Re: [PATCH 1/2] strbuf: use st_add3() in strbuf_grow()

[Junio C Hamano <gitster@pobox.com>]

René Scharfe <l.s.r@web.de> writes:

> Simplify the code by calling st_add3() to do overflow checks instead of
> open-coding it.  This changes the error message to include the offending
> summands, which can be helpful when tracking down the cause.
>
> Signed-off-by: René Scharfe <l.s.r@web.de>
> ---
>  strbuf.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/strbuf.c b/strbuf.c
> index 3e04addc22..bb04d3910e 100644
> --- a/strbuf.c
> +++ b/strbuf.c
> @@ -106,12 +106,9 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
>  void strbuf_grow(struct strbuf *sb, size_t extra)
>  {
>  	int new_buf = !sb->alloc;
> -	if (unsigned_add_overflows(extra, 1) ||
> -	    unsigned_add_overflows(sb->len, extra + 1))
> -		die("you want to use way too much memory");
>  	if (new_buf)
>  		sb->buf = NULL;
> -	ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
> +	ALLOC_GROW(sb->buf, st_add3(sb->len, extra, 1), sb->alloc);

ALLOC_GROW() being a macro that references its second argument three
times, doesn't this rewrite rely on the compiler being clever enough
to notice that checking for the same overflow three times is
pointless and does it only once?  I guess the original has the same
issue already, so this may not be so bad but it makes me feel a bit
queasy.

>  	if (new_buf)
>  		sb->buf[0] = '\0';
>  }