Re: [PATCH 7/7] xen/bitops: Delete find_first_set_bit()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/03/2024 3:59 pm, Jan Beulich wrote:
> On 13.03.2024 18:27, Andrew Cooper wrote:
>> --- a/xen/arch/x86/include/asm/bitops.h
>> +++ b/xen/arch/x86/include/asm/bitops.h
>> @@ -401,18 +401,6 @@ static always_inline unsigned int __scanbit(unsigned 
>> long val, unsigned int max)
>>      r__;                                                                    
>> \
>>  })
>>  
>> -/**
>> - * find_first_set_bit - find the first set bit in @word
>> - * @word: the word to search
>> - * 
>> - * Returns the bit-number of the first set bit. The input must *not* be 
>> zero.
>> - */
>> -static inline unsigned int find_first_set_bit(unsigned long word)
>> -{
>> -    asm ( "rep; bsf %1,%0" : "=r" (word) : "rm" (word) );
>> -    return (unsigned int)word;
>> -}
> And you think it's okay to no longer use TZCNT like this when available,
> where the output doesn't have to have its value set up front?

This is a particularly evil piece of inline asm.

It is interpreted as BSF or TZCNT depending on the BMI instruction set
(Haswell/Piledriver era).  Furthermore there are errata on some Intel
systems where REP BSF behaves as per TZCNT *even* when BMI isn't enumerated.

Which means this piece of asm suffers from all of an undefined output
register, undefined CF behaviour, and differing ZF behaviour (I believe)
depending on which hardware you're running on.

The only thing the REP prefix is getting you is a deterministic 0 in the
destination register, on some hardware only, for code which has already
violated the input safety condition.  As a piece of defence in depth,
then perhaps it's useful.

But following up from the other thread,
https://gcc.gnu.org/pipermail/gcc/2024-March/243475.html is form where
the compiler can (and does!) simplify back to the plain BSF form when it
can prove that this is safe.


The only case where using TZCNT is helpful is when we're compiling for
x86_64-v3 and there is no need to work around BSF's undefined behaviour.

Even with x86's arch_ffs() now split nicely based on whether the
compiler knows BSF is safe or not, an alternative to swap between BSF
and TZCNT probably isn't a win; you still have to cover up 6 or 7 bytes
of the -1 setup, which you can't do with leading prefixes on the TZCNT
itself.

All CPUs with BMI can swallow the double-instruction data dependency
without breaking a sweat, at which point you're trading off (at best) a
1-cycle improvement vs the setup costs of the alternative.  If there is
any real improvement to be had here, it's marginal enough that I'm not
sure it's worth doing.

~Andrew