[COMMITTED] path solver: Solve PHI imports first for ranges.

Aldy Hernandez aldyh@redhat.com
Sat Nov 13 09:41:02 GMT 2021

On Sat, Nov 13, 2021 at 1:51 AM Andrew MacLeod <amacleod@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:


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


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