[PR42630] reset debug insns referencing unroll-expanded vars

[Richard Guenther]

On Fri, Jan 8, 2010 at 1:16 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Fri, Jan 8, 2010 at 8:21 AM, Alexandre Oliva <aoliva@redhat.com> wrote:
>> -fvariable-expansion-in-unroller wants to turn:
>>
>> <loop>
>> ...
>>  R = R + 1
>> ...
>> <endloop>
>>
>> into
>>
>>  R' = 0
>> <loop>
>> ...
>>  R = R + 1
>> ...
>> <unrolled>
>> ...
>>  R' = R' + 1
>> ...
>> <endloop>
>>  R = R + R'
>>
>> It requires R to be referenced in a single insn.  This means a debug
>> insn would disable the optimization.  That was easy to fix.
>>
>> Not so easy was figuring out how to adjust the debug insns.  Consider:
>>
>> <loop>
>> ...
>>  R = R + 1
>>  # debug var => R
>> ...
>> <endloop>
>>  # debug var => R
>>
>> We could correctly expand this to:
>>
>>  R' = 0
>> <loop>
>> ...
>>  R = R + 1
>>  # debug var => R + R'
>> ...
>> <unrolled>
>> ...
>>  R' = R' + 1
>>  # debug var => R + R'
>> ...
>> <endloop>
>>  R = R + R'
>>  # debug var => R
>>
>> However, taking note of which debug insns will require updating because
>> of each expanded var, and performing the updates at the end, turned out
>> to require working too hard.  I decided to just reset the debug insns
>> instead, which at least fixes the incorrect debug info we'd get if we
>> left them alone referencing the original R.
>>
>> Ok to install if it passes regstrapping?
>
> You need to free body in reset_debug_uses_in_loop.
>
> Otherwise ok - even though I don't like a 2nd pass over all basic blocks
> in the loop too much.  I suppose you couldn't have avoided copying
> the debug insn during the unrolling instead?

Having a 2nd look there should be exactly one such debug insn, so
why not reset it in referenced_in_one_insn_in_loop_p instead,
moving the mode checks before the call.   Also referenced_in_one_insn_in_loop_p
also needs to free body.

Richard.

> Thanks,
> Richard.
>
>>
>>
>> --
>> Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
>> You must be the change you wish to see in the world. -- Gandhi
>> Be Free! -- http://FSFLA.org/   FSF Latin America board member
>> Free Software Evangelist      Red Hat Brazil Compiler Engineer
>>
>>
>