Re: [PATCH 7/9] arm: replace arbitrary divisions

[Christoffer Dall <christoffer.dall@linaro.org>]

On Mon, Oct 14, 2013 at 06:23:33PM +0200, Andrew Jones wrote:
> arm can't do arbitrary divisions without software support. Usually
> libgcc would jump in here, but depending on the toolchain used that may
> or may not work. Luckily, we only care about two types of divisions.
> Divide by 10 and divide by 16. Divide by 16 is already covered by gcc
> since it's a power of two. Divide by 10 can be hacked up using a
> multiplication and shift.

Isn't this just a matter of supplying a few libc implementations to
handle div_by_0 and that sort?  I'm pretty sure we had that working in
the kvm-selftest for ARM thingy that allowed you to use the standard '/'
operator in C code.... I suspect there will be more users of this
eventually.

Or wait, do you mean 'long long' operations by arbitrary? In that case,
I'm less sure...  A library implementation to support the operator would
still be preferred IMHO.

> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  lib/divmod.h | 20 ++++++++++++++++++++
>  lib/printf.c | 27 ++++++++++++++-------------
>  2 files changed, 34 insertions(+), 13 deletions(-)
>  create mode 100644 lib/divmod.h
> 
> diff --git a/lib/divmod.h b/lib/divmod.h
> new file mode 100644
> index 0000000000000..c0f04d7d8386d
> --- /dev/null
> +++ b/lib/divmod.h
> @@ -0,0 +1,20 @@
> +#ifndef _DIVMOD_H_
> +#define _DIVMOD_H_
> +#ifdef __arm__
> +static inline long long div10(long long n)
> +{
> +    /*
> +     * multiply n by 2^32/10 and the result of n/10
> +     * will be in the upper word
> +     */
> +    return (n * 0x1999999aU) >> 32;
> +}
> +static inline int mod10(long long n)
> +{
> +    return n - div10(n) * 10;
> +}
> +#else
> +#define div10(n) ((n) / 10)
> +#define mod10(n) ((n) % 10)
> +#endif
> +#endif
> diff --git a/lib/printf.c b/lib/printf.c
> index 867eb1926f742..9fb8133868c7a 100644
> --- a/lib/printf.c
> +++ b/lib/printf.c
> @@ -1,4 +1,5 @@
>  #include "libcflat.h"
> +#include "divmod.h"
>  
>  typedef struct pstream {
>      char *buffer;
> @@ -23,7 +24,7 @@ void print_str(pstream_t *p, const char *s)
>  
>  static char digits[16] = "0123456789abcdef";
>  
> -void print_int(pstream_t *ps, long long n, int base)
> +void print_int(pstream_t *ps, long long n)
>  {
>      char buf[sizeof(long) * 3 + 2], *p = buf;
>      int s = 0, i;
> @@ -34,8 +35,8 @@ void print_int(pstream_t *ps, long long n, int base)
>      }
>  
>      while (n) {
> -	*p++ = digits[n % base];
> -	n /= base;
> +	*p++ = digits[mod10(n)];
> +	n = div10(n);
>      }
>  
>      if (s)
> @@ -57,14 +58,14 @@ void print_int(pstream_t *ps, long long n, int base)
>      print_str(ps, buf);
>  }
>  
> -void print_unsigned(pstream_t *ps, unsigned long long n, int base)
> +void print_unsigned(pstream_t *ps, unsigned long long n)
>  {
>      char buf[sizeof(long) * 3 + 1], *p = buf;
>      int i;
>  
>      while (n) {
> -	*p++ = digits[n % base];
> -	n /= base;
> +	*p++ = digits[n % 16];
> +	n /= 16;
>      }
>  
>      if (p == buf)
> @@ -116,32 +117,32 @@ int vsnprintf(char *buf, int size, const char *fmt, va_list va)
>  	case 'd':
>  	    switch (nlong) {
>  	    case 0:
> -		print_int(&s, va_arg(va, int), 10);
> +		print_int(&s, va_arg(va, int));
>  		break;
>  	    case 1:
> -		print_int(&s, va_arg(va, long), 10);
> +		print_int(&s, va_arg(va, long));
>  		break;
>  	    default:
> -		print_int(&s, va_arg(va, long long), 10);
> +		print_int(&s, va_arg(va, long long));
>  		break;
>  	    }
>  	    break;
>  	case 'x':
>  	    switch (nlong) {
>  	    case 0:
> -		print_unsigned(&s, va_arg(va, unsigned), 16);
> +		print_unsigned(&s, va_arg(va, unsigned));
>  		break;
>  	    case 1:
> -		print_unsigned(&s, va_arg(va, unsigned long), 16);
> +		print_unsigned(&s, va_arg(va, unsigned long));
>  		break;
>  	    default:
> -		print_unsigned(&s, va_arg(va, unsigned long long), 16);
> +		print_unsigned(&s, va_arg(va, unsigned long long));
>  		break;
>  	    }
>  	    break;
>  	case 'p':
>  	    print_str(&s, "0x");
> -	    print_unsigned(&s, (unsigned long)va_arg(va, void *), 16);
> +	    print_unsigned(&s, (unsigned long)va_arg(va, void *));
>  	    break;
>  	case 's':
>  	    print_str(&s, va_arg(va, const char *));
> -- 
> 1.8.1.4
>