Subject: Re: [PATCH] Properly align memory allocations and temporary buffers

From: Junio C Hamano <[email-masked]>
Date: Fri, 07 Jan 2022 11:30:52 -0800

"brian m. carlson" <[email-masked]> writes:

> I'm not opposed to a small amount of finagling for this case, but I am
> very much opposed to defining your C ABI in an intentionally difficult
> way.

I was looking at git_qsort_s() today and thought that its use of
"char buf[1k]" on stack is something we would write for a system
without alloca().

We already have xalloca() wrapper defined in the compat-util header.
Perhaps it is a good idea to use it instead?

Both the true alloca() and xmalloc() we use as a fallback (when
<alloca.h> is not available) are supposed to return a block of
memory that is suitably aligned for any use, no?  

 compat/qsort_s.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git c/compat/qsort_s.c w/compat/qsort_s.c
index 52d1f0a73d..63660a4304 100644
--- c/compat/qsort_s.c
+++ w/compat/qsort_s.c
@@ -49,21 +49,23 @@ int git_qsort_s(void *b, size_t n, size_t s,
 		int (*cmp)(const void *, const void *, void *), void *ctx)
 {
 	const size_t size = st_mult(n, s);
-	char buf[1024];
+	char *tmp;
+	int use_alloca;
 
 	if (!n)
 		return 0;
 	if (!b || !cmp)
 		return -1;
 
-	if (size < sizeof(buf)) {
-		/* The temporary array fits on the small on-stack buffer. */
-		msort_with_tmp(b, n, s, cmp, buf, ctx);
-	} else {
-		/* It's somewhat large, so malloc it.  */
-		char *tmp = xmalloc(size);
-		msort_with_tmp(b, n, s, cmp, tmp, ctx);
+	use_alloca = (size < 1024);
+
+	tmp = use_alloca ? xalloca(size) : xmalloc(size);
+
+	msort_with_tmp(b, n, s, cmp, tmp, ctx);
+
+	if (use_alloca)
+		xalloca_free(tmp);
+	else
 		free(tmp);
-	}
 	return 0;
 }