[PATCH] Replaced loop with __builtin_ctzl for detecting first 0 in bitnamp

[goldstein.w.n at gmail.com]

# HG changeset patch
# User Noah Goldstein <goldstein.w.n at gmail.com>
# Date 1606497081 18000
#      Fri Nov 27 12:11:21 2020 -0500
# Node ID 7ec2fc7b29d6614df28152dd4a895e6139138890
# Parent  90cc7194e993f8d722347e9f46a00f65dffc3935
Replaced loop with __builtin_ctzl for detecting first 0 in bitnamp
No particular pressing reason for this change other than the performance benefit it yields. Converts a loop that could take 63 iterations (and had a branch) to 5 instructions

diff -r 90cc7194e993 -r 7ec2fc7b29d6 src/core/ngx_slab.c
--- a/src/core/ngx_slab.c	Fri Nov 27 00:01:20 2020 +0300
+++ b/src/core/ngx_slab.c	Fri Nov 27 12:11:21 2020 -0500
@@ -235,36 +235,33 @@
 
                 if (bitmap[n] != NGX_SLAB_BUSY) {
 
-                    for (m = 1, i = 0; m; m <<= 1, i++) {
-                        if (bitmap[n] & m) {
-                            continue;
+                    m = ~bitmap[n];
+                    i = __builtin_ctzl(m);
+
+                    bitmap[n] = ~(m & (m - 1));
+
+                    i = (n * 8 * sizeof(uintptr_t) + i) << shift;
+
+                    p = (uintptr_t) bitmap + i;
+
+                    pool->stats[slot].used++;
+
+                    if (bitmap[n] == NGX_SLAB_BUSY) {
+                        for (n = n + 1; n < map; n++) {
+                            if (bitmap[n] != NGX_SLAB_BUSY) {
+                                goto done;
+                            }
                         }
 
-                        bitmap[n] |= m;
-
-                        i = (n * 8 * sizeof(uintptr_t) + i) << shift;
-
-                        p = (uintptr_t) bitmap + i;
-
-                        pool->stats[slot].used++;
+                        prev = ngx_slab_page_prev(page);
+                        prev->next = page->next;
+                        page->next->prev = page->prev;
 
-                        if (bitmap[n] == NGX_SLAB_BUSY) {
-                            for (n = n + 1; n < map; n++) {
-                                if (bitmap[n] != NGX_SLAB_BUSY) {
-                                    goto done;
-                                }
-                            }
+                        page->next = NULL;
+                        page->prev = NGX_SLAB_SMALL;
+                    }
 
-                            prev = ngx_slab_page_prev(page);
-                            prev->next = page->next;
-                            page->next->prev = page->prev;
-
-                            page->next = NULL;
-                            page->prev = NGX_SLAB_SMALL;
-                        }
-
-                        goto done;
-                    }
+                    goto done;
                 }
             }