linux-kernel - [PATCH 2/2] uapi: Refactor __GENMASK

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Message-ID: <20250211162412.477655-3-richard120310@gmail.com>
Date: Wed, 12 Feb 2025 00:24:12 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: yury.norov@...il.com
Cc: anshuman.khandual@....com,
	arnd@...db.de,
	linux-kernel@...r.kernel.org,
	jserv@...s.ncku.edu.tw,
	skhan@...uxfoundation.org,
	I Hsin Cheng <richard120310@...il.com>
Subject: [PATCH 2/2] uapi: Refactor __GENMASK_ULL() for speed-up

The calculation of "((~_ULL(0)) - (_ULL(1) << (l)) + 1)" is to generate
a bitmask with "l" trailing zeroes, which is equivalent to
"(~_ULL(0) << (l))".

Refactor the calculation so the number of arithmetic instruction will be
reduced from 3 to 1.

Signed-off-by: I Hsin Cheng <richard120310@...il.com>
---
Test is done to show the speed-up we can get from reducing the number of
instruction. The test machine runs with 6.9.0-0-generic kernel on x86_64
architecture with processor AMD Ryzen 7 5700X3D 8-Core Processor.

The test executes in the form of kernel module.

Test result:
[452858.156718] new __GENMASK_ULL() : 5908678
[452858.156722] old __GENMASK_ULL() : 5950998

Test script snippet:
/* switch to __BITS_PER_LONG_LONG when testing __GENMASK_ULL() */
\#define __GENMASK_ULL_NEW(h, l) \
        ((~_ULL(0) << (l)) & \
         (~_ULL(0) >> (__BITS_PER_LONG_LONG - 1 - (h))))

int init_module(void)
{
    ktime_t start, end, total1 = 0, total2 = 0;
    for (int k = 0; k < 100; k++) {
        for (int i = 0; i < __BITS_PER_LONG_LONG; i++) {
            for (int j = 0; j <= i; j++) {
                unsigned result1, result2;

                start = ktime_get();
                result1 = __GENMASK_ULL_NEW(i, j);
                end = ktime_get();
                total1 += (end - start);

                start = ktime_get();
                result2 = __GENMASK_ULL(i, j);
                end = ktime_get();
                total2 += (end - start);

                if (result1 != result2) {
                    pr_info("Wrong calculation of GENMASK_ULL_NEW()\n");
                    return 0;
                }
            }
        }
    }

    pr_info("new __GENMASK_ULL() : %lld\n", total1);
    pr_info("old __GENMASK_ULL() : %lld\n", total2);

    return 0;
}
---
 include/uapi/linux/bits.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/bits.h b/include/uapi/linux/bits.h
index 8fc7fea65288..965f6eb91ff3 100644
--- a/include/uapi/linux/bits.h
+++ b/include/uapi/linux/bits.h
@@ -9,7 +9,7 @@
          (~_UL(0) >> (__BITS_PER_LONG - 1 - (h))))
 
 #define __GENMASK_ULL(h, l) \
-        (((~_ULL(0)) - (_ULL(1) << (l)) + 1) & \
+	((~_ULL(0) << (l)) & \
          (~_ULL(0) >> (__BITS_PER_LONG_LONG - 1 - (h))))
 
 #define __GENMASK_U128(h, l) \
-- 
2.43.0