> > Second footnote: The live variable lists are updated during the simplify
> > phase when I verify that the all of the liveness information is correct.
> > Previously, I hadn't needed to do this, I just assumed that the values
> > filled in in backend.fun were correct. (Steve and I wrestled with this at
> > the end of the summer when we first added the liveness calculation to
> > backend.fun. It's correct, although in some cases it overly conservative,
> > claiming that variables are live into a block when they don't need to be.)
>
> Shoot. If you let me know where it's still conservative, I'll look into it.
Here's where I found some differences in the liveness calculations. These
programs were all run with -detect-overflow false to avoid all of the
overflow blocks. Unfortunately, I don't see a common error in these
cases.
regression/real.sml
This one is due to a missed CPS optimization:
fun L_1055(x_14390) =
case x_14390 of
false => L_889
| true => L_889
In MachineOutput, this became a Switch, which was optimized at translation
to a single jump to L_889. Thus, x_14390 (which became RI(0)) is no
longer live in.
benchmarks/barnes-hut.sml *
There were a number of differences in the liveness.
Here's the problematic CPS functions:
fun L_376(x_17179, x_17180) =
let
val x_16282 = Word32_fromInt(x_15515)
val x_16283 = Word32_fromInt(x_17175)
val x_16284 = Word32_andb(x_16282, x_16283)
val x_16279 = MLton_eq(x_16284, global_82)
fun L_700() =
let
val x_16286 = Int_add(x_17179, global_12)
in
x_17180
end
fun L_699() =
let
val x_16288 = Int_add(x_17179, global_12)
val x_16289 = Word32_fromInt(env_126)
val x_16290 = Word32_fromInt(x_16288)
val x_16287 = Word32_lt(x_16290, global_79)
fun L_703(x_16291) =
let
val x_16293 = Word32_toIntX(x_16291)
fun L_704() =
let
val x_16295 = Word32_toIntX(x_16291)
val x_16296 = Int_add(x_17180, x_16295)
val x_16297 = Int_add(x_17179, global_12)
in
x_16296
end
fun L_705() =
raise [global_24]
val x_16292 = Int_lt(x_16293, global_9)
in
case x_16292 of
false => L_704 | true => L_705
end
fun L_701() =
let
val x_16298 = Word32_arshift(x_16289, global_78)
in
L_703(x_16298)
end
fun L_702() =
let
val x_16299 = Word32_arshift(x_16289, x_16290)
in
L_703(x_16299)
end
in
case x_16287 of
false => L_701 | true => L_702
end
in
case x_16279 of
false => L_699 | true => L_700
end
The basic issues seems to be with L_704, where barnes-hut.reg shows that
x_16297 is unused, but x_17179 is assigned to RI(5), and the backend
reports that RI(5) is live into L_704. This is propagated back and the
backend reports that L_703, L_702, and L_701 all have RI(5) live in.
The same error occurs with L_700, where x_16289 is unused.
benchmarks/mlyacc.sml
fun L_7055() =
let
val x_72292 = Array_length(data_36)
fun L_7056() =
let
val x_72293 = Array_sub(data_36, x_65181)
in
L_5885(x_72293)
end
fun L_7057() =
raise [global_39]
val x_72291 = Int_geu(x_65181, x_72292)
in
case x_72291 of
false => L_7056 | true => L_7057
end
The variable x_72292 is assigned to RI(2), which is reported as live
into L_7056.
benchmarks/zern.sml
The CPS is a little bit big for this one, so here's the C backend output:
L_278:
/* live: L_278 [RI(3),RI(4),RI(1),RI(2)] */
RI(5) = Int_sub(RI(3), 1);
RI(6) = Int_add(RI(4), 1000000);
L_279:
/* live: L_279 [RI(5),RI(2),RI(1)] */
RI(6) = Int_lt(RI(5), 0);
BZ(RI(6), L_280)
Note that RI(4) shouldn't really be live into L_278, because
RI(6) = Int_add(RI(4), 1000000);
is dead code.
Here's the important part of the CPS:
fun L_279(x_5081, x_5082) =
let
fun L_280() = ... (big function, neither x_5081 nor x_5082 is free) ...
fun L_281() =
L_197(global_9)
val x_5039 = Int_lt(x_5082, global_3)
in
case x_5039 of
false => L_280 | true => L_281
end
fun L_277() = L_279(x_5037, x_5036)
fun L_278() =
let
val x_5075 = Int_sub(x_5036, global_5)
val x_5076 = Int_add(x_5037, global_7)
in
L_279(x_5076, x_5075)
end
The issue seems to be that x_5081 is really useless in L_279, but it
wasn't eliminated by the CPS shrinker. On the otherhand, the register
allocator established that x_5039 and x_5081 could share the same
pseudo-register.
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