linux-kernel - [PATCH 08/14] lib/Kconfig: introduce FAST

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Date:   Wed, 17 Feb 2021 20:05:06 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>, linux-m68k@...ts.linux-m68k.org,
        linux-arch@...r.kernel.org, linux-sh@...r.kernel.org,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jianpeng Ma <jianpeng.ma@...el.com>,
        Joe Perches <joe@...ches.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Rich Felker <dalias@...c.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: [PATCH 08/14] lib/Kconfig: introduce FAST_PATH option

This series introduces fast paths for find_bit() routines. It is
beneficial for typical systems, but those who limited in I-cache
may be concerned about increasing the .text size of the Image.

To address this concern, one can disable FAST_PATH option in the config
and some save memory.

The effect of this option on my arm64 next-20210217 build is:

Before:
	Sections:
	Idx Name          Size      VMA               LMA               File off  Algn
	  0 .head.text    00010000  ffff800010000000  ffff800010000000  00010000  2**16
			  CONTENTS, ALLOC, LOAD, READONLY, CODE
	  1 .text         0115e3a8  ffff800010010000  ffff800010010000  00020000  2**16
			  CONTENTS, ALLOC, LOAD, READONLY, CODE
	  2 .got.plt      00000018  ffff80001116e3a8  ffff80001116e3a8  0117e3a8  2**3
			  CONTENTS, ALLOC, LOAD, DATA
	  3 .rodata       007a72ca  ffff800011170000  ffff800011170000  01180000  2**12
			  CONTENTS, ALLOC, LOAD, DATA
	  ...

After:
	Sections:
	Idx Name          Size      VMA               LMA               File off  Algn
	  0 .head.text    00010000  ffff800010000000  ffff800010000000  00010000  2**16
			  CONTENTS, ALLOC, LOAD, READONLY, CODE
	  1 .text         011623a8  ffff800010010000  ffff800010010000  00020000  2**16
			  CONTENTS, ALLOC, LOAD, READONLY, CODE
	  2 .got.plt      00000018  ffff8000111723a8  ffff8000111723a8  011823a8  2**3
			  CONTENTS, ALLOC, LOAD, DATA
	  3 .rodata       007a772a  ffff800011180000  ffff800011180000  01190000  2**12
			  CONTENTS, ALLOC, LOAD, DATA
	  ...

Notice that this is the cumulive effect on already existing fast paths
controlled by SMALL_CONST() together with ones added by this series.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 include/asm-generic/bitsperlong.h | 4 ++++
 lib/Kconfig                       | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
index 0eeb77544f1d..209e531074c1 100644
--- a/include/asm-generic/bitsperlong.h
+++ b/include/asm-generic/bitsperlong.h
@@ -23,6 +23,10 @@
 #define BITS_PER_LONG_LONG 64
 #endif
 
+#ifdef CONFIG_FAST_PATH
 #define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG)
+#else
+#define SMALL_CONST(n) (0)
+#endif
 
 #endif /* __ASM_GENERIC_BITS_PER_LONG */
diff --git a/lib/Kconfig b/lib/Kconfig
index a38cc61256f1..7a1b9c8d2a32 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -39,6 +39,13 @@ config PACKING
 
 	  When in doubt, say N.
 
+config FAST_PATH
+	bool "Enable fast path code generation"
+	default y
+	help
+	  This option enables fast path optimization with the cost of increasing
+	  the text section.
+
 config BITREVERSE
 	tristate
 
-- 
2.25.1