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From: Junio C Hamano <gitster@pobox.com>
To: "brian m. carlson" <sandals@crustytoothpaste.net>
Cc: "Jessica Clarke" <jrtc27@jrtc27.com>,
	"Taylor Blau" <me@ttaylorr.com>, "René Scharfe" <l.s.r@web.de>,
	git@vger.kernel.org
Subject: Re: [PATCH] Properly align memory allocations and temporary buffers
Date: Fri, 07 Jan 2022 11:30:52 -0800	[thread overview]
Message-ID: <xmqq7dbb9wcz.fsf@gitster.g> (raw)
In-Reply-To: <Ydea3EO7wrYc9/Ij@camp.crustytoothpaste.net> (brian m. carlson's message of "Fri, 7 Jan 2022 01:43:56 +0000")

"brian m. carlson" <sandals@crustytoothpaste.net> 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;
 }

  parent reply	other threads:[~2022-01-07 19:31 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-05 13:23 [PATCH] Properly align memory allocations and temporary buffers Jessica Clarke
2022-01-06 21:46 ` Taylor Blau
2022-01-06 21:56   ` Jessica Clarke
2022-01-06 22:27   ` Junio C Hamano
2022-01-06 22:56     ` Jessica Clarke
2022-01-07  0:10       ` Junio C Hamano
2022-01-07  0:22         ` Jessica Clarke
2022-01-07  0:31         ` brian m. carlson
2022-01-07  0:39           ` Jessica Clarke
2022-01-07  1:43             ` brian m. carlson
2022-01-07  2:08               ` Jessica Clarke
2022-01-07  2:11                 ` Jessica Clarke
2022-01-07 19:30               ` Junio C Hamano [this message]
2022-01-07 19:33                 ` Jessica Clarke
2022-01-07 20:56                 ` René Scharfe
2022-01-07 21:30                   ` Junio C Hamano
2022-01-07 23:30                     ` René Scharfe
2022-01-08  0:18                       ` Elijah Newren
2022-01-06 23:22 ` brian m. carlson
2022-01-06 23:31   ` Jessica Clarke
2022-01-07 14:57 ` Philip Oakley
2022-01-07 16:08 ` René Scharfe
2022-01-07 16:21   ` Jessica Clarke
2022-01-12 13:58 ` Jessica Clarke
2022-01-12 15:47   ` René Scharfe
2022-01-12 15:49     ` Jessica Clarke
2022-01-23 15:24 ` [PATCH v2] mem-pool: Don't assume uintmax_t is aligned enough for all types Jessica Clarke
2022-01-23 20:17   ` Junio C Hamano
2022-01-23 20:23     ` Jessica Clarke
2022-01-23 20:28       ` Junio C Hamano
2022-01-23 20:33   ` [PATCH v3] " Jessica Clarke
2022-01-24 17:11     ` Junio C Hamano

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