[PATCH v2 3/4] s390/bitops: Use __assume() for __flogr() inline assembly return value

[Heiko Carstens <hca@linux.ibm.com>]

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>
---
 arch/s390/include/asm/bitops.h | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/s390/include/asm/bitops.h b/arch/s390/include/asm/bitops.h
index a1dd72b16f54..5ff069fe9526 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;
 	}
 }
 
-- 
2.48.1