Re: PR 30194: temporary workaround

["Daniel Berlin" <dberlin at dberlin dot org>]

On 12/14/06, Diego Novillo <dnovillo@redhat.com> wrote:
> gcc.dg/pr19633-1.c has started exposing random failures after the
> mem-ssa merge.  We are making different partitioning decisions because
> the order of the symbols in the partitioned alias set changed.  So,
> sometimes we get one SFT into a partition which just happens to cross
> a call-site, and if it's added to the partition, we miss the
> optimization that removes the call to link_error.
>
> In the failing case, the SFT associated with b.s.w is added to MPT.69.
> So that we end up with:
>
>   <L1>:;
>     # MPT.69_8 = VDEF <MPT.69_7(D)> { MPT.69 }
>     b.s.w = 3;
>     # VUSE <SFT.54_9(D), SFT.55_10(D), SFT.56_11(D)> { SFT.54 SFT.55 SFT.56 SFT.62 }
>     # SFT.62_13 = VDEF <SFT.62_12(D)>
>     # MPT.69_14 = VDEF <MPT.69_8> { SFT.62 MPT.69 }
>     x = bar1 (*c_1, *c_1);
>     # VUSE <MPT.69_14> { MPT.69 }
>     D.1993_6 = b.s.w;
>
> Notice that the read from b.s.w will read from the call-clobbered
> partition MPT.69.  So, we never propagate the value '3' to the read
> from b.s.w.

> But if aliases are stored in different order in the alias set
> (different DECL_UID assignment), we just happen not to choose this SFT
> for partitioning, leaving the IL as:
>
>  <L1>:;
>     # SFT.61_8 = VDEF <SFT.61_7(D)> { SFT.61 }
>     b.s.w = 3;
>     # VUSE <SFT.58_9(D), SFT.59_10(D), SFT.60_11(D), SFT.61_8> { SFT.58 SFT.59 SFT.60 SFT.61 }
>     # MPT.69_13 = VDEF <MPT.69_12(D)> { MPT.69 }
>     x = bar1 (*c_1, *c_1);
>     # VUSE <SFT.61_8> { SFT.61 }
>     D.1990_6 = b.s.w;
>
> I will workaround this by increasing partitioning thresholds on this
> test, for now, but I will leave the PR assigned to me while I figure
> out better static partitioning heuristics.

I assume you are thinking of two things here:
1. Prioritze putting call clobbered vars into partitions ahead of
putting non-call clobbered vars into partitions
2.  Put call clobbered vars into call clobbered partitions and
non-call clobbered vars into non-call clobbered partitions, to avoid
adding more call clobbering than necessary.

>