Malloc cleanups and comments (#18257)

Author: juj

system/include/compat/malloc.h

@@ -8,7 +8,7 @@ extern "C" {
 #endif
 
 /* This version of struct mallinfo must match the one in
-   system/lib/dlmallo.c.  */
+   system/lib/dlmalloc.c.  */
 
 struct mallinfo {
   int arena;    /* total space allocated from system */

system/lib/emmalloc.c

@@ -228,7 +228,11 @@ static int compute_free_list_bucket(size_t allocSize)
 {
   if (allocSize < 128) return (allocSize >> 3) - 1;
   int clz = __builtin_clz(allocSize);
-  int bucketIndex = (clz > 19) ? 110 - (clz<<2) + ((allocSize >> (29-clz)) ^ 4) : MIN(71 - (clz<<1) + ((allocSize >> (30-clz)) ^ 2), NUM_FREE_BUCKETS-1);
+  int bucketIndex =
+    (clz > 19)
+      ?     110 - (clz<<2) + ((allocSize >> (29-clz)) ^ 4)
+      : MIN( 71 - (clz<<1) + ((allocSize >> (30-clz)) ^ 2), NUM_FREE_BUCKETS-1);
+
   assert(bucketIndex >= 0);
   assert(bucketIndex < NUM_FREE_BUCKETS);
   return bucketIndex;
@@ -584,7 +588,7 @@ static void *attempt_allocate(Region *freeRegion, size_t alignment, size_t size)
   // from the list of free regions: whatever slop remains will be later added back to the free region pool.
   unlink_from_free_list(freeRegion);
 
-  // Before we proceed further, fix up the boundary of this region and the region that precedes this one,
+  // Before we proceed further, fix up the boundary between this and the preceding region,
   // so that the boundary between the two regions happens at a right spot for the payload to be aligned.
   if (payloadStartPtr != payloadStartPtrAligned)
   {
@@ -739,8 +743,7 @@ static void *allocate_memory(size_t alignment, size_t size)
       bucketMask ^= 1;
     }
     // Instead of recomputing bucketMask from scratch at the end of each loop, it is updated as we go,
-    // to avoid undefined behavior with (x >> 32)/(x >> 64) when bucketIndex reaches 32/64, (the shift would comes out as a no-op instead of 0).
-
+    // to avoid undefined behavior with (x >> 32)/(x >> 64) when bucketIndex reaches 32/64, (the shift would come out as a no-op instead of 0).
     assert((bucketIndex == NUM_FREE_BUCKETS && bucketMask == 0) || (bucketMask == freeRegionBucketsUsed >> bucketIndex));
   }
 
@@ -753,6 +756,7 @@ static void *allocate_memory(size_t alignment, size_t size)
   int largestBucketIndex = NUM_FREE_BUCKETS - 1 - __builtin_clzll(freeRegionBucketsUsed);
   // freeRegion will be null if there is absolutely no memory left. (all buckets are 100% used)
   Region *freeRegion = freeRegionBucketsUsed ? freeRegionBuckets[largestBucketIndex].next : 0;
+  // The 30 first free region buckets cover memory blocks < 2048 bytes, so skip looking at those here (too small)
   if (freeRegionBucketsUsed >> 30)
   {
     // Look only at a constant number of regions in this bucket max, to avoid bad worst case behavior.