Re: [PATCH] xen/vpci: Improve code generation in mask_write()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Mar 15, 2024 at 12:13:22PM +0000, Andrew Cooper wrote:
> The use of __clear_bit() forces dmask to be spilled to the stack, and
> interferes with the compiler heuristcs for some upcoming improvements to the
> ffs() code generation.
> 
> First, shrink dmask to just the active vectors by making out the upper bits.
> This replaces the "i < msi->vectors" part of the loop condition.
> 
> Next, use a simple while() loop with "clear bottom bit" expressed in plane C,
> which affords the optimiser a far better understanding of what the loop is
> doing.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.