avoid linear-time if/else special cases in sysconf

the previous handling of cases that could not fit in the 16-bit table
or which required non-constant results was extremely ugly and could
not scale. the new code remaps these keys into a contiguous range
that's efficient for a switch statement.

diff --git a/src/conf/sysconf.c b/src/conf/sysconf.c
index 12e8136..e225c3d 100644 (file)
--- a/src/conf/sysconf.c
+++ b/src/conf/sysconf.c
@@ -6,15 +6,21 @@
 #include "syscall.h"
 #include "libc.h"
 
-#define VER (-2)
-#define OFLOW (-3)
-#define CPUCNT (-4)
+#define JT(x) (-256|(x))
+#define VER JT(1)
+#define JT_ARG_MAX JT(2)
+#define JT_MQ_PRIO_MAX JT(3)
+#define JT_PAGE_SIZE JT(4)
+#define JT_SEM_VALUE_MAX JT(5)
+#define JT_NPROCESSORS_CONF JT(6)
+#define JT_NPROCESSORS_ONLN JT(7)
+
 #define RLIM(x) (-32768|(RLIMIT_ ## x))
 
 long sysconf(int name)
 {
        static const short values[] = {
-               [_SC_ARG_MAX] = OFLOW,
+               [_SC_ARG_MAX] = JT_ARG_MAX,
                [_SC_CHILD_MAX] = RLIM(NPROC),
                [_SC_CLK_TCK] = 100,
                [_SC_NGROUPS_MAX] = 32,
@@ -42,12 +48,12 @@ long sysconf(int name)
                [_SC_AIO_PRIO_DELTA_MAX] = 0, /* ?? */
                [_SC_DELAYTIMER_MAX] = _POSIX_DELAYTIMER_MAX,
                [_SC_MQ_OPEN_MAX] = -1,
-               [_SC_MQ_PRIO_MAX] = OFLOW,
+               [_SC_MQ_PRIO_MAX] = JT_MQ_PRIO_MAX,
                [_SC_VERSION] = VER,
-               [_SC_PAGE_SIZE] = OFLOW,
+               [_SC_PAGE_SIZE] = JT_PAGE_SIZE,
                [_SC_RTSIG_MAX] = _NSIG - 1 - 31 - 3,
                [_SC_SEM_NSEMS_MAX] = SEM_NSEMS_MAX,
-               [_SC_SEM_VALUE_MAX] = OFLOW,
+               [_SC_SEM_VALUE_MAX] = JT_SEM_VALUE_MAX,
                [_SC_SIGQUEUE_MAX] = -1,
                [_SC_TIMER_MAX] = -1,
                [_SC_BC_BASE_MAX] = _POSIX2_BC_BASE_MAX,
@@ -97,8 +103,8 @@ long sysconf(int name)
                [_SC_THREAD_PRIO_INHERIT] = -1,
                [_SC_THREAD_PRIO_PROTECT] = -1,
                [_SC_THREAD_PROCESS_SHARED] = VER,
-               [_SC_NPROCESSORS_CONF] = CPUCNT,
-               [_SC_NPROCESSORS_ONLN] = CPUCNT,
+               [_SC_NPROCESSORS_CONF] = JT_NPROCESSORS_CONF,
+               [_SC_NPROCESSORS_ONLN] = JT_NPROCESSORS_ONLN,
                [_SC_PHYS_PAGES] = -1,
                [_SC_AVPHYS_PAGES] = -1,
                [_SC_ATEXIT_MAX] = -1,
@@ -215,28 +221,37 @@ long sysconf(int name)
                [_SC_THREAD_ROBUST_PRIO_INHERIT] = -1,
                [_SC_THREAD_ROBUST_PRIO_PROTECT] = -1,
        };
+
        if (name > sizeof(values)/sizeof(values[0])) {
                errno = EINVAL;
                return -1;
-       } else if (values[name] == VER) {
+       } else if (values[name] >= -1) {
+               return values[name];
+       } else if (values[name] < -256) {
+               long lim[2];
+               __syscall(SYS_getrlimit, values[name]&16383, lim);
+               return lim[0] < 0 ? LONG_MAX : lim[0];
+       }
+
+       switch ((unsigned char)values[name]) {
+       case VER & 255:
                return _POSIX_VERSION;
-       } else if (values[name] == OFLOW) {
-               if (name == _SC_ARG_MAX) return ARG_MAX;
-               if (name == _SC_SEM_VALUE_MAX) return SEM_VALUE_MAX;
-               if (name == _SC_MQ_PRIO_MAX) return MQ_PRIO_MAX;
-               /* name == _SC_PAGE_SIZE */
+       case JT_ARG_MAX & 255:
+               return ARG_MAX;
+       case JT_MQ_PRIO_MAX & 255:
+               return MQ_PRIO_MAX;
+       case JT_PAGE_SIZE & 255:
                return PAGE_SIZE;
-       } else if (values[name] == CPUCNT) {
+       case JT_SEM_VALUE_MAX & 255:
+               return SEM_VALUE_MAX;
+       case JT_NPROCESSORS_CONF & 255:
+       case JT_NPROCESSORS_ONLN & 255: ;
                unsigned char set[128] = {1};
                int i, cnt;
                __syscall(SYS_sched_getaffinity, 0, sizeof set, set);
                for (i=cnt=0; i<sizeof set; i++)
                        for (; set[i]; set[i]&=set[i]-1, cnt++);
                return cnt;
-       } else if (values[name] < OFLOW) {
-               long lim[2];
-               __syscall(SYS_getrlimit, values[name]&16383, lim);
-               return lim[0] < 0 ? LONG_MAX : lim[0];
        }
        return values[name];
 }