Re: [Qemu-devel] [PATCH 13/12] qapi: refine human printing of sizes

[Eric Blake]

On 01/30/2014 09:42 AM, Paolo Bonzini wrote:
> This fixes several bugs or shortcomings of the previous pretty-printer.
> In particular:
> 
> * use PRIu64 instead of casting to long long
> 
> * the exact value is included too
> 
> * the correct unit of measure (MiB, GiB, etc.) is used.  PiB and EiB
> are added too.
> 
> * due to an off-by-one error, 512*2^30 was printed as 0.500MiB rather than
> 512MiB.  floor(log2(val)) is equal to 63 - clz(val), while the code used 64.
> 
> * The desired specification is %g rather than %f, which always uses three
> decimals in the current code.  However %g would switch to scientific
> notation when the integer part is >= 1000 (e.g. 1000*2^30).  To keep the
> code simple, switch to the higher power when the integer part is >= 1000;
> overflow is avoided by using frexp instead of clz.
> 
> Suggested-by: Eric Blake <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  qapi/string-output-visitor.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 

>  
> -    out = g_strdup_printf("%0.03f%c", (double)val/div, suffixes[i]);
> +    out = g_strdup_printf("%"PRIu64" (%0.3g %c%s)", val,

For 1152, the old code would print 1.125k, while the new code prints
only 1.12k.  But that's acceptable in my eyes (Remember, %0.3g is how
many significant digits, where both 0.999 and 1.01 have 3 digits).

I also checked that overflow is not possible, and checked several border
cases.  Much nicer!

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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