rts: Fix AdjustorPool bitmap manipulation

Previously the implementation of bitmap_first_unset assumed that
`__builtin_clz` would accept `uint8_t` however it apparently rather
extends its argument to `unsigned int`.

To fix this we simply revert to a naive implementation since handling
the various corner cases with `clz` is quite tricky. This should be
fine given that AdjustorPool isn't particularly hot. Ideally we would
have a single, optimised bitmap implementation in the RTS but I'll leave
this for future work.

Fixes #21838.

(cherry picked from commit dfa95eccf7b45a8961e034710d5e3ca45a36bf5e)

1 files changed, 14 insertions, 14 deletions

diff --git a/rts/adjustor/AdjustorPool.c b/rts/adjustor/AdjustorPool.c
index 9458c67246..146afc5b40 100644
--- a/rts/adjustor/AdjustorPool.c
+++ b/rts/adjustor/AdjustorPool.c
@@ -167,20 +167,6 @@ new_adjustor_pool(
     return pool;
 }
 
-/* Return the index of the first unset bit of the given bitmap or
- * length_in_bits. */
-static size_t
-bitmap_first_unset(uint8_t *bitmap, size_t length_in_bits, size_t start_idx)
-{
-    for (size_t i = start_idx; i < length_in_bits; i += 8) {
-        uint8_t x = bitmap[i / 8];
-        if (x != 0xff) {
-            return i + __builtin_clz(~x);
-        }
-    }
-    return length_in_bits;
-}
-
 static void
 bitmap_set(uint8_t *bitmap, size_t idx, bool value)
 {
@@ -202,6 +188,19 @@ bitmap_get(uint8_t *bitmap, size_t idx)
     return bitmap[word_n] & bit;
 }
 
+/* Return the index of the first unset bit of the given bitmap or
+ * length_in_bits if all bits are set. */
+static size_t
+bitmap_first_unset(uint8_t *bitmap, size_t length_in_bits, size_t start_idx)
+{
+    for (size_t i = start_idx; i < length_in_bits; i ++) {
+        if (bitmap_get(bitmap, i) == 0) {
+            return i;
+        }
+    }
+    return length_in_bits;
+}
+
 static void *
 get_context(struct AdjustorChunk *chunk, size_t slot_idx)
 {
@@ -224,6 +223,7 @@ alloc_adjustor(struct AdjustorPool *pool, void *context)
     chunk = pool->free_list;
     slot_idx = chunk->first_free;
     ASSERT(slot_idx < pool->chunk_slots);
+    ASSERT(bitmap_get(chunk->slot_bitmap, slot_idx) == 0);
     bitmap_set(chunk->slot_bitmap, slot_idx, 1);
 
     // advance first_free