[PATCH v2] malloc: send freed small chunks to smallbin

[k4lizen <k4lizen@proton.me>]

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Large chunks get added to the unsorted bin since
sorting them takes time, for small chunks the
benefit of adding them to the unsorted bin is
non-existant, actually hurting performance.

Splitting and malloc_consolidate still add small
chunks to unsorted, but we can hint the compiler
that that is a relatively rare occurance.
Benchmarking shows this to be consistently good.
---
malloc/malloc.c | 59 +++++++++++++++++++++++++++++++++----------------
1 file changed, 40 insertions(+), 19 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index bcb6e5b83c..ad77cd083e 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -4156,9 +4156,9 @@ _int_malloc (mstate av, size_t bytes)
#endif
}

- /* place chunk in bin */
-
- if (in_smallbin_range (size))
+ /* Place chunk in bin. Only malloc_consolidate() and splitting can put
+ small chunks into the unsorted bin. */
+ if (__glibc_unlikely (in_smallbin_range (size)))
{
victim_index = smallbin_index (size);
bck = bin_at (av, victim_index);
@@ -4723,23 +4723,45 @@ _int_free_create_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size,
} else
clear_inuse_bit_at_offset(nextchunk, 0);

- /*
- Place the chunk in unsorted chunk list. Chunks are
- not placed into regular bins until after they have
- been given one chance to be used in malloc.
- */
+ mchunkptr bck, fwd;

- mchunkptr bck = unsorted_chunks (av);
- mchunkptr fwd = bck->fd;
- if (__glibc_unlikely (fwd->bk != bck))
- malloc_printerr ("free(): corrupted unsorted chunks");
- p->fd = fwd;
+ if(!in_smallbin_range (size))
+ {
+ /*
+ Place large chunks in unsorted chunk list. Large chunks are
+ not placed into regular bins until after they have
+ been given one chance to be used in malloc.
+
+ This branch is first in the if-statement to help branch
+ prediction on consecutive adjacent frees.
+ */
+
+ bck = unsorted_chunks (av);
+ fwd = bck->fd;
+ if (__glibc_unlikely (fwd->bk != bck))
+ malloc_printerr ("free(): corrupted unsorted chunks");
+ p->fd_nextsize = NULL;
+ p->bk_nextsize = NULL;
+ }
+ else
+ {
+ /*
+ Place small chunks directly in their smallbin, so they
+ don't pollute the unsorted bin.
+ */
+
+ int chunk_index = smallbin_index (size);
+ bck = bin_at (av, chunk_index);
+ fwd = bck->fd;
+
+ if (__glibc_unlikely (fwd->bk != bck))
+ malloc_printerr ("free(): chunks in smallbin corrupted");
+
+ mark_bin (av, chunk_index);
+ }
+
p->bk = bck;
- if (!in_smallbin_range(size))
- {
- p->fd_nextsize = NULL;
- p->bk_nextsize = NULL;
- }
+ p->fd = fwd;
bck->fd = p;
fwd->bk = p;

@@ -4748,7 +4770,6 @@ _int_free_create_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size,

check_free_chunk(av, p);
}
-
else
{
/* If the chunk borders the current high end of memory,
--
2.45.2