From 743096a4964bf921f7b39cc7b739fd4fab6ad39c Mon Sep 17 00:00:00 2001
From: Stefan Eissing <stefan@eissing.org>
Date: Wed, 12 Aug 2026 15:49:39 +0200
Subject: [PATCH] uint-bset: add slot0 member

Add a fixed slot0 member for uint32_bset to have no allocations for
sizes < 64. This is a common use case for curl_easy_perform().

Closes #22561
---
 lib/multi.c           |  2 ++
 lib/uint-bset.c       | 21 +++++++++++++++------
 lib/uint-bset.h       |  1 +
 tests/unit/unit3211.c |  5 +++++
 4 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/lib/multi.c b/lib/multi.c
index dd4b9bfe8a..013aefc63f 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -432,6 +432,8 @@ static CURLMcode multi_xfers_add(struct Curl_multi *multi,
         /* make it a 64 multiple, since our bitsets grow by that and
          * small (easy_multi) grows to at least 64 on first resize. */
         new_size = (((used + min_unused) + 63) / 64) * 64;
+        if(new_size < 256) /* don't be too shy about it */
+          new_size = 256;
       }
     }
   }
diff --git a/lib/uint-bset.c b/lib/uint-bset.c
index 55aedb234b..f734f86ffd 100644
--- a/lib/uint-bset.c
+++ b/lib/uint-bset.c
@@ -44,14 +44,22 @@ CURLcode Curl_uint32_bset_resize(struct uint32_bset *bset, uint32_t nmax)
 
   DEBUGASSERT(bset->init == CURL_UINT32_BSET_MAGIC);
   if(nslots != bset->nslots) {
-    uint64_t *slots = curlx_calloc(nslots, sizeof(uint64_t));
-    if(!slots)
-      return CURLE_OUT_OF_MEMORY;
+    uint64_t *slots;
+    if(nslots > 1) {
+      slots = curlx_calloc(nslots, sizeof(uint64_t));
+      if(!slots)
+        return CURLE_OUT_OF_MEMORY;
+    }
+    else {
+      bset->slot0 = 0;
+      slots = &bset->slot0;
+    }
 
-    if(bset->slots) {
+    if((bset->slots != slots) && nslots && bset->nslots) {
       memcpy(slots, bset->slots,
              (CURLMIN(nslots, bset->nslots) * sizeof(uint64_t)));
-      curlx_free(bset->slots);
+      if(bset->slots != &bset->slot0)
+        curlx_free(bset->slots);
     }
     bset->slots = slots;
     bset->nslots = nslots;
@@ -63,7 +71,8 @@ CURLcode Curl_uint32_bset_resize(struct uint32_bset *bset, uint32_t nmax)
 void Curl_uint32_bset_destroy(struct uint32_bset *bset)
 {
   DEBUGASSERT(bset->init == CURL_UINT32_BSET_MAGIC);
-  curlx_free(bset->slots);
+  if(bset->slots != &bset->slot0)
+    curlx_free(bset->slots);
   memset(bset, 0, sizeof(*bset));
 }
 
diff --git a/lib/uint-bset.h b/lib/uint-bset.h
index ce0450c53c..d4e2972ad1 100644
--- a/lib/uint-bset.h
+++ b/lib/uint-bset.h
@@ -39,6 +39,7 @@
 
 struct uint32_bset {
   uint64_t *slots;
+  uint64_t slot0;
   uint32_t nslots;
   uint32_t first_slot_used;
 #ifdef DEBUGBUILD
diff --git a/tests/unit/unit3211.c b/tests/unit/unit3211.c
index 0d1dde200d..fea99301a1 100644
--- a/tests/unit/unit3211.c
+++ b/tests/unit/unit3211.c
@@ -126,6 +126,10 @@ static CURLcode test_unit3211(const char *arg)
 {
   UNITTEST_BEGIN_SIMPLE
 
+  static const uint32_t s0[] = {
+    /* spread numbers, some at slot edges */
+    0, 1, 4, 5, 8, 13, 17, 23, 24, 63,
+  };
   static const uint32_t s1[] = {
     /* spread numbers, some at slot edges */
     0, 1, 4, 17, 63, 64, 65, 66, 90, 99,
@@ -142,6 +146,7 @@ static CURLcode test_unit3211(const char *arg)
     120, 121, 122, 123, 124, 125, 126, 127,
   };
 
+  check_set("s0", 64, s0, CURL_ARRAYSIZE(s1));
   check_set("s1", 100, s1, CURL_ARRAYSIZE(s1));
   check_set("s2", 1000, s2, CURL_ARRAYSIZE(s2));