[Bug target/81365] New: GCC miscompiles swap

["kaufmann at cs dot uni-potsdam.de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81365

            Bug ID: 81365
           Summary: GCC miscompiles swap
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kaufmann at cs dot uni-potsdam.de
  Target Milestone: ---

Created attachment 41700
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41700&action=edit
Minimal test case

For the attached code, gcc miscompiles the call to swap in Foo::swapToHead()
with the result that the value of 'a' is lost.

gcc-7.1 miscompiles the code at -O2 and above (-O1 and above if std::swap is
used) at least with x86_64-pc-linux-gnu.

gcc-6.3 compiles the code correctly.

### Relevant sequence of instructions:
mov    edx,edx
lea    rcx,[rdi+0x8]                   ; tail[0]
mov    DWORD PTR [rdi+rdx*4+0x10],esi  ; store to head_[1] - This does not seem
to be right
mov    eax,DWORD PTR [rdi+rdx*4+0x10]
mov    DWORD PTR [rcx],eax             ; now head_[1] and tail[0] are the same

### COMMAND:
g++ -O3  bug-min.ii
./a.out

### EXPECTED OUTPUT:
(rc=0)

### ACTUAL OUTPUT:
x.swapToHead(Lit::make( 2), 1) && "Lit 2 went missing"
Aborted

### g++ -v
Using built-in specs.
COLLECT_GCC=/home/wv/bin/linux/64/gcc-7.1/bin/g++
COLLECT_LTO_WRAPPER=/home/wv/bin/linux/64/gcc-7.1/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../../sources/gcc-7.1.0/configure
--prefix=/home/wv/bin/linux/64/gcc-7.1 --enable-languages=c,c++
Thread model: posix
gcc version 7.1.0 (GCC)