git: 62d555c4e828 - stable/13 - fread: improve performance for unbuffered reads

[Pedro F. Giffuni]

The branch stable/13 has been updated by pfg:

URL: https://cgit.FreeBSD.org/src/commit/?id=62d555c4e82852bf80b1d833a31ff4dc7833f4cd

commit 62d555c4e82852bf80b1d833a31ff4dc7833f4cd
Author:     Pedro F. Giffuni <pfg at FreeBSD.org>
AuthorDate: 2021-05-31 01:48:38 +0000
Commit:     Pedro F. Giffuni <pfg at FreeBSD.org>
CommitDate: 2021-06-15 21:26:31 +0000

    fread: improve performance for unbuffered reads
    
    We can use the buffer passed to fread(3) directly in the FILE *.
    The buffer needs to be reset before each call to __srefill().
    This preserves the expected behavior in all cases.
    
    The change was found originally in OpenBSD and later adopted by NetBSD.
    
    MFC after:      2 weeks
    Obtained from:  OpenBSD (CVS 1.18)
    
    Differential Revision:  https://reviews.freebsd.org/D30548
---
 lib/libc/stdio/fread.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/lib/libc/stdio/fread.c b/lib/libc/stdio/fread.c
index c12bcf1148b7..11f8d13f0caf 100644
--- a/lib/libc/stdio/fread.c
+++ b/lib/libc/stdio/fread.c
@@ -99,6 +99,35 @@ __fread(void * __restrict buf, size_t size, size_t count, FILE * __restrict fp)
 		fp->_r = 0;
 	total = resid;
 	p = buf;
+
+	/*
+	 * If we're unbuffered we know that the buffer in fp is empty so
+	 * we can read directly into buf.  This is much faster than a
+	 * series of one byte reads into fp->_nbuf.
+	 */
+	if ((fp->_flags & __SNBF) != 0 && buf != NULL) {
+		while (resid > 0) {
+			/* set up the buffer */
+			fp->_bf._base = fp->_p = p;
+			fp->_bf._size = resid;
+
+			if (__srefill(fp)) {
+				/* no more input: return partial result */
+				count = (total - resid) / size;
+				break;
+			}
+			p += fp->_r;
+			resid -= fp->_r;
+		}
+
+		/* restore the old buffer (see __smakebuf) */
+		fp->_bf._base = fp->_p = fp->_nbuf;
+		fp->_bf._size = 1;
+		fp->_r = 0;
+
+		return (count);
+	}
+
 	while (resid > (r = fp->_r)) {
 		(void)memcpy((void *)p, (void *)fp->_p, (size_t)r);
 		fp->_p += r;