From: Ondřej Surý <ondrej@isc.org>
Date: Thu, 12 Oct 2023 07:20:42 +0000 (+0200)
Subject: Use mul and div instead of bitshifts to calculate srtt
X-Git-Tag: v9.19.18~47^2~1
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=91f3b0edee9873b3f727bc9fc372a362b59480b1;p=thirdparty%2Fbind9.git

Use mul and div instead of bitshifts to calculate srtt

There was a microoptimization for smoothing srtt with bitshifts.  Revert
the code to use * 98 / 100, it doesn't really make that difference on
modern CPUs, for comparison here:

    muldiv:
	    imul    eax, edi, 98
	    imul    rax, rax, 1374389535
	    shr     rax, 37
	    ret
    shift:
	    mov     eax, edi
	    sal     eax, 9
	    sub     eax, edi
	    shr     eax, 9
	    ret
---

diff --git a/lib/dns/adb.c b/lib/dns/adb.c
index a589519f6ee..65904e0f5d6 100644
--- a/lib/dns/adb.c
+++ b/lib/dns/adb.c
@@ -3052,10 +3052,8 @@ adjustsrtt(dns_adbaddrinfo_t *addr, unsigned int rtt, unsigned int factor,
 
 	if (factor == DNS_ADB_RTTADJAGE) {
 		if (atomic_load(&addr->entry->lastage) != now) {
-			new_srtt = addr->entry->srtt;
-			new_srtt <<= 9;
-			new_srtt -= addr->entry->srtt;
-			new_srtt >>= 9;
+			new_srtt = (uint64_t)atomic_load(&addr->entry->srtt) *
+				   98 / 100;
 			atomic_store(&addr->entry->lastage, now);
 			atomic_store(&addr->entry->srtt, new_srtt);
 			addr->srtt = new_srtt;