[Bug rtl-optimization/71976] New: [avr] insn-combiner deletes a live 64-bit shift

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71976

            Bug ID: 71976
           Summary: [avr] insn-combiner deletes a live 64-bit shift
           Product: gcc
           Version: 6.1.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---
            Target: avr

Created attachment 38953
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38953&action=edit
bug-combin.c: C test case

In the following test case, combine.c removes an 64-bit shift and generates
wrong code.

== Compile ==

$ avr-gcc -mmcu=atmega8 bug-combin.c -S -dp -Os -fdump-rtl-combine-details
-fdump-rtl-ud_dce

== configure ==

Target: avr
Configured with: ../../gcc.gnu.org/gcc-6-branch/configure --target=avr
--prefix=/local/gnu/install/gcc-6-avr-mingw32 --host=i386-mingw32
--build=x86_64-linux-gnu --enable-languages=c,c++ --disable-nls
--disable-shared --enable-lto --with-dwarf2 --with-gnu-ld --with-gnu-as
Thread model: single
gcc version 6.1.1 20160627 (GCC)