Re: [PATCH v2 01/19] add strbuf_set operations

Michael,

On Wed, Jun 11, 2014 at 02:05:35PM +0200, Michael Haggerty wrote:

On 06/10/2014 12:19 AM, Jeremiah Mahler wrote:

quoted

Currently, the data in a strbuf is modified using add operations.  To
set the buffer to some data a reset must be performed before an add.

...

quoted

diff --git a/strbuf.c b/strbuf.c
index ac62982..9d64b00 100644
--- a/strbuf.c
+++ b/strbuf.c

@@ -189,6 +189,27 @@ void strbuf_splice(struct strbuf *sb, size_t pos, size_t len,
 	strbuf_setlen(sb, sb->len + dlen - len);
 }
 
+void strbuf_set(struct strbuf *sb, const void *data, size_t len)
+{
+	strbuf_reset(sb);
+	strbuf_add(sb, data, len);
+}
+

I never know how much intelligence to attribute to modern compilers, but
it seems like even after optimization this function will be doing more
work than necessary:

    strbuf_reset(sb)
    -> strbuf_setlen(sb, 0)
       -> sb->len = 0
       -> sb->buf[len] = '\0'
    -> strbuf_add(sb, data, len)
       -> strbuf_grow(sb, len)
          -> ...lots of stuff...
       -> memcpy(sb->buf + sb->len, data, len)
       -> strbuf_setlen(sb, sb->len + len)
          -> sb->len = len
          -> sb->buf[len] = '\0'

If there were a function like strbuf_grow_to(sb, len):

void strbuf_grow_to(struct strbuf *sb, size_t len)
{
	int new_buf = !sb->alloc;
	if (unsigned_add_overflows(len, 1))
		die("you want to use way too much memory");
	if (new_buf)
		sb->buf = NULL;
	ALLOC_GROW(sb->buf, len + 1, sb->alloc);
	if (new_buf)
		sb->buf[0] = '\0';
}

grow_to() which could reduce in size, interesting.

(strbuf_grow() could call it:

static inline void strbuf_grow(struct strbuf *sb, size_t extra)
{
	if (unsigned_add_overflows(sb->len, extra))
		die("you want to use way too much memory");
	strbuf_grow_to(sb, sb->len + extra);
}

), then your function could be minimized to

void strbuf_set(struct strbuf *sb, const void *data, size_t len)
{
	strbuf_grow_to(sb, len);
	memcpy(sb->buf, data, len);
	strbuf_setlen(sb, len);
}

I think strbuf_grow_to() would be useful in other situations too.

This is just an idea; I'm not saying that you have to make this change.

I like your idea.  I am leaning towards doing the un-optimized
strbuf_set operations first, then optimizing in a later patch.

quoted

[...]

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/

-- 
Jeremiah Mahler
jmmahler@gmail.com
http://github.com/jmahler