[Bug 1860876] [NEW] wrong code generated with -O3 and -mssse3

[Andrew Wesie]

Public bug reported:

When compiling a program with -O3 and -mssse3, gcc 9 may generate code
with incorrect behavior. The gcc master branch does not have this issue,
so I was able to bisect the fix to PR92420
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92420). This fix should be
backported to gcc 9, at a minimum.

I also minimized the issue to a single file which is attached. The
commit for the PR referenced above also contains its own test case.

Tested on Ubuntu 19.10 with 9.2.1-9ubuntu2 and Ubuntu 20.04 with
9.2.1-25ubuntu1.

** Affects: gcc-9 (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "Test case based on broken program due to compiler"
   https://bugs.launchpad.net/bugs/1860876/+attachment/5322921/+files/gcc-test1.c

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https://bugs.launchpad.net/bugs/1860876

Title:
  wrong code generated with -O3 and -mssse3

Status in gcc-9 package in Ubuntu:
  New

Bug description:
  When compiling a program with -O3 and -mssse3, gcc 9 may generate code
  with incorrect behavior. The gcc master branch does not have this
  issue, so I was able to bisect the fix to PR92420
  (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92420). This fix should
  be backported to gcc 9, at a minimum.

  I also minimized the issue to a single file which is attached. The
  commit for the PR referenced above also contains its own test case.

  Tested on Ubuntu 19.10 with 9.2.1-9ubuntu2 and Ubuntu 20.04 with
  9.2.1-25ubuntu1.

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