A new gimple pass (LRS: live range shrinking) to reduce register pressure

Xinliang David Li davidxl@google.com
Thu Jan 1 18:33:00 GMT 2009


On Thu, Jan 1, 2009 at 9:02 AM, Steven Bosscher <stevenb.gcc@gmail.com> wrote:
> Xinliang David Li wrote:
>> * An iterative data flow analysis (live use references). The result
>> of the analysis is used to estimate the register pressure as well
>> as the impact (cost/benefit) of code motions on the register pressure.
>> The data flow result can be easily updated under various transformations.
>> * An upward code motion pass to shrink live ranges
>> * A downward code motion pass to perform subtree scheduling (to reduce
>> overlapping live ranges). Multiple use trees are also scheduled downward
>> if profitable
>> * A forward data flow analysis to compute reaching virtual defs -- the
>> result of this analysis is used for legality check for downward motion
>> of statements with virtual uses
>> * An expression tree reassociation pass to enable more opportunities
>> for overlapping live range reduction (this is complementary to the
>> existing reassociation pass, but with a different objective).
>
> Are all these analyses/transformations interdependent?

All the transformations share the same data flow analysis result.

>
> Is there a reason for not just adding the reassociation
> transformations to tree-ssa-reassoc.c?

1) see above -- there is dependency on the data flow result
2) the optimization objectives are different and therefore the tree
operand reordering scheme may be conflict. The one in
tree-ssa-reassoc.c is based on operand ranks in order to expose
opportunities for simplification (folding), factorization, PRE/LICM,
and  to reduce dependence height. The one in LRS is to maximizing
freedom for lrs.

>
>
>
>> +sbitmap_a_and_not_b (sbitmap dst, const_sbitmap a, const_sbitmap b)
>
> See sbitmap_difference().

They are similar -- the latter is more general and has more checks
(unnecessary when sizes are the same).
>
>
>
>> @@ -1095,4 +1155,3 @@ sbitmap_popcount (const_sbitmap a, unsig
>>      }
>>    return count;
>>  }
>> -
>
> Redundant white space change.
>
>
>
>> +DEFPARAM (PARAM_REG_PRESSURE_MIN_BB,
>> +       "reg-pressure-min-bb",
>> +          "The parmater to control the behavior of the lrs shrinking"
>> +          "phase",
>> +          80, 1, 10000)
>
> The description is not very useful (for all 3 params). You should also
> add documentation for the new params in doc/invoke.texi.

Ok.

>
>
>
>> Index: passes.c
>> ===================================================================
>> --- passes.c  (revision 142954)
>> +++ passes.c  (working copy)
>> @@ -674,6 +674,7 @@ init_optimization_passes (void)
>>        NEXT_PASS (pass_cse_reciprocals);
>>        NEXT_PASS (pass_convert_to_rsqrt);
>>        NEXT_PASS (pass_reassoc);
>> +      NEXT_PASS (pass_lrs);
>>        NEXT_PASS (pass_vrp);
>>        NEXT_PASS (pass_dominator);
>>        /* The only const/copy propagation opportunities left after
>
> If you run the pass here, you still have tracer and forwprop (and some
> more) running after live range shrinking.  With profile-guided
> optimization, tracer restructures the CFG completely, so this would
> change the live ranges completely too. Therefore I'd run this pass
> after tracer. And forwprop propagates SSA names, maybe extending live
> ranges, so you probably want to run it after the last fwprop too.

Good point.


>
> In fact, I would expect you want to run this as the last pass, maybe
> even after uncprop.
>
>
>
>> +/* The function returns the unique id of the gimple STMT.  */
>> +
>> +static inline int
>> +get_stmt_idx_in_region (gimple stmt)
>> +{
>> +  return gimple_uid (stmt);
>> +}
>
> Don't know what else to say then wtf.
>

More readable code ? I can make it a macro if it makes you happier :)


>
>> +is_gimple_assign_rhs_reassociable (enum tree_code rhs_code,
>
> ISTR we have similar checks in tree-ssa-reassoc.c.  Maybe move the
> function there and export it (unless you can move all the reassoc code
> to that file, of course - that'd be the preferred solution ;-)
>

Yes, I thought about that -- some code factorization is needed.

> I don't see the relation between all the functions in the new
> tree-ssa-lrs.c file.  You probably want to group functions and add
> some overviews for the different phases/passes that you implement.
> Bonus if you can split things per-pass into separate files...

Yes, Richard has the same comment.

>
> How does this new pass (do these new passes?) impact compile time performance?
>

The fixed overhead is the loop recognition. The rest depends on the
existence of large inner loops (or standlone bbs) in functions which
is usually not so common.  I did not see difference in compiler
bootstrap time in my earlier experiment.

Thanks,

David


> Gr.
> Steven
>



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