util/idalloc: add lowest_free_idx to avoid iterating from 0

lowest_free_idx is a conservative estimation of the lowest index
where a free id can be found.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6600>

src/util/u_idalloc.c

@@ -71,18 +71,21 @@ util_idalloc_alloc(struct util_idalloc *buf)
 {
    unsigned num_elements = buf->num_elements;
 
-   for (unsigned i = 0; i < num_elements / 32; i++) {
+   for (unsigned i = buf->lowest_free_idx; i < num_elements / 32; i++) {
       if (buf->data[i] == 0xffffffff)
          continue;
 
       unsigned bit = ffs(~buf->data[i]) - 1;
       buf->data[i] |= 1u << bit;
+      buf->lowest_free_idx = i;
       return i * 32 + bit;
    }
 
    /* No slots available, resize and return the first free. */
    util_idalloc_resize(buf, num_elements * 2);
 
+   buf->lowest_free_idx = num_elements / 32;
+
    buf->data[num_elements / 32] |= 1 << (num_elements % 32);
 
    return num_elements;
@@ -92,7 +95,9 @@ void
 util_idalloc_free(struct util_idalloc *buf, unsigned id)
 {
    assert(id < buf->num_elements);
-   buf->data[id / 32] &= ~(1 << (id % 32));
+   unsigned idx = id / 32;
+   buf->lowest_free_idx = MIN2(idx, buf->lowest_free_idx);
+   buf->data[idx] &= ~(1 << (id % 32));
 }
 
 void

src/util/u_idalloc.h

@@ -38,6 +38,7 @@ struct util_idalloc
 {
    uint32_t *data;
    unsigned num_elements;
+   unsigned lowest_free_idx;
 };
 
 void