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

[Richard Biener]

On November 13, 2021 10:41:02 AM GMT+01:00, Aldy Hernandez <aldyh@redhat.com> wrote:
>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:
>
>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. 

Richard. 
>
>Thoughts?
>Aldy
>