linux-kernel - [PATCH] interconnect: Optimize kbps_to

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Message-Id: <20250930043055.2200322-1-visitorckw@gmail.com>
Date: Tue, 30 Sep 2025 12:30:55 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: djakov@...nel.org
Cc: jserv@...s.ncku.edu.tw,
	marscheng@...gle.com,
	wllee@...gle.com,
	linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Kuan-Wei Chiu <visitorckw@...il.com>
Subject: [PATCH] interconnect: Optimize kbps_to_icc() macro

The current expansion of kbps_to_icc() introduces unnecessary logic
when compiled from a general expression. Rewriting it allows compilers
to emit shorter and more efficient code across architectures.

For example, with gcc -O2:

arm64:

old:
        tst     x0, 7
        add     w1, w0, 7
        cset    w2, ne
        cmp     w0, 0
        csel    w0, w1, w0, lt
        add     w0, w2, w0, asr 3

new:
        add     w1, w0, 14
        adds    w0, w0, 7
        csel    w0, w1, w0, mi
        asr     w0, w0, 3

x86-64:

old:
        xor     eax, eax
        test    dil, 7
        lea     edx, [rdi+7]
        setne   al
        test    edi, edi
        cmovns  edx, edi
        sar     edx, 3
        add     eax, edx

new:
        lea     eax, [rdi+14]
        add     edi, 7
        cmovns  eax, edi
        sar     eax, 3

In both cases the old form relies on extra test and compare
instructions (tst, test, cmp) combined with conditional moves or sets,
while the new form uses fewer instructions by folding the addition and
flag update together (adds on arm64, add on x86).

This reduces the instruction sequence, prevents multiple evaluations of
x when it is an expression or a function call, and keeps the macro
simpler.

Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
---
 include/linux/interconnect.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/interconnect.h b/include/linux/interconnect.h
index e4b8808823ad..4b12821528a6 100644
--- a/include/linux/interconnect.h
+++ b/include/linux/interconnect.h
@@ -16,7 +16,7 @@
 #define MBps_to_icc(x)	((x) * 1000)
 #define GBps_to_icc(x)	((x) * 1000 * 1000)
 #define bps_to_icc(x)	(1)
-#define kbps_to_icc(x)	((x) / 8 + ((x) % 8 ? 1 : 0))
+#define kbps_to_icc(x)	(((x) + 7) / 8)
 #define Mbps_to_icc(x)	((x) * 1000 / 8)
 #define Gbps_to_icc(x)	((x) * 1000 * 1000 / 8)
 
-- 
2.34.1