Quick and dirty stack allocation

casperisfine · Oct 29, 2024 · b8db61b · b8db61b
1 parent 5b4b24e
commit b8db61b
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Showing 2 changed files with 29 additions and 4 deletions.
diff --git a/ext/json/ext/fbuffer/fbuffer.h b/ext/json/ext/fbuffer/fbuffer.h
@@ -9,6 +9,8 @@ typedef struct FBufferStruct {
     char *ptr;
     unsigned long len;
     unsigned long capa;
+    char on_stack;
+    char spilled;
 } FBuffer;
 
 #define FBUFFER_INITIAL_LENGTH_DEFAULT 1024
@@ -43,13 +45,14 @@ static FBuffer *fbuffer_alloc(unsigned long initial_length)
     fb = ALLOC(FBuffer);
     memset((void *) fb, 0, sizeof(FBuffer));
     fb->initial_length = initial_length;
+    fb->spilled = TRUE;
     return fb;
 }
 
 static void fbuffer_free(FBuffer *fb)
 {
-    if (fb->ptr) ruby_xfree(fb->ptr);
-    ruby_xfree(fb);
+    if (fb->ptr && fb->spilled) ruby_xfree(fb->ptr);
+    if (!fb->on_stack) ruby_xfree(fb);
 }
 
 #ifndef JSON_GENERATOR
@@ -72,7 +75,14 @@ static inline void fbuffer_inc_capa(FBuffer *fb, unsigned long requested)
         for (required = fb->capa; requested > required - fb->len; required <<= 1);
 
         if (required > fb->capa) {
-            REALLOC_N(fb->ptr, char, required);
+            if (fb->spilled) {
+                REALLOC_N(fb->ptr, char, required);
+            } else {
+                const char *old_buffer = fb->ptr;
+                fb->ptr = ALLOC_N(char, required);
+                fb->spilled = true;
+                MEMCPY(fb->ptr, old_buffer, char, fb->len);
+            }
             fb->capa = required;
         }
     }

diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
@@ -946,9 +946,24 @@ static VALUE generate_json_rescue(VALUE d, VALUE exc)
     return Qundef;
 }
 
+struct StackFBuffer {
+    FBuffer fbuffer;
+    char stack_buffer[FBUFFER_INITIAL_LENGTH_DEFAULT - sizeof(FBuffer)];
+};
+
 static VALUE cState_partial_generate(VALUE self, VALUE obj)
 {
-    FBuffer *buffer = cState_prepare_buffer(self);
+    struct StackFBuffer stack_buffer;
+    FBuffer *buffer = &stack_buffer.fbuffer;
+    // FBuffer *buffer = alloca(FBUFFER_INITIAL_LENGTH_DEFAULT); // alloca cause crashes?
+    buffer->on_stack = TRUE;
+    buffer->spilled = FALSE;
+    buffer->initial_length = buffer->capa = (FBUFFER_INITIAL_LENGTH_DEFAULT - sizeof(FBuffer));
+    buffer->len = 0;
+    buffer->ptr = stack_buffer.stack_buffer;
+
+    // fprintf(stderr, "alloca(%ld)\n", FBUFFER_INITIAL_LENGTH_DEFAULT);
+    // FBuffer *buffer = cState_prepare_buffer(self);
     GET_STATE(self);
 
     struct generate_json_data data = {