[Bug rtl-optimization/124435] New: [16 Regression] wrong code with signed short shift right at -O1 and above

[zsojka at seznam dot cz via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124435

            Bug ID: 124435
           Summary: [16 Regression] wrong code with signed short shift
                    right at -O1 and above
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: mips64el-unknown-linux-gnuabi64

Created attachment 63870
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=63870&action=edit
reduced testcase

Output:
$ mips64el-unknown-linux-gnuabi64-gcc -O1 testcase.c -static
$ qemu-mips64el -- ./a.out 
qemu: uncaught target signal 6 (Aborted) - core dumped
Aborted                    qemu-mips64el -- ./a.out

The difference between OK (gcc-15) to BAD ASM is:
@@ -29,9 +29,8 @@
        daddiu  $2,$2,255
        and     $2,$4,$2
        or      $2,$2,$3
-       dsll    $2,$2,48
        jr      $31
-       dsra    $2,$2,58
+       dsra    $2,$2,10

        .set    macro
        .set    reorder


eg. gcc-16 omits the shift left first, so it seems the "garbage" is shifted in
during the right shift.


$ mips64el-unknown-linux-gnuabi64-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-mips64el/bin/mips64el-unknown-linux-gnuabi64-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-20260309124537-r16-7957-ge1eb2472c965a1-checking-yes-rtl-df-extra-mips64el/bin/../libexec/gcc/mips64el-unknown-linux-gnuabi64/16.0.1/lto-wrapper
Target: mips64el-unknown-linux-gnuabi64
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--enable-libsanitizer --with-sysroot=/usr/mips64el-unknown-linux-gnuabi64
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=mips64el-unknown-linux-gnuabi64
--with-ld=/usr/bin/mips64el-unknown-linux-gnuabi64-ld
--with-as=/usr/bin/mips64el-unknown-linux-gnuabi64-as --disable-multilib
--with-abi=64 --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-20260309124537-r16-7957-ge1eb2472c965a1-checking-yes-rtl-df-extra-mips64el
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.0.1 20260309 (experimental) (GCC)