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rpclib: slow floating point math and libm

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In release-6 rpc/rpc-lib (libgfrpc) added the function
get_rightmost_set_bit() which calls log2(3), a call that takes
a floating point parameter and returns a floating point.

It's used thusly:
    right_most_unset_bit = get_rightmost_set_bit(...);

(So is it really the right-most unset bit, or the right-most set bit?)

It's unclear to me whether this is in the data path or not. If it is,
it's rather scary to think about integer-to-float and float-to-integer
conversions and slow calls to libm functions in the data path.

gcc and clang have __builtin_ctz() which returns the same result as
get_rightmost_set_bit(), and does it substantially faster. Approx
20M iterations of get_rightmost_set_bit() took ~33sec of wall clock
time on my devel machine, while 20M iterations of __builtin_ctz()
took < 9sec; get_rightmost_set_bit() is 3x slower than __builtin_ctz().

And as a side benefit, we can again eliminate the need to link libgfrpc
with libm.

Change-Id: If9e7e80874577c52223f8125b385fc930de20699
fixes: bz#1692957
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
This commit is contained in:
Kaleb S. KEITHLEY 2019-03-28 09:36:33 -04:00 committed by Shyamsundar Ranganathan
parent c5bc21ebbc
commit 27a96f1f34
1 changed files with 2 additions and 8 deletions
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10
rpc/rpc-lib/src/rpcsvc.c
View File

@ -36,7 +36,6 @@
#include <fnmatch.h>
#include <stdarg.h>
#include <stdio.h>
#include <math.h>
#include <dlfcn.h>
#ifdef IPV6_DEFAULT
@ -89,12 +88,6 @@ rpcsvc_toggle_queue_status(rpcsvc_program_t *prog,
return;
}
static int
get_rightmost_set_bit(int n)
{
return log2(n & -n);
}
int
rpcsvc_get_free_queue_index(rpcsvc_program_t *prog)
{
@ -109,7 +102,8 @@ rpcsvc_get_free_queue_index(rpcsvc_program_t *prog)
right_most_unset_bit = 0;
break;
} else {
right_most_unset_bit = get_rightmost_set_bit(
/* get_rightmost_set_bit (sic)*/
right_most_unset_bit = __builtin_ctz(
~prog->request_queue_status[i]);
if (right_most_unset_bit < 8) {
break;