[Bug target/101023] New: [11/12 Regression] wrong code with -mstackrealign

["zsojka at seznam dot cz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101023

            Bug ID: 101023
           Summary: [11/12 Regression] wrong code with -mstackrealign
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 50981
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50981&action=edit
auto-reduced testcase (from OpenTTD sources)

Compiler output:
$ x86_64-pc-linux-gnu-g++ -O2 -mtune=opteron -mstackrealign
--param=hot-bb-frequency-fraction=1 testcase.C -S

The offending code is:
.L8:
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movq    %r12, -8(%rbp)
        pushq   %rax

the "push" overwrites r12 stored just one instruction above; when returning:
        movq    -8(%rbp), %r12
        xorl    %eax, %eax
        leave
        .cfi_def_cfa 7, 8
        ret

the wrong data is restored to r12

I wasn't able yet to generate an executable testcase, but I can try to if
needed. (it shouldn't be that hard, just r12 needs to be used by the caller
during the call)

$ x86_64-pc-linux-gnu-g++ -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-g++
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-r12-1366-20210610095751-g6fcba9ef23e-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --with-cloog --with-ppl --with-isl
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-r12-1366-20210610095751-g6fcba9ef23e-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20210610 (experimental) (GCC)