From 7d254c81400128923d1952909bf22b1784e540fc Mon Sep 17 00:00:00 2001
From: Nicholas Noll <nbnoll@eml.cc>
Date: Tue, 21 Apr 2020 13:29:38 -0700
Subject: feat: added simple memory arena allocator to libn

---
 sys/libn/memory.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)

(limited to 'sys/libn/memory.c')

diff --git a/sys/libn/memory.c b/sys/libn/memory.c
index d3deef7..6b715d9 100644
--- a/sys/libn/memory.c
+++ b/sys/libn/memory.c
@@ -48,3 +48,92 @@ _bufpop(void *buf, int i, vlong eltsize)
     _bufHdr(buf)->len--;
 }
 
+// -------------------------------------------------------------------------
+// Arena allocator
+
+#define ARENA_ALIGN 8
+#define ARENA_BLOCK_SIZE 1024 * 1024
+
+#define ALIGN_DOWN(n, a) ((n) & ~((a)-1))
+#define ALIGN_UP(n, a) ALIGN_DOWN((n) + (a)-1, (a))
+#define ALIGN_DOWN_PTR(p, a) ((void*)ALIGN_DOWN((uintptr)(p), (a)))
+#define ALIGN_UP_PTR(p, a) ((void*)ALIGN_UP((uintptr)(p), (a)))
+
+struct Block 
+{
+    struct Block *next;
+    byte   buf[];
+};
+
+struct mem·Arena
+{
+    mem·Allocator heap;
+    byte  *off;
+    byte  *end;
+    struct Block *curr;
+    struct Block first;
+}; 
+
+mem·Arena*
+mem·newarena(mem·Allocator from)
+{
+    mem·Arena *a  = from.alloc(sizeof(*a) + ARENA_BLOCK_SIZE);
+    a->heap       = from;
+    a->off        = a->first.buf;
+    a->end        = a->first.buf + ARENA_BLOCK_SIZE;
+    a->curr       = &a->first;
+    a->first.next = nil;
+
+    return a;
+}
+
+void
+grow(mem·Arena *a, vlong min)
+{
+    uintptr size;
+    struct Block *blk;
+
+    size   = ALIGN_UP(MAX(min, ARENA_BLOCK_SIZE), ARENA_ALIGN);
+    blk    = a->heap.alloc(sizeof(*blk) + size);
+    a->off = blk->buf;
+    a->end = a->off + size;
+
+    Assert(a->curr->next == nil);
+    Assert(a->off == ALIGN_DOWN_PTR(a->off, ARENA_ALIGN));
+
+    a->curr->next = blk;
+    a->curr = blk;
+}
+
+void*
+mem·arenaalloc(mem·Arena *a, ulong size)
+{
+    void *ptr;
+    if (size > (ulong)(a->end - a->off)) {
+        grow(a, size);
+        Assert(size <= (uintptr)(a->end - a->off));
+    }
+
+    ptr    = a->off;
+    a->off = ALIGN_UP_PTR(a->off + size, ARENA_ALIGN);
+
+    Assert(a->off <= a->end);
+    Assert(ptr == ALIGN_DOWN_PTR(ptr, ARENA_ALIGN));
+
+    return ptr;
+}
+
+void
+mem·freearena(mem·Arena *a)
+{
+    struct Block *it, *next;
+
+    it = &a->first;
+    while (it != nil) {
+        next = it->next;
+        a->heap.free(it);
+        it = next;
+    }
+
+    a->heap.free(a);
+}
-- 
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