On Sat, Nov 13, 2021 at 2:26 PM Richard Biener <richard.guent...@gmail.com> wrote: > > On November 13, 2021 10:41:02 AM GMT+01:00, Aldy Hernandez <al...@redhat.com> > wrote: > >On Sat, Nov 13, 2021 at 1:51 AM Andrew MacLeod <amacl...@redhat.com> wrote: > >> > >> On 11/12/21 14:50, Richard Biener via Gcc-patches wrote: > >> > On November 12, 2021 8:46:25 PM GMT+01:00, Aldy Hernandez via > >> > Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > >> >> PHIs must be resolved first while solving ranges in a block, > >> >> regardless of where they appear in the import bitmap. We went through > >> >> a similar exercise for the relational code, but missed these. > >> > Must not all stmts be resolved in program order (for optimality at > >> > least)? > >> > >> Generally,Imports are live on entry values to a block, so their order is > >> not particularly important.. they are all simultaneous. PHIs are also > >> considered imports for data flow purposes, but they happen before the > >> first stmt, all simultaneously... they need to be distinguished because > >> phi arguments can refer to other phi defs which may be in this block > >> live around a back edge, and we need to be sure we get the right version. > >> > >> we should look closer to be sure this isn't an accidental fix that > >> leaves the root problem . we need to be sure *all* the PHI arguments > >> are resolved from outside this block. whats the testcase? > > > >The testcase is the simpler testcase from the PR: > > > >https://gcc.gnu.org/bugzilla/attachment.cgi?id=51776 > > > >The gist is on a path coming in from BB13: > > > > # n_42 = PHI <m_31(13), addr_14(D)(4)> > > # m_31 = PHI <0(13), m_16(4)> > > > >We were solving m_31 first and putting it in the cache, and then the > >calculation for n_42 picked up this cached m_31 incorrectly. > > > >With my patch we do the PHIs first, in whatever gphi_iterator order > >uses, which I assume is the order in the IL above. > > > >However, if PHIs must be resolved simultaneously, then perhaps we need > >to tweak this. Suppose we flip the definitions: > > > > # m_31 = PHI <0(13), m_16(4)> > > # n_42 = PHI <m_31(13), addr_14(D)(4)> > > > >I assume the definition of n_42 should pick up the incoming m_31(13), > >not one defined in the other PHI. In which case, we could resolve all > >the PHIs first, but put them in the cache after we're done with all of > >them. > > PHI order is irrelevant, they are executed in parallel, thus arguments pick > up the old value irrespective of order. >
Ughh, yeah. Just noticed, per my follow-up patch for PR103222. Tested on x86-64 & ppc64le Linux, and pushed. Thanks. Aldy
