[Bug target/66648] New: [4.9/5/6 regression] incorrect memcpy expansion with unrolled_loop strategy at -O2

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66648

            Bug ID: 66648
           Summary: [4.9/5/6 regression] incorrect memcpy expansion with
                    unrolled_loop strategy at -O2
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ebotcazou at gcc dot gnu.org
  Target Milestone: ---
            Target: x86_64-*-linux

Created attachment 35840
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35840&action=edit
Testcase

One of the numerous paths of ix86_expand_set_or_movmem using the unrolled_loop
strategy can generate wrong code (buffer overflow) at -O2, when VRP is able to
compute a minimum copying size.

Compile and run the attached testcase with -O2
-mstringop-strategy=unrolled_loop -mtune=nocona on a x86-64/Linux machine.