Make dlmalloc a little faster

This change also documents the libc arena allocator.

examples/hello4.c

@@ -0,0 +1,17 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to this file,    │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include "libc/math.h"
+#include "libc/stdio/stdio.h"
+
+int main(int argc, char *argv[]) {
+  volatile double x = 123;
+  printf("cos(%g) is %g\n", x, cos(x));
+  return 0;
+}

libc/mem/arena.c

@@ -126,7 +126,7 @@ static dontinline bool __arena_grow(size_t offset, size_t request) {
   return false;
 }
 
-static void *__arena_alloc(size_t a, size_t n) {
+static inline void *__arena_alloc(size_t a, size_t n) {
   size_t o;
   if (!n) n = 1;
   o = ROUNDUP(__arena.offset[__arena.depth] + sizeof(size_t), a);
@@ -299,6 +299,25 @@ static void __arena_init(void) {
   atexit(__arena_destroy);
 }
 
+/**
+ * Pushes memory arena.
+ *
+ * This allocator gives a ~3x performance boost over dlmalloc, mostly
+ * because it isn't thread safe and it doesn't do defragmentation.
+ *
+ * Calling this function will push a new arena. It may be called
+ * multiple times from the main thread recursively. The first time it's
+ * called, it hooks all the regular memory allocation functions. Any
+ * allocations that were made previously outside the arena, will be
+ * passed on to the previous hooks. Then, the basic idea, is rather than
+ * bothering with free() you can just call __arena_pop() to bulk free.
+ *
+ * Arena allocations also have a slight size advantage, since 32-bit
+ * pointers are always used. The maximum amount of arena memory is
+ * 805,175,296 bytes.
+ *
+ * @see __arena_pop()
+ */
 void __arena_push(void) {
   if (UNLIKELY(!__arena.once)) {
     __arena_init();
@@ -313,6 +332,15 @@ void __arena_push(void) {
   ++__arena.depth;
 }
 
+/**
+ * Pops memory arena.
+ *
+ * This pops the most recently created arena, freeing all the memory
+ * that was allocated between the push and pop arena calls. If this is
+ * the last arena on the stack, then the old malloc hooks are restored.
+ *
+ * @see __arena_push()
+ */
 void __arena_pop(void) {
   size_t a, b, greed;
   __arena_check();

libc/x/xloadzd.c

@@ -20,6 +20,7 @@
 #include "libc/fmt/leb128.h"
 #include "libc/intrin/lockcmpxchg.h"
 #include "libc/nexgen32e/crc32.h"
+#include "libc/runtime/internal.h"
 #include "libc/runtime/runtime.h"
 #include "libc/x/x.h"
 #include "third_party/zlib/zlib.h"
@@ -47,16 +48,7 @@ void *xloadzd(bool *o, void **t, const void *p, size_t n, size_t m, size_t c,
   int64_t x, y;
   assert(z == 2 || z == 4);
   b = q = malloc(m);
-  zs.zfree = 0;
+  __inflate(q, m, p, n);
-  zs.zalloc = 0;
-  zs.next_in = p;
-  zs.avail_in = n;
-  zs.total_in = n;
-  zs.avail_out = m;
-  zs.total_out = m;
-  zs.next_out = (void *)q;
-  inflateInit2(&zs, -MAX_WBITS);
-  inflate(&zs, Z_NO_FLUSH);
   r = memalign(z, c * z);
   for (x = i = 0; i < c; ++i) {
     b += unzleb64(b, 10, &y);

third_party/dlmalloc/dlmalloc.c

@@ -1,4 +1,5 @@
 #include "libc/assert.h"
+#include "libc/bits/likely.h"
 #include "libc/bits/weaken.h"
 #include "libc/calls/calls.h"
 #include "libc/dce.h"
@@ -22,7 +23,7 @@
 #define HAVE_MMAP 1
 #define HAVE_MREMAP 0
 #define HAVE_MORECORE 0
-#define USE_LOCKS 1
+#define USE_SPIN_LOCKS 1
 #define MORECORE_CONTIGUOUS 0
 #define MALLOC_INSPECT_ALL 1
 
@@ -820,12 +821,7 @@ void dlfree(void* mem) {
 void* dlcalloc(size_t n_elements, size_t elem_size) {
   void* mem;
   size_t req = 0;
-  if (n_elements != 0) {
+  if (__builtin_mul_overflow(n_elements, elem_size, &req)) req = -1;
-    req = n_elements * elem_size;
-    if (((n_elements | elem_size) & ~(size_t)0xffff) &&
-        (req / n_elements != elem_size))
-      req = MAX_SIZE_T; /* force downstream failure on overflow */
-  }
   mem = dlmalloc(req);
   if (mem != 0 && calloc_must_clear(mem2chunk(mem)))
     bzero(mem, req);
@@ -1216,7 +1212,7 @@ void* dlrealloc(void* oldmem, size_t bytes) {
   if (oldmem == 0) {
     mem = dlmalloc(bytes);
   }
-  else if (bytes >= MAX_REQUEST) {
+  else if (UNLIKELY(bytes >= MAX_REQUEST)) {
     MALLOC_FAILURE_ACTION;
   }
 #ifdef REALLOC_ZERO_BYTES_FREES

third_party/dlmalloc/dlmalloc.mk

@@ -50,6 +50,13 @@ $(THIRD_PARTY_DLMALLOC_A).pkg:					\
 		$(THIRD_PARTY_DLMALLOC_A_OBJS)			\
 		$(foreach x,$(THIRD_PARTY_DLMALLOC_A_DIRECTDEPS),$($(x)_A).pkg)
 
+# README file recommends -O3
+# It does double performance in default mode
+o//third_party/dlmalloc/dlmalloc.o				\
+o/rel/third_party/dlmalloc/dlmalloc.o:				\
+		OVERRIDE_CFLAGS +=				\
+			-O3
+
 # we can't use address sanitizer because:
 #   address sanitizer depends on dlmalloc
 o/$(MODE)/third_party/dlmalloc/dlmalloc.o:			\