From: Heiko Carstens <hca@linux.ibm.com>
Date: Tue, 16 Sep 2025 13:48:02 +0000 (+0200)
Subject: s390/bitops: Use __assume() for __flogr() inline assembly return value
X-Git-Tag: v6.18-rc1~207^2~9
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=79161603952c842eb22313f2060051b359b0a592;p=thirdparty%2Flinux.git

s390/bitops: Use __assume() for __flogr() inline assembly return value

Use __assume() to tell compilers that the output operand of the __flogr()
inline assembly contains a value in the range of 0..64. This allows to
optimize the logical AND operation away.

This reduces the kernel image size by 2804 bytes (defconfig, gcc 15.2.0).

Suggested-by: Juergen Christ <jchrist@linux.ibm.com>
Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
---

diff --git a/arch/s390/include/asm/bitops.h b/arch/s390/include/asm/bitops.h
index 9bc70acbac9ec..ac94672db8170 100644
--- a/arch/s390/include/asm/bitops.h
+++ b/arch/s390/include/asm/bitops.h
@@ -132,9 +132,10 @@ static inline bool test_bit_inv(unsigned long nr,
  */
 static __always_inline unsigned char __flogr(unsigned long word)
 {
-	if (__builtin_constant_p(word)) {
-		unsigned long bit = 0;
+	unsigned long bit;
 
+	if (__builtin_constant_p(word)) {
+		bit = 0;
 		if (!word)
 			return 64;
 		if (!(word & 0xffffffff00000000UL)) {
@@ -169,7 +170,14 @@ static __always_inline unsigned char __flogr(unsigned long word)
 		asm volatile(
 			"       flogr   %[rp],%[rp]\n"
 			: [rp] "+d" (rp.pair) : : "cc");
-		return rp.even & 127;
+		bit = rp.even;
+		/*
+		 * The result of the flogr instruction is a value in the range
+		 * of 0..64. Let the compiler know that the AND operation can
+		 * be optimized away.
+		 */
+		__assume(bit <= 64);
+		return bit & 127;
 	}
 }