Use bitwise test instead of popcount to check if one bit is set

We don't need to determine the total number of bits set to determine if
exactly one is set.

Additionally, on x86_64 without any -march=* flag, __builtin_popcount
will get compiled to a function call to the compiler runtime (on gcc),
or a long sequence of bit operations (on clang).

Signed-off-by: Michael Forney <mforney@mforney.org>

configure.ac

@@ -77,7 +77,6 @@ AS_IF([test "x$ac_cv_func_secure_getenv" = xno -a \
 ])
 
 AX_GCC_BUILTIN(__builtin_expect)
-AX_GCC_BUILTIN(__builtin_popcount)
 
 # Some tests use Linux-specific headers
 AC_CHECK_HEADER([linux/input.h])

meson.build

@@ -76,9 +76,6 @@ endif
 if cc.links('int main(){if(__builtin_expect(1<0,0)){}}', name: '__builtin_expect')
     configh_data.set('HAVE___BUILTIN_EXPECT', 1)
 endif
-if cc.links('int main(){__builtin_popcount(1);}', name: '__builtin_popcount')
-    configh_data.set('HAVE___BUILTIN_POPCOUNT', 1)
-endif
 if cc.has_header_symbol('unistd.h', 'eaccess', prefix: '#define _GNU_SOURCE')
     configh_data.set('HAVE_EACCESS', 1)
 endif

src/state.c

@@ -1373,7 +1373,7 @@ key_get_consumed(struct xkb_state *state, const struct xkb_key *key,
             if (XkbLevelsSameSyms(level, no_mods_level))
                 continue;
 
-            if (entry == matching_entry || my_popcount(entry->mods.mask) == 1)
+            if (entry == matching_entry || one_bit_set(entry->mods.mask))
                 consumed |= entry->mods.mask & ~entry->preserve.mask;
         }
         break;

src/utils.h

@@ -186,18 +186,10 @@ msb_pos(uint32_t mask)
     return pos;
 }
 
-// Avoid conflict with other popcount()s.
 static inline int
-my_popcount(uint32_t x)
+one_bit_set(uint32_t x)
 {
-    int count;
-#if defined(HAVE___BUILTIN_POPCOUNT)
-    count = __builtin_popcount(x);
-#else
-    for (count = 0; x; count++)
-        x &= x - 1;
-#endif
-    return count;
+    return x && (x & (x - 1)) == 0;
 }
 
 bool