Re: [Qemu-devel] [PATCH v4 1/6] target-ppc: Implement unsigned quadword left/right shift and unit tests

[Eric Blake <eblake@redhat.com>]

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On 12/19/2016 10:47 AM, Jose Ricardo Ziviani wrote:
> This commit implements functions to right and left shifts and the
> unittest for them. Such functions is needed due to instructions
> that requires them.
> 
> Today, there is already a right shift implementation in int128.h
> but it's designed for signed numbers.
> 
> Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> ---

> +static const test_data test_ltable[] = {
> +    { 1223ULL, 0, 1223ULL,   0, 0, false },
> +    { 1ULL,    0, 2ULL,   0, 1, false },
> +    { 1ULL,    0, 4ULL,   0, 2, false },
> +    { 1ULL,    0, 16ULL,  0, 4, false },
> +    { 1ULL,    0, 256ULL, 0, 8, false },
> +    { 1ULL,    0, 65536ULL, 0, 16, false },
> +    { 1ULL,    0, 2147483648ULL, 0, 31, false },
> +    { 1ULL,    0, 35184372088832ULL, 0, 45, false },
> +    { 1ULL,    0, 1152921504606846976ULL, 0, 60, false },
> +    { 1ULL,    0, 0, 1ULL, 64, false },
> +    { 1ULL,    0, 0, 65536ULL, 80, false },
> +    { 1ULL,    0, 0, 9223372036854775808ULL, 127, false },

I concur with the request to write these tests in hex.

> +    { 0ULL,    1, 0, 0, 64, true },
> +    { 0x8888888888888888ULL, 0x9999999999999999ULL,
> +        0x8000000000000000ULL, 0x9888888888888888ULL, 60, true },
> +    { 0x8888888888888888ULL, 0x9999999999999999ULL,
> +        0, 0x8888888888888888ULL, 64, true },
> +    { 0x8ULL, 0, 0, 0x8ULL, 64, false },
> +    { 0x8ULL, 0, 0, 0x8000000000000000ULL, 124, false },
> +    { 0x1ULL, 0, 0, 0x4000000000000000ULL, 126, false },
> +    { 0x1ULL, 0, 0, 0x8000000000000000ULL, 127, false },
> +    { 0x1ULL, 0, 0x1ULL, 0, 128, true },

Do we really want this to be well-defined behavior?  Or would it be
better to require shift to be in the bounded range [0,127] and assert()
that it is always in range?  At least your testsuite ensures that if we
want it to be well-defined, we won't go breaking it.

> +    { 0, 0, 0ULL, 0, 200, false },

If you are going to support shifts larger than 127, your testsuite
should include a shift of a non-zero number.  Also, if you are going to
implicitly truncate the shift value into range, then accepting a signed
shift might be nicer (as there are cases where it is easier to code a
shift by -1 than it is a shift by 127).

> +++ b/util/host-utils.c
> @@ -26,6 +26,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu/host-utils.h"
>  
> +#ifndef CONFIG_INT128
>  /* Long integer helpers */
>  static inline void mul64(uint64_t *plow, uint64_t *phigh,
>                           uint64_t a, uint64_t b)
> @@ -158,4 +159,47 @@ int divs128(int64_t *plow, int64_t *phigh, int64_t divisor)
>  
>      return overflow;
>  }
> +#endif

How is the addition of this #ifndef related to the rest of the patch?  I
almost wonder if it needs two patches (one to compile the file
regardless of 128-bit support, the other to add new 128-bit shifts); if
not, mentioning it in the commit message doesn't hurt.

>  
> +void urshift(uint64_t *plow, uint64_t *phigh, uint32_t shift)

Comments on the function contract would be much appreciated (for
example, what range must shift belong to, and the fact that the shift is
modifying the value in-place, and that the result is always zero-extended).

> +{
> +    shift &= 127;

This says you allow any shift value (whether negative or beyond 127);
either the testsuite must cover this, or you should tighten the contract
and assert that the callers pass a value in range.

> +    uint64_t h = *phigh >> (shift & 63);
> +    if (shift == 0) {
> +        return;

Depending on the compiler, this may waste the work of computing h; maybe
you can float this conditional first.

> +    } else if (shift >= 64) {
> +        *plow = h;
> +        *phigh = 0;
> +    } else {
> +        *plow = (*plow >> (shift & 63)) | (*phigh << (64 - (shift & 63)));
> +        *phigh = h;
> +    }

At any rate, the math looks correct.

> +}
> +
> +void ulshift(uint64_t *plow, uint64_t *phigh, uint32_t shift, bool *overflow)

Again, doc comments are useful, including what overflow represents, and
a repeat of the question on whether a signed shift amount makes sense if
you intend to allow silent truncation of the shift value.

> +{
> +    uint64_t low = *plow;
> +    uint64_t high = *phigh;
> +
> +    if (shift > 127 && (low | high)) {
> +        *overflow = true;
> +    }
> +    shift &= 127;
> +
> +    if (shift == 0) {
> +        return;
> +    }
> +
> +    urshift(&low, &high, 128 - shift);
> +    if (low > 0 || high > 0) {

Can't this be written 'if (low | high)' as above?

> +        *overflow = true;
> +    }
> +
> +    if (shift >= 64) {
> +        *phigh = *plow << (shift & 63);
> +        *plow = 0;
> +    } else {
> +        *phigh = (*plow >> (64 - (shift & 63))) | (*phigh << (shift & 63));
> +        *plow = *plow << shift;
> +    }
> +}
> 

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


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