linux-kernel - [PATCH] x86/hweight: Fix and improve __arch

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Message-ID: <20250310200817.33581-1-ubizjak@gmail.com>
Date: Mon, 10 Mar 2025 21:08:04 +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] x86/hweight: Fix and improve __arch_hweight{32,64}() assembly

a) Use ASM_CALL_CONSTRAINT to prevent inline asm that includes call
instruction from being scheduled before the frame pointer gets set
up by the containing function, causing objtool to print a "call
without frame pointer save/setup" warning.

b) 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.

c) Use named operands in inline asm.

More inlining causes slight increase in the code size:

   text    data     bss     dec     hex filename
27261832        4640296  814660 32716788        1f337f4 vmlinux-new.o
27261222        4640320  814660 32716202        1f335aa vmlinux-old.o

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>
---
 arch/x86/include/asm/arch_hweight.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/arch_hweight.h b/arch/x86/include/asm/arch_hweight.h
index ba88edd0d58b..20b0633744e4 100644
--- a/arch/x86/include/asm/arch_hweight.h
+++ b/arch/x86/include/asm/arch_hweight.h
@@ -16,10 +16,10 @@ static __always_inline unsigned int __arch_hweight32(unsigned int w)
 {
 	unsigned int res;
 
-	asm (ALTERNATIVE("call __sw_hweight32", "popcntl %1, %0", X86_FEATURE_POPCNT)
-			 : "="REG_OUT (res)
-			 : REG_IN (w));
-
+	asm_inline (ALTERNATIVE("call __sw_hweight32",
+				"popcntl %[val], %[cnt]", X86_FEATURE_POPCNT)
+			 : [cnt] "="REG_OUT (res), ASM_CALL_CONSTRAINT
+			 : [val] REG_IN (w));
 	return res;
 }
 
@@ -44,10 +44,10 @@ static __always_inline unsigned long __arch_hweight64(__u64 w)
 {
 	unsigned long res;
 
-	asm (ALTERNATIVE("call __sw_hweight64", "popcntq %1, %0", X86_FEATURE_POPCNT)
-			 : "="REG_OUT (res)
-			 : REG_IN (w));
-
+	asm_inline (ALTERNATIVE("call __sw_hweight64",
+				"popcntq %[val], %[cnt]", X86_FEATURE_POPCNT)
+			 : [cnt] "="REG_OUT (res), ASM_CALL_CONSTRAINT
+			 : [val] REG_IN (w));
 	return res;
 }
 #endif /* CONFIG_X86_32 */
-- 
2.42.0