[PATCH] Workaround for ffs() on LP64 targets

[Sebastian Huber]

On 27/07/17 14:48, Corinna Vinschen wrote:

> On Jul 27 14:33, Sebastian Huber wrote:
>> On 27/07/17 14:27, Corinna Vinschen wrote:
>>
>>> On Jul 27 13:24, Sebastian Huber wrote:
>>>> On 27/07/17 13:13, Eric Blake wrote:
>>>>
>>>>> On 07/27/2017 03:06 AM, Sebastian Huber wrote:
>>>>>> Signed-off-by: Sebastian Huber<sebastian.huber@embedded-brains.de>
>>>>>> ---
>>>>>>     newlib/libc/misc/ffs.c | 11 +++++++++++
>>>>>>     1 file changed, 11 insertions(+)
>>>>>>
>>>>>> diff --git a/newlib/libc/misc/ffs.c b/newlib/libc/misc/ffs.c
>>>>>> index ba5700920..a09cbd3bb 100644
>>>>>> --- a/newlib/libc/misc/ffs.c
>>>>>> +++ b/newlib/libc/misc/ffs.c
>>>>>> @@ -31,6 +31,17 @@ No supporting OS subroutines are required.  */
>>>>>>     int
>>>>>>     ffs(int i)
>>>>>>     {
>>>>>> +#ifdef __LP64__
>>>>>> +	/* GCC would expand the __builtin_ffs() to ffs() in this case */
>>>>>> +	int bit;
>>>>>> +
>>>>>> +	if (i == 0)
>>>>>> +		return (0);
>>>>>> +	for (bit = 1; !(i & 1); bit++)
>>>>>> +		i = (unsigned int)i >> 1;
>>>>>> +	return (bit);
>>>>> If we're going to open-code it to work around the compiler creating an
>>>>> infloop recursion to ffs(), at least code a straight-line version
>>>>> without branches, rather than the painfully slow bit-by-bit loop.
>>>>> There's plenty of examples on the web of writing ffs() by using
>>>>> bit-twiddling without branching.
>>>> This is roughly the same implementation we had before. I do not intend to
>>>> optimize this.
>>> Still, __LP64__ is unacceptable.  Cygwin would be affected by this as
>>> well and would have to revert to its former own ffs implementation.
>>>
>>> Reverting to a C-based implementation should only be performed on a
>>> case-by-case basis.
>> Yes, so maybe something like this
>>
>> #if defined(__LP64__) && defined(__riscv)
>>
>> or a target-specific ffs.c file similar to memcpy.c, etc.
> I'm inclined to favor a target-specific file.  This would also allow
> to implement the replacement in assembler easily.

Ok, good. So, this is something for the new RISC-V port.

-- 
Sebastian Huber, embedded brains GmbH

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