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
