From ddaf1f62e3090335e6973911062a9cf70673c06f Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 26 Mar 2021 12:38:11 +0000 Subject: [PATCH] csum-file: make hashwrite() more readable The hashwrite() method takes an input buffer and updates a hashfile's hash function while writing the data to a file. To avoid overuse of flushes, the hashfile has an internal buffer and most writes will use memcpy() to transfer data from the input 'buf' to the hashfile's buffer of size 8 * 1024 bytes. Logic introduced by a8032d12 (sha1write: don't copy full sized buffers, 2008-09-02) reduces the number of memcpy() calls when the input buffer is sufficiently longer than the hashfile's buffer, causing nr to be the length of the full buffer. In these cases, the input buffer is used directly in chunks equal to the hashfile's buffer size. This method caught my attention while investigating some performance issues, but it turns out that these performance issues were noise within the variance of the experiment. However, during this investigation, I inspected hashwrite() and misunderstood it, even after looking closely and trying to make it faster. This change simply reorganizes some parts of the loop within hashwrite() to make it clear that each batch either uses memcpy() to the hashfile's buffer or writes directly from the input buffer. The previous code relied on indirection through local variables and essentially inlined the implementation of hashflush() to reduce lines of code. Helped-by: Jeff King Helped-by: Junio C Hamano Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- csum-file.c | 33 ++++++++++++++++++--------------- 1 file changed, 18 insertions(+), 15 deletions(-) diff --git a/csum-file.c b/csum-file.c index 0f35fa5ee47..7510950fa3e 100644 --- a/csum-file.c +++ b/csum-file.c @@ -89,32 +89,35 @@ int finalize_hashfile(struct hashfile *f, unsigned char *result, unsigned int fl void hashwrite(struct hashfile *f, const void *buf, unsigned int count) { while (count) { - unsigned offset = f->offset; - unsigned left = sizeof(f->buffer) - offset; + unsigned left = sizeof(f->buffer) - f->offset; unsigned nr = count > left ? left : count; - const 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; + /* + * Flush a full batch worth of data directly + * from the input, skipping the memcpy() to + * the hashfile's buffer. In this block, + * f->offset is necessarily zero. + */ + the_hash_algo->update_fn(&f->ctx, buf, nr); + flush(f, buf, nr); } else { - memcpy(f->buffer + offset, buf, nr); - data = f->buffer; + /* + * Copy to the hashfile's buffer, flushing only + * if it became full. + */ + memcpy(f->buffer + f->offset, buf, nr); + f->offset += nr; + left -= nr; + if (!left) + hashflush(f); } count -= nr; - offset += nr; buf = (char *) buf + nr; - left -= nr; - if (!left) { - the_hash_algo->update_fn(&f->ctx, data, offset); - flush(f, data, offset); - offset = 0; - } - f->offset = offset; } }