Re: [PATCH v3 1/3] lib: find_*_bit reimplementation

From: Rasmus Villemoes
Date: Fri Feb 13 2015 - 05:13:51 EST

On Fri, Feb 13 2015, "George Spelvin" <linux@xxxxxxxxxxx> wrote:

>> the main loop is 20--3b. The test instruction at 2e seems to be
>> redundant. The same at 37: the sub instruction already sets plenty of
>> flags that could be used, so explicitly comparing %rbx to -1 seems
>> redundant.
> Er... I think you hand-edited that code; it's wrong. The loop assumes that
> %rbx is in units of words, but the prologue sets it up in units of bits.

No, but I messed up the source by hand :-) My DIV_ROUND_UP macro was
bogus. Well spotted. Fixing that I still see the redundant cmp and
test, though.

> The mov to %rcx is also redundant, since it could be eliminated with
> some minor rescheduling.
> The code generation I *want* for that function is:
> # addr in %rdi, size in %rsi
> movl %esi, %ecx
> leaq 0x3f(%rsi), %rax
> negl %ecx
> movq $-1, %rdx
> shrq $6, %rax
> shrq %cl, %rdx
> jmp 2f
> 1:
> movq $-1, %rdx
> 2:
> subq $1, %rax
> jc 3f
> andq (%rdi,%rax,8), %rdx
> jeq 1b
> bsrq %rdx, %rdx
> salq $6, %rax
> addq %rdx, %rax
> ret
> 3:
> movq %rsi, %rax
> retq

Nice. But I don't think find_last_bit is important enough to warrant
arch-specific versions.

So, where are we with this? Have we reached some kind of consensus?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at