linux-kernel - [PATCH v3 3/3] x86/hweight: Use asm

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Message-ID: <20250312123905.149298-3-ubizjak@gmail.com>
Date: Wed, 12 Mar 2025 13:38:45 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v3 3/3] x86/hweight: Use asm_inline in inline asm()

Use asm_inline to instruct the compiler that the size of asm()
is the minimum size of one instruction, ignoring how many instructions
the compiler thinks it is. ALTERNATIVE macro that expands to several
pseudo directives causes instruction length estimate to count
more than 20 instructions.

bloat-o-meter reports slight reduction of the code size
for x86_64 defconfig object file, compiled with gcc-14.2:

add/remove: 6/12 grow/shrink: 59/50 up/down: 3389/-3560 (-171)
Total: Before=22734393, After=22734222, chg -0.00%

where 29 instances of code blocks involving POPCNT now gets inlined,
resulting in the removal of several functions:

format_is_yuv_semiplanar.part.isra            41       -     -41
cdclk_divider                                 69       -     -69
intel_joiner_adjust_timings                  140       -    -140
nl80211_send_wowlan_tcp_caps                 369       -    -369
nl80211_send_iftype_data                     579       -    -579
__do_sys_pidfd_send_signal                   809       -    -809

One noticeable change is:

pcpu_page_first_chunk                       1075    1060     -15

Where the compiler now inlines 4 more instances of POPCNT insns,
but still manages to compile to a function with smaller code size.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
v2: Use bloat-o-meter to assess code size changes.
v3: Split patch into three separate patches.
---
 arch/x86/include/asm/arch_hweight.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/arch_hweight.h b/arch/x86/include/asm/arch_hweight.h
index f233eb00f41f..b5982b94bdba 100644
--- a/arch/x86/include/asm/arch_hweight.h
+++ b/arch/x86/include/asm/arch_hweight.h
@@ -16,7 +16,8 @@ static __always_inline unsigned int __arch_hweight32(unsigned int w)
 {
 	unsigned int res;
 
-	asm (ALTERNATIVE("call __sw_hweight32", "popcntl %[val], %[cnt]", X86_FEATURE_POPCNT)
+	asm_inline (ALTERNATIVE("call __sw_hweight32",
+				"popcntl %[val], %[cnt]", X86_FEATURE_POPCNT)
 			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
 
@@ -44,7 +45,8 @@ static __always_inline unsigned long __arch_hweight64(__u64 w)
 {
 	unsigned long res;
 
-	asm (ALTERNATIVE("call __sw_hweight64", "popcntq %[val], %[cnt]", X86_FEATURE_POPCNT)
+	asm_inline (ALTERNATIVE("call __sw_hweight64",
+				"popcntq %[val], %[cnt]", X86_FEATURE_POPCNT)
 			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
 
-- 
2.48.1