Re: [PATCH v2 05/13] xen/bitops: Implement generic_f?sl() in lib/

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/05/2024 9:44 am, Jan Beulich wrote:
> On 24.05.2024 22:03, Andrew Cooper wrote:
>> generic_f?s() being static inline is the cause of lots of the complexity
>> between the common and arch-specific bitops.h
>>
>> They appear to be static inline for constant-folding reasons (ARM uses them
>> for this), but there are better ways to achieve the same effect.
>>
>> It is presumptuous that an unrolled binary search is the right algorithm to
>> use on all microarchitectures.  Indeed, it's not for the eventual users, but
>> that can be addressed at a later point.
>>
>> It is also nonsense to implement the int form as the base primitive and
>> construct the long form from 2x int in 64-bit builds, when it's just one 
>> extra
>> step to operate at the native register width.
>>
>> Therefore, implement generic_f?sl() in lib/.  They're not actually needed in
>> x86/ARM/PPC by the end of the cleanup (i.e. the functions will be dropped by
>> the linker), and they're only expected be needed by RISC-V on hardware which
>> lacks the Zbb extension.
>>
>> Implement generic_fls() in terms of generic_flsl() for now, but this will be
>> cleaned up in due course.
>>
>> Provide basic runtime testing using __constructor inside the lib/ file.  This
>> is important, as it means testing runs if and only if generic_f?sl() are used
>> elsewhere in Xen.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

> with a suggestion and a question.
>
>> I suspect we want to swap CONFIG_DEBUG for CONFIG_BOOT_UNIT_TESTS in due
>> course.  These ought to be able to be used in a release build too.
> +1

Actually - I might as well do this now.  Start as we mean to go on.

>
>> --- /dev/null
>> +++ b/xen/lib/generic-ffsl.c
>> @@ -0,0 +1,65 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +
>> +#include <xen/bitops.h>
>> +#include <xen/boot-check.h>
>> +#include <xen/init.h>
>> +
>> +unsigned int generic_ffsl(unsigned long x)
>> +{
>> +    unsigned int r = 1;
>> +
>> +    if ( !x )
>> +        return 0;
>> +
>> +#if BITS_PER_LONG > 32
> To be future-proof, perhaps ahead of this
>
> #if BITS_PER_LONG > 64
> # error "..."
> #endif
>
> or a functionally similar BUILD_BUG_ON()?

Good point.  I'll fold this in to both files.

>
>> --- /dev/null
>> +++ b/xen/lib/generic-flsl.c
>> @@ -0,0 +1,68 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +
>> +#include <xen/bitops.h>
>> +#include <xen/boot-check.h>
>> +#include <xen/init.h>
>> +
>> +/* Mask of type UL with the upper x bits set. */
>> +#define UPPER_MASK(x) (~0UL << (BITS_PER_LONG - (x)))
>> +
>> +unsigned int generic_flsl(unsigned long x)
>> +{
>> +    unsigned int r = BITS_PER_LONG;
>> +
>> +    if ( !x )
>> +        return 0;
>> +
>> +#if BITS_PER_LONG > 32
>> +    if ( !(x & UPPER_MASK(32)) )
>> +    {
>> +        x <<= 32;
>> +        r -= 32;
>> +    }
>> +#endif
>> +    if ( !(x & UPPER_MASK(16)) )
>> +    {
>> +        x <<= 16;
>> +        r -= 16;
>> +    }
>> +    if ( !(x & UPPER_MASK(8)) )
>> +    {
>> +        x <<= 8;
>> +        r -= 8;
>> +    }
>> +    if ( !(x & UPPER_MASK(4)) )
>> +    {
>> +        x <<= 4;
>> +        r -= 4;
>> +    }
>> +    if ( !(x & UPPER_MASK(2)) )
>> +    {
>> +        x <<= 2;
>> +        r -= 2;
>> +    }
>> +    if ( !(x & UPPER_MASK(1)) )
>> +    {
>> +        x <<= 1;
>> +        r -= 1;
>> +    }
>> +
>> +    return r;
>> +}
> While, as you say, the expectation is for this code to not commonly come
> into actual use, I still find the algorithm a little inefficient in terms
> of the constants used, specifically considering how they would need
> instantiating in resulting assembly. It may be that Arm's fancy constant-
> move insns can actually efficiently synthesize them, but I think on most
> other architectures it would be more efficient (and presumably no less
> efficient on Arm) to shift the "remaining" value right, thus allowing for
> successively smaller (and hence easier to instantiate) constants to be
> used.

ARM can only synthesise UPPER_MASK(16) and narrower masks, I think.

That said, I'm not concerned about the (in)efficiency seeing as this
doesn't get included in x86/ARM/PPC builds by the end of the series.

It's RISC-V which matters, and I'm pretty sure this is the wrong
algorithm to be using.

Incidentally, this algorithm is terrible for superscalar pipelines,
because each branch is inherently unpredictable.

Both these files want rewriting based on an analysis of the H-capable
Zbb-incapable RISC-V cores which exist.

I expect that what we actually want is the De Bruijn form which is an
O(1) algorithm, given a decent hardware multiplier.  If not, there's a
loop form which I expect would still be better than this.

~Andrew