_TAIL_STEP_CYCLEP (tail, check_quit) \
? (cycle) : (void) 0)
#define FOR_EACH_TAIL_BASIC(tail, stepper) \
for (struct for_each_tail_internal li = { tail, 2, 0, 2 }; \
CONSP (tail); stepper)
#define FOR_EACH_TAIL_BASIC(tail, stepper) \
for (struct for_each_tail_internal li = { tail, 0 }; \
CONSP (tail) || (likely (NILP (tail)), false); \
stepper)
/* Step TAIL and return whether a cycle has been detected.
If CHECK_QUIT then check for quit occasionally. */
#define FOR_EACH_TAIL_STEP_CYCLEP(tail, check_quit) \
((tail) = XCDR (tail), \
((--li.q != 0 \
|| ((check_quit) ? maybe_quit () : (void) 0, 0 < --li.n) \
|| (li.q = li.n = li.max <<= 1, li.n >>= USHRT_WIDTH, \
li.tortoise = (tail), false)) \
&& BASE_EQ (tail, li.tortoise)))
/* How many iterations until we start advancing the tortoise, which is
necessary to detect lists which end in a cycle but whose initial
elements aren't part of the cycle. Also, number of iterations until
we check for quits. Must be a power of two. */
#define FOR_EACH_TAIL_THRESHOLD 4096
static_assert (POWER_OF_2 (FOR_EACH_TAIL_THRESHOLD));
/* Step TAIL and return whether a cycle has been detected. If
CHECK_QUIT then check for quit occasionally. We only check for quits
once every 4096 iterations, and we don't advance the tortoise until
that happens for the first time.
The C comma operator is used here to avoid statement expressions: the
expression evaluates to the value of BASE_EQ (tail, li.tortoise), but
has additional side effects, possibly exiting nonlocally. */
#define FOR_EACH_TAIL_STEP_CYCLEP(tail, check_quit) \
((tail) = XCDR (tail), \
(! likely (!BASE_EQ (tail, li.tortoise)) \
|| (! likely (((++li.l) & (FOR_EACH_TAIL_THRESHOLD-1)) != 0) \
&& (((check_quit) ? maybe_quit () : (void) 0, false) \
|| (POWER_OF_2 (li.l) && (li.tortoise = (tail), false))))))
/* Do a `for' loop over alist values. */