From a8032d12241f2226a6ab98272b12a1d98ca446a5 Mon Sep 17 00:00:00 2001
From: Nicolas Pitre <nico@cam.org>
Date: Tue, 2 Sep 2008 10:22:20 -0400
Subject: [PATCH] sha1write: don't copy full sized buffers

No need to memcpy() source buffer data when we might just process the
data in place instead of accumulating it into a separate buffer.
This is the case when a whole buffer would have been copied, summed,
written out and then discarded right away.

Also move the CRC32 processing within the loop so the data is more likely
to remain in the L1 CPU cache between the CRC32 sum, SHA1 sum and the
write call.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 csum-file.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/csum-file.c b/csum-file.c
index 28389541a..bb70c75ee 100644
--- a/csum-file.c
+++ b/csum-file.c
@@ -11,10 +11,8 @@
 #include "progress.h"
 #include "csum-file.h"
 
-static void sha1flush(struct sha1file *f, unsigned int count)
+static void sha1flush(struct sha1file *f, void *buf, unsigned int count)
 {
-	void *buf = f->buffer;
-
 	for (;;) {
 		int ret = xwrite(f->fd, buf, count);
 		if (ret > 0) {
@@ -39,7 +37,7 @@ int sha1close(struct sha1file *f, unsigned char *result, unsigned int flags)
 
 	if (offset) {
 		SHA1_Update(&f->ctx, f->buffer, offset);
-		sha1flush(f, offset);
+		sha1flush(f, f->buffer, offset);
 		f->offset = 0;
 	}
 	SHA1_Final(f->buffer, &f->ctx);
@@ -47,7 +45,7 @@ int sha1close(struct sha1file *f, unsigned char *result, unsigned int flags)
 		hashcpy(result, f->buffer);
 	if (flags & (CSUM_CLOSE | CSUM_FSYNC)) {
 		/* write checksum and close fd */
-		sha1flush(f, 20);
+		sha1flush(f, f->buffer, 20);
 		if (flags & CSUM_FSYNC)
 			fsync_or_die(f->fd, f->name);
 		if (close(f->fd))
@@ -62,21 +60,30 @@ int sha1close(struct sha1file *f, unsigned char *result, unsigned int flags)
 
 int sha1write(struct sha1file *f, void *buf, unsigned int count)
 {
-	if (f->do_crc)
-		f->crc32 = crc32(f->crc32, buf, count);
 	while (count) {
 		unsigned offset = f->offset;
 		unsigned left = sizeof(f->buffer) - offset;
 		unsigned nr = count > left ? left : count;
+		void *data;
+
+		if (f->do_crc)
+			f->crc32 = crc32(f->crc32, buf, nr);
+
+		if (nr == sizeof(f->buffer)) {
+			/* process full buffer directly without copy */
+			data = buf;
+		} else {
+			memcpy(f->buffer + offset, buf, nr);
+			data = f->buffer;
+		}
 
-		memcpy(f->buffer + offset, buf, nr);
 		count -= nr;
 		offset += nr;
 		buf = (char *) buf + nr;
 		left -= nr;
 		if (!left) {
-			SHA1_Update(&f->ctx, f->buffer, offset);
-			sha1flush(f, offset);
+			SHA1_Update(&f->ctx, data, offset);
+			sha1flush(f, data, offset);
 			offset = 0;
 		}
 		f->offset = offset;
-- 
2.26.2