Ping**2
On Fri, May 17, 2013 at 9:01 PM, Steven Bosscher <stevenb....@gmail.com> wrote:
> Ping
>
> On Sun, May 12, 2013 at 5:45 PM, Steven Bosscher wrote:
>> Hello,
>>
>> While working on a sched-deps based delay slot scheduler, I've come to
>> the conclusion that the dependencies themselves must indicate whether
>> the dependent ref is delayed. So I started hacking sched-deps and ran
>> into trouble... It turns out there is a problem introduced along with
>> DEP_POSTPONED last year, but the real problem is the complicated ds_t
>> representation and the unclear documentation. The first *6* bits on a
>> ds_t were reserved for the dependence type, and two more bits were
>> reserved for HARD_DEP and DEP_CANCELLED:
>>
>> -/* First '6' stands for 4 dep type bits and the HARD_DEP and DEP_CANCELLED
>> - bits.
>> - Second '4' stands for BEGIN_{DATA, CONTROL}, BE_IN_{DATA, CONTROL}
>> - dep weakness. */
>> -#define BITS_PER_DEP_WEAK ((BITS_PER_DEP_STATUS - 6) / 4)
>>
>> But DEP_POSTPONED adds another bit:
>>
>> /* Instruction has non-speculative dependence. This bit represents the
>> property of an instruction - not the one of a dependence.
>> Therefore, it can appear only in the TODO_SPEC field of an instruction. */
>> #define HARD_DEP (DEP_CONTROL << 1)
>>
>> /* Set in the TODO_SPEC field of an instruction for which new_ready
>> has decided not to schedule it speculatively. */
>> #define DEP_POSTPONED (HARD_DEP << 1)
>>
>> /* Set if a dependency is cancelled via speculation. */
>> #define DEP_CANCELLED (DEP_POSTPONED << 1)
>>
>> I wanted to add another flag, DEP_DELAYED, and optimistically just
>> added another bit, the compiler started warning, etc.
>>
>>
>> So far we seem to've gotten away with this because the sign bit on a
>> ds_t was unused:
>>
>> /* We exclude sign bit. */
>> #define BITS_PER_DEP_STATUS (HOST_BITS_PER_INT - 1)
>>
>>
>> The attached patch extends the ds_t documentation to clarify in a
>> comment how all the bits are used. I've made ds_t and dw_t unsigned
>> int, because ds_t is just a collection of bits, and dw_t is unsigned.
>> The target hooks I had to update are all only used by ia64.
>>
>> I opportunistically reserved the one left-over bit for my own purposes ;-)
>>
>> Bootstrapped&tested on ia64-unknown-linux-gnu and on
>> powerpc64-unknown-linux-gnu unix-{,-m32).
>> OK for trunk?
>>
>> Ciao!
>> Steven
