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

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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>
This commit is contained in:
Heiko Carstens 2025-09-16 15:48:02 +02:00 committed by Alexander Gordeev
parent f72e2cff13
commit 7916160395
1 changed files with 11 additions and 3 deletions
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14
arch/s390/include/asm/bitops.h
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@ -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;
}
}