On 02/23/2018 08:29 AM, Peter Maydell wrote:
> On 17 February 2018 at 18:22, Richard Henderson
> <[email protected]> wrote:
>> Signed-off-by: Richard Henderson <[email protected]>
>> ---
>
>> diff --git a/target/arm/sve_helper.c b/target/arm/sve_helper.c
>> index 86cd792cdf..ae433861f8 100644
>> --- a/target/arm/sve_helper.c
>> +++ b/target/arm/sve_helper.c
>> @@ -46,14 +46,14 @@
>> *
>> * The return value has bit 31 set if N is set, bit 1 set if Z is clear,
>> * and bit 0 set if C is set.
>> - *
>> - * This is an iterative function, called for each Pd and Pg word
>> - * moving forward.
>> */
>>
>> /* For no G bits set, NZCV = C. */
>> #define PREDTEST_INIT 1
>>
>> +/* This is an iterative function, called for each Pd and Pg word
>> + * moving forward.
>> + */
>
> Why move this comment?
Meant to fold this to the first. But moving so that I can separately
document...
>> +/* This is an iterative function, called for each Pd and Pg word
>> + * moving backward.
>> + */
>> +static uint32_t iter_predtest_bwd(uint64_t d, uint64_t g, uint32_t flags)
... this.
>> + do {
>> \
>> + uint64_t out = 0, pg;
>> \
>> + do {
>> \
>> + i -= sizeof(TYPE), out <<= sizeof(TYPE);
>> \
>> + TYPE nn = *(TYPE *)(vn + H(i));
>> \
>> + TYPE mm = *(TYPE *)(vm + H(i));
>> \
>> + out |= nn OP mm;
>> \
>> + } while (i & 63);
>> \
>> + pg = *(uint64_t *)(vg + (i >> 3)) & MASK;
>> \
>> + out &= pg;
>> \
>> + *(uint64_t *)(vd + (i >> 3)) = out;
>> \
>> + flags = iter_predtest_bwd(out, pg, flags);
>> \
>> + } while (i > 0);
>> \
>> + return flags;
>> \
>> +}
>
> Why do we iterate backwards through the vector? As far as I can
> see the pseudocode iterates forwards, and I don't think it
> makes a difference to the result which way we go.
You're right, it does not make a difference to the result which way we iterate.
Of the several different ways I've written loops over predicates, this is my
favorite. It has several points in its favor:
1) Operate on full uint64_t predicate units instead
of uint8_t or uint16_t sub-units. This means
1a) No big-endian adjustment required,
1b) Fewer memory loads.
2) No separate loop tail; it is shared with the main loop body.
3) A sub-point specific to predicate output, but the main loop
gets to run un-predicated. Here the governing predicate is
applied at the end: out &= pg.
r~
