Re: [patch][rfc] make tree PRE code-size aware

[Richard Guenther <rguenther at suse dot de>]

On Thu, 4 Dec 2008, Steven Bosscher wrote:

> On Thu, Dec 4, 2008 at 11:41 AM, Richard Guenther <rguenther@suse.de> wrote:
> > I think it's reasonable.
> >
> > +                 /* If the available value is not a NAME, PREing this
> > +                    value will probably result in a copy on the edge
> > +                    to assign the expression to a register.  */
> > +                 if (edoubleprime->kind != NAME)
> > +                   inserts_needed++;
> >
> > Any reason to not handle constants the same as NAME?  I'll throw the
> > patch on vangelis.
> 
> Because constants killed code size more than anything else on CSiBE,
> benchmark "compiler", file "vam.c", function trace().
> 
> What happens, is that PRE creates new PHIs with constants in
> situations like this:
> 
> { pred ENTRY }
> BB1
> if (...) goto BB2 else goto BB3
> { succ BB2, BB3 }
> 
> { pred BB1 }
> BB2
> ...
> if (...) goto BB4 else goto BB5
> { succ BB4, BB5 }
> 
> { pred BB1 }
> BB3
> ...
> goto BB5
> { succ BB5 }
> 
> { pred BB2 }
> BB5
> b_3 = ...
> ...
> goto BB5
> { succ BB5 }
> 
> { pred BB2, BB3, BB4 }
> BB5
> b_4 = PHI<10,20,b_3>
> b_5 = b_4 + 4
> ...
> b_6 = b_4 + 8
> ...
> b_7 = b_4 + 12
> ...
> b_8 = b_4 + 16
> ...
> (...etc...)
> { succ EXIT }
> 
> 
> 
> where you notice that the constants in the PHI are available, and the
> additions to b_4 can also be made available as constants when PRE
> transforms the above to:
> 
> 
> { pred ENTRY }
> BB1
> if (...) goto BB2 else goto BB3
> { succ BB2, BB3 }
> 
> { pred BB1 }
> BB2
> ...
> if (...) goto BB4 else goto BB5
> { succ BB4, BB5 }
> 
> { pred BB1 }
> BB3
> ...
> goto BB5
> { succ BB5 }
> 
> { pred BB2 }
> BB5
> b_3 = ...
> b_9 = b_3 + 4
> b_10 = b_3 + 8
> b_11 = b_3 + 12
> b_12 = b_3 + 16
> ...
> goto BB5
> { succ BB5 }
> 
> { pred BB2, BB3, BB4 }
> BB5
> b_4 = PHI<10,20,b_3>
> b_13 = PHI<14,24,b_9>
> b_14 = PHI<18,28,b_10>
> b_15 = PHI<22,32,b_11>
> b_16 = PHI<26,36,b_12>
> ...
> (...etc...)
> { succ EXIT }
> 
> 
> On going out of SSA, we need to insert assignments for the constants
> for b_4, b_13, b_14, b_15, and b_16. Notice how register pressure has
> just gone up on entry to BB5 from 1 before PRE to 5 after this
> transformation.
> 
> In short: Constants are *NOT* free.
> 
> In the case of vam.c:trace(), PRE creates many dozens of new PHIs like
> this.  The result is many spills (even on ia64).  Code size more than
> doubles, and I imagine performance also takes a bad hit.
> 
> Therefore, in my patch, I assumed anything that is not a NAME needs an
> insertion.
> 
> Does that answer your question?

Yes.

Thanks,
Richard.