Re: [Qemu-devel] [PATCH 0/5] replace qemu_fls() with pow2ceil()/pow2floor()

[Paolo Bonzini]

On 23/07/2015 13:08, Peter Maydell wrote:
> We have a qemu_fls() function which is just a silly wrapper
> around clz32() and which is used in only a handful of places
> in the codebase. It turns out that all of those are really
> trying to round up or down to a power of 2, which is something
> we have utility functions for. This series replaces all
> the qemu_fls() calls with pow2ceil() or pow2floor(), and then
> removes the now-unused function.

The series looks good, but I'd prefer (especially for patch 4 which is
in a fast path) if pow2ceil and pow2floor were made inline.

BTW,

    return 1ULL << (64 - nlz);

can be implemented as "rotate_right(1ULL, nlz)" (the latter is (1ULL <<
(64 - nlz)) | (1ULL >> nlz), which only differs in the undefined case
nlz == 0).  However, this is probably something for the compiler to
implement, not really something to do in our sources.

Paolo

> For the case where you really want to do bit counting rather
> than just power-of-2 rounding, you should use the clz/clo
> functions directly.
> 
> (I've set myself a little goal of "try to do one cleanup a
> day"; that may be a bit ambitious, so we'll see...)
> 
> Peter Maydell (5):
>   hw/pci: Use pow2ceil() rather than hand-calculation
>   hw/virtio/virtio-pci: Use pow2ceil() rather than hand-calculation
>   hw/block/nvme.c: Use pow2ceil() rather than hand-calculation
>   exec.c: Use pow2floor() rather than hand-calculation
>   Remove unused qemu_fls function
> 
>  exec.c                 | 4 +---
>  hw/block/nvme.c        | 2 +-
>  hw/pci/msix.c          | 4 +---
>  hw/pci/pci.c           | 4 +---
>  hw/virtio/virtio-pci.c | 4 +---
>  include/qemu-common.h  | 1 -
>  util/cutils.c          | 5 -----
>  7 files changed, 5 insertions(+), 19 deletions(-)
>