Re: [Qemu-trivial] [Qemu-devel] [PATCH 19/29] disas: use QEMU_IS_ALIGNED macro

[Eric Blake <eblake@redhat.com>]

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On 07/23/2017 09:52 AM, Paolo Bonzini wrote:

> 
> But I agree that QEMU_IS_ALIGNED looks weird here.  I think it should
> mostly be used when the argument is a pointer, to hide the cast.  Uses
> for non-pointer arguments should be decided on a one-by-one basis; "is
> the first argument an address" is definitely a good thing to consider.
> Another might be "is the second argument a variable", because "i % n ==
> 0" is less efficient than "(i & (n - 1)) == 0".

How often is QEMU_IS_ALIGNED used on non-power-of-2?  Would it be worth
rewriting it into bit-wise ops, at the expense of declaring that any
code aligning to other values must open-code their own %?  Can we have
the compiler help us ensure that the second argument is a power-of-2?
Conversely, if the compiler is given a constant for the second argument,
and is not optimizing modulo power of two into bitwise ops (at least at
CFLAGS=-O2), then that's a compiler bug.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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