Save a couple loads/stores in sweep pages (JuliaLang#49263)

We did a load/store on every iteration. Keep a temporary in a register
instead. It's a very small difference but it's visible in vtune.

src/gc-debug.c

@@ -563,11 +563,11 @@ JL_NO_ASAN static void gc_scrub_range(char *low, char *high)
         // Find the age bit
         char *page_begin = gc_page_data(tag) + GC_PAGE_OFFSET;
         int obj_id = (((char*)tag) - page_begin) / osize;
-        uint8_t *ages = pg->ages + obj_id / 8;
+        uint32_t *ages = pg->ages + obj_id / 32;
         // Force this to be a young object to save some memory
         // (especially on 32bit where it's more likely to have pointer-like
         //  bit patterns)
-        *ages &= ~(1 << (obj_id % 8));
+        *ages &= ~(1 << (obj_id % 32));
         memset(tag, 0xff, osize);
         // set mark to GC_MARKED (young and marked)
         tag->bits.gc = GC_MARKED;

src/gc.c

@@ -976,8 +976,8 @@ STATIC_INLINE void gc_setmark_pool_(jl_ptls_t ptls, jl_taggedvalue_t *o,
             page->has_young = 1;
             char *page_begin = gc_page_data(o) + GC_PAGE_OFFSET;
             int obj_id = (((char*)o) - page_begin) / page->osize;
-            uint8_t *ages = page->ages + obj_id / 8;
-            jl_atomic_fetch_and_relaxed((_Atomic(uint8_t)*)ages, ~(1 << (obj_id % 8)));
+            uint32_t *ages = page->ages + obj_id / 32;
+            jl_atomic_fetch_and_relaxed((_Atomic(uint32_t)*)ages, ~(1 << (obj_id % 32)));
         }
     }
     objprofile_count(jl_typeof(jl_valueof(o)),
@@ -1406,7 +1406,7 @@ static NOINLINE jl_taggedvalue_t *add_page(jl_gc_pool_t *p) JL_NOTSAFEPOINT
     jl_ptls_t ptls = jl_current_task->ptls;
     jl_gc_pagemeta_t *pg = jl_gc_alloc_page();
     pg->osize = p->osize;
-    pg->ages = (uint8_t*)malloc_s(GC_PAGE_SZ / 8 / p->osize + 1);
+    pg->ages = (uint32_t*)malloc_s(LLT_ALIGN(GC_PAGE_SZ / 8 / p->osize + 1, sizeof(uint32_t)));
     pg->thread_n = ptls->tid;
     jl_taggedvalue_t *fl = reset_page(ptls, p, pg, NULL);
     p->newpages = fl;
@@ -1506,7 +1506,7 @@ int64_t lazy_freed_pages = 0;
 static jl_taggedvalue_t **sweep_page(jl_gc_pool_t *p, jl_gc_pagemeta_t *pg, jl_taggedvalue_t **pfl, int sweep_full, int osize) JL_NOTSAFEPOINT
 {
     char *data = pg->data;
-    uint8_t *ages = pg->ages;
+    uint32_t *ages = pg->ages;
     jl_taggedvalue_t *v = (jl_taggedvalue_t*)(data + GC_PAGE_OFFSET);
     char *lim = (char*)v + GC_PAGE_SZ - GC_PAGE_OFFSET - osize;
     size_t old_nfree = pg->nfree;
@@ -1557,18 +1557,25 @@ static jl_taggedvalue_t **sweep_page(jl_gc_pool_t *p, jl_gc_pagemeta_t *pg, jl_t
         int16_t prev_nold = 0;
         int pg_nfree = 0;
         jl_taggedvalue_t **pfl_begin = NULL;
-        uint8_t msk = 1; // mask for the age bit in the current age byte
+        uint32_t msk = 1; // mask for the age bit in the current age byte
+        uint32_t age = *ages;
         while ((char*)v <= lim) {
+            if (!msk) {
+                msk = 1;
+                *ages = age;
+                ages++;
+                age = *ages;
+            }
             int bits = v->bits.gc;
             if (!gc_marked(bits)) {
                 *pfl = v;
                 pfl = &v->next;
                 pfl_begin = pfl_begin ? pfl_begin : pfl;
                 pg_nfree++;
-                *ages &= ~msk;
+                age &= ~msk;
             }
             else { // marked young or old
-                if (*ages & msk || bits == GC_OLD_MARKED) { // old enough
+                if (age & msk || bits == GC_OLD_MARKED) { // old enough
                     // `!age && bits == GC_OLD_MARKED` is possible for
                     // non-first-class objects like array buffers
                     // (they may get promoted by jl_gc_wb_buf for example,
@@ -1584,17 +1591,13 @@ static jl_taggedvalue_t **sweep_page(jl_gc_pool_t *p, jl_gc_pagemeta_t *pg, jl_t
                     has_young = 1;
                 }
                 has_marked |= gc_marked(bits);
-                *ages |= msk;
+                age |= msk;
                 freedall = 0;
             }
             v = (jl_taggedvalue_t*)((char*)v + osize);
             msk <<= 1;
-            if (!msk) {
-                msk = 1;
-                ages++;
-            }
         }
-
+        *ages = age;
         assert(!freedall);
         pg->has_marked = has_marked;
         pg->has_young = has_young;
@@ -4017,7 +4020,7 @@ JL_DLLEXPORT jl_value_t *jl_gc_internal_obj_base_ptr(void *p)
             goto valid_object;
         // We know now that the age bit reflects liveness status during
         // the last sweep and that the cell has not been reused since.
-        if (!(meta->ages[obj_id / 8] & (1 << (obj_id % 8)))) {
+        if (!(meta->ages[obj_id / 32] & (1 << (obj_id % 32)))) {
             return NULL;
         }
         // Not a freelist entry, therefore a valid object.

src/gc.h

@@ -9,6 +9,7 @@
 #ifndef JL_GC_H
 #define JL_GC_H
 
+#include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
 #include <strings.h>
@@ -170,7 +171,7 @@ typedef struct {
     uint16_t fl_end_offset;   // offset of last free object in this page
     uint16_t thread_n;        // thread id of the heap that owns this page
     char *data;
-    uint8_t *ages;
+    uint32_t *ages;
 } jl_gc_pagemeta_t;
 
 // Page layout: