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

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 30 Jan 2014 17:42:25 +0100
Paolo Bonzini <pbonzini@redhat.com> 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 <eblake@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  qapi/string-output-visitor.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
> index 67a8798..95dd8fa 100644
> --- a/qapi/string-output-visitor.c
> +++ b/qapi/string-output-visitor.c
> @@ -15,6 +15,7 @@
>  #include "qapi/visitor-impl.h"
>  #include "qapi/qmp/qerror.h"
>  #include "qemu/host-utils.h"
> +#include <math.h>
>  
>  struct StringOutputVisitor
>  {
> @@ -47,30 +48,30 @@ static void print_type_size(Visitor *v, uint64_t *obj, const char *name,
>                             Error **errp)
>  {
>      StringOutputVisitor *sov = DO_UPCAST(StringOutputVisitor, visitor, v);
> -    static const char suffixes[] = { 'B', 'K', 'M', 'G', 'T' };
> +    static const char suffixes[] = { 'B', 'K', 'M', 'G', 'T', 'P', 'E' };
>      uint64_t div, val;
>      char *out;
>      int i;
>  
>      if (!sov->human) {
> -        out = g_strdup_printf("%llu", (long long) *obj);
> +        out = g_strdup_printf("%"PRIu64, *obj);
>          string_output_set(sov, out);
>          return;
>      }
>  
>      val = *obj;
>  
> -    /* Compute floor(log2(val)).  */
> -    i = 64 - clz64(val);
> -
> -    /* Find the power of 1024 that we'll display as the units.  */
> -    i /= 10;
> -    if (i >= ARRAY_SIZE(suffixes)) {
> -        i = ARRAY_SIZE(suffixes) - 1;
> -    }
> +    /* The exponent (returned in i) minus one gives us
> +     * floor(log2(val * 1024 / 1000).  The correction makes us
> +     * switch to the higher power when the integer part is >= 1000.
> +     */
> +    frexp(val / (1000.0 / 1024.0), &i);
> +    i = (i - 1) / 10;
> +    assert(i < ARRAY_SIZE(suffixes));
>      div = 1ULL << (i * 10);
>  
> -    out = g_strdup_printf("%0.03f%c", (double)val/div, suffixes[i]);
> +    out = g_strdup_printf("%"PRIu64" (%0.3g %c%s)", val,
> +			  (double)val/div, suffixes[i], i ? "iB" : "");
ERROR: code indent should never use tabs
#81: FILE: qapi/string-output-visitor.c:74:
+^I^I^I  (double)val/div, suffixes[i], i ? "iB" : "");$

>      string_output_set(sov, out);
>  }
>