[Bug rtl-optimization/60749] New: combine is overly cautious when operating on volatile memory references

["amylaar at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60749

            Bug ID: 60749
           Summary: combine is overly cautious when operating on volatile
                    memory references
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amylaar at gcc dot gnu.org
            Blocks: 53938

Curtesy of volatile_ok / init_recog_no_volatile, combine will
reject any combination that involves a volatile memref in the combined
pattern.

In particular, if any narrow memory location is read on a
WORD_REGISTER_OPERATIONS target, the zero/sign extension can't be combined
with a memory read, even if a suitably extending memory load instruction is
available - unless that pattern gets specifically written to accept
volatile memrefs, shunning the standard memory_operand and
general_operand predicates.

combine already needs to do special checks to make sure it doesn't
slip up when handling such patterns (E.g. see PR51374), so what good
does init_recog_non_volatile do combine these days?

At the very least, I think we should allow combinations involving a single
memref with unchanged mode before and after combination - that woud cover
the zero and sign extending loads.