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shared: explicitly implement binary search in NM_UTILS_STRING_TABLE_LOOKUP_DEFINE*()

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When looking at nm_utils_array_find_binary_search(), we see that binary
search really isn't complicated. In nm_utils_array_find_binary_search()
it looks complicated, because that is our general purpose function which
accepts arbitrary lists, uses an opaque compare function, accepts a user
data argument, and returns the insertion position.

This is unnecessary for the narrow purpose in NM_UTILS_STRING_TABLE_LOOKUP_DEFINE*().
When we inline the binary search, it can be simplified, and the remaining
parts is simple enough to prefer this duplication of binary search over
using our general purpose function.

Also, this gives the compiler more chance for optimization. For
example, we can use "unsigned" as index type instead of gssize, because
we know (at compile time), that this type will always be large enough
for our LIST. Also, we can directly call strcmp().

The result is that the macro's implementation is also fast in the best
case (where the needle is found with only one strcmp()) and in the cases
where there is a small number of items to search.
It thus alleviates concerns that using the macro might be slower than
an optimized implementation.

The binary size of the defined function increases slightly (from 112
bytes to 192 bytes, on x86_64, GCC, -O2). But according to my tests it
is slightly and measurably faster.
This commit is contained in:
Thomas Haller 2020-02-13 11:17:22 +01:00
parent 760551e3fc
commit 17d9b852c8
1 changed files with 22 additions and 11 deletions
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33
shared/nm-glib-aux/nm-shared-utils.h
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@ -1565,8 +1565,6 @@ fcn_name (const char *name) \
static gboolean checked = FALSE; \
int i; \
\
G_STATIC_ASSERT (G_N_ELEMENTS (LIST) > 1); \
\
if (!checked) { \
checked = TRUE; \
\
@ -1579,16 +1577,29 @@ fcn_name (const char *name) \
} \
\
if (G_LIKELY (name)) { \
gssize idx; \
G_STATIC_ASSERT (G_N_ELEMENTS (LIST) > 1); \
G_STATIC_ASSERT (G_N_ELEMENTS (LIST) < G_MAXUINT / 2u - 10u); \
unsigned imin = 0; \
unsigned imax = (G_N_ELEMENTS (LIST) - 1); \
unsigned imid = (G_N_ELEMENTS (LIST) - 1) / 2; \
\
idx = nm_utils_array_find_binary_search (LIST, \
sizeof (LIST[0]), \
G_N_ELEMENTS (LIST), \
&name, \
nm_strcmp_p_with_data, \
NULL); \
if (G_LIKELY (idx >= 0)) \
return LIST[idx].value; \
for (;;) { \
const int cmp = strcmp (LIST[imid].name, name); \
\
if (G_UNLIKELY (cmp == 0)) \
return LIST[imid].value; \
\
if (cmp < 0) \
imin = imid + 1u; \
else \
imax = imid - 1u; \
\
if (G_UNLIKELY (imin > imax)) \
break; \
\
/* integer overflow cannot happen, because LIST is shorter than G_MAXUINT/2. */ \
imid = (imin + imax) / 2u;\
} \
} \
\
{ unknown_val_cmd; } \