From: Vladimír Čunát <vladimir.cunat@nic.cz>
Date: Mon, 9 Mar 2020 14:39:15 +0000 (+0100)
Subject: lib/generic/array: improve the growth strategy
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=db7024393c864c36e2885d3c81bb4f9eda0f11e9;p=thirdparty%2Fknot-resolver.git

lib/generic/array: improve the growth strategy

For large arrays it was linear, so huge amount of +1 would need
quadratic time together.  I've been hating that for a long time,
but now I finally have a use case where it makes a large difference.
The one from GCC looks good to me (theoretically) and it surely has
lots of practical deployment.

CI scan-build: I still have no idea about these array allocation
errors; I had just given up and believe they're false alarms.
---

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6cc339b58..1c870cebd 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -241,7 +241,7 @@ lint:scan-build:
   script:
     - export SCANBUILD="scan-build --status-bugs -no-failure-reports $(./scripts/get-scanbuild-args.sh)"
     - ninja -C build_ci* scan-build || true
-    - test "$(ls build_ci*/meson-logs/scanbuild/*/report-*.html | wc -l)" = 38 # we have this many errors ATM :-)
+    - test "$(ls build_ci*/meson-logs/scanbuild/*/report-*.html | wc -l)" = 43 # we have this many errors ATM :-)
 
 lint:tidy:
   <<: *test
diff --git a/lib/generic/array.h b/lib/generic/array.h
index e29a3fbe2..a3b2ba1bf 100644
--- a/lib/generic/array.h
+++ b/lib/generic/array.h
@@ -53,14 +53,25 @@
 #pragma once
 #include <stdlib.h>
 
-/** Simplified Qt containers growth strategy. */
-static inline size_t array_next_count(size_t want)
+/** Growth strategy agorithm from GCC libstdc++ (also GPL 3+)
+    https://github.com/gcc-mirror/gcc/blob/releases/gcc-9.2.0/gcc/vec.c#L159
+ */
+static inline size_t array_next_count(size_t alloc, size_t desired)
 {
-	if (want < 2048) {
-		return (want < 20) ? want + 4 : want * 2;
-	} else {
-		return want + 2048;
-	}
+	/* Exponential growth. */
+	if (!alloc)
+		alloc = 4;
+	else if (alloc < 16)
+		/* Double when small.  */
+		alloc = alloc * 2;
+	else
+		/* Grow slower when large.  */
+		alloc = (alloc * 3 / 2);
+
+	/* If this is still too small, set it to the right size. */
+	if (alloc < desired)
+		alloc = desired;
+	return alloc;
 }
 
 /** @internal Incremental memory reservation */
@@ -70,7 +81,7 @@ static inline int array_std_reserve(void *baton, char **mem, size_t elm_size, si
 		return 0;
 	}
 	/* Simplified Qt containers growth strategy */
-	size_t next_size = array_next_count(want);
+	size_t next_size = array_next_count(*have, want);
 	void *mem_new = realloc(*mem, next_size * elm_size);
 	if (mem_new != NULL) {
 		*mem = mem_new;
diff --git a/lib/utils.c b/lib/utils.c
index 36d2feee0..ae76a30f1 100644
--- a/lib/utils.c
+++ b/lib/utils.c
@@ -219,7 +219,7 @@ int kr_memreserve(void *baton, char **mem, size_t elm_size, size_t want, size_t
         return 0;
     } else {
         knot_mm_t *pool = baton;
-        size_t next_size = array_next_count(want);
+        size_t next_size = array_next_count(*have, want);
         void *mem_new = mm_alloc(pool, next_size * elm_size);
         if (mem_new != NULL) {
 	    if (*mem) { /* 0-length memcpy from NULL isn't technically OK */