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On 11/07/11 09:24, Richard Guenther wrote:
> On Mon, Nov 7, 2011 at 5:07 PM, Jeff Law <l...@redhat.com> wrote:
>
>>>>
>>>> I also wonder if instead of a new pass control-dependent DCE
>>>> can be taught of this?
> It could. I'm not sure it really buys us anything except maybe
> being able to reuse the pdom & control-dependency analysis.
>
> It actually more naturally belongs in cprop because it's cprop
> that exposes NULL pointers in useful ways (in memory dereferences
> and as PHI args).
>
>> Heh, indeed, that as well. I suppose every pass that handles
>> unexecutable edges in some way might benefit from this. Like
>> for
>
>> if (i) j_1 = 1; else { j_2 = 2; *0 = ...; } j_3 = PHI <j_1, j_2>
>
>> we could CCP j_1 = 1 to j_3 ...
Yea, but we already do that... We'd propagate the constant 1 into the
PHI. DOM, CCP and probably VRP all do this..
>
>> Even without actually removing the edge and the trapping stmt.
I'm guessing you mean we could propagate j_1's value to the uses of
j_3. This is true, but misses the primary effect of the patch, namely
to remove the controlling conditional branch which leads to the *0 (if
(i)) in your sample.
Those controlling conditions *are* executed at runtime and we want to
remove them if at all possible. Once that's done all the things you'd
expect/want to happen do so naturally with the existing optimizers.
>
>> For DCE the most interesting part would be to end the block at
>> the point of the trap - thus - replace the store with a
>> __builtin_trap () which then trivially makes all code w/o
>> side-effects on the path dead.
But we're still stuck with the conditional leading to the path with
the __builtin_trap. That's what we want to avoid since those
conditionals are executed at runtime.
Jeff
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