[Bug target/51861] New: Incorrect generated code with __builtin_unreachable() on MIPS

["maarten at treewalker dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51861

             Bug #: 51861
           Summary: Incorrect generated code with __builtin_unreachable()
                    on MIPS
    Classification: Unclassified
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: maarten@treewalker.org
              Host: i686-linux
            Target: mipsel-linux
             Build: i686-linux


Created attachment 26326
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26326
Minimal test case

If I compile the attached test case for little endian MIPS with the options
"-O2 -march=mips32", the generated code will hang at runtime instead of
returning the correct value (30).

Minor changes to the code can produce a correct answer, a segfault, or various
wrong answers. The test case is relatively large for a minimal test case (about
30 lines), but I couldn't reduce it any further.

The original problem also occurs with GCC 4.6.1; I haven't tested the test case
with that version.

The problem occurs at -O2 and -O3, not at lower optimization levels; I haven't
tested higher levels.

Without the "-march=mips32" option I get the wrong answer (20), but no hang.

When compiling for x86_64 the problem does not occur.

The problem occurs with both the C and the C++ compiler.

When compiling to an object file, the fetch_value() function is just 5
instructions when the generated code is bad, while it is over 20 instructions
when the generated code is correct (for example, at -O1 or if the unused "case
4:" is commented out). So perhaps an invalid optimization is performed.

For easier testing, you can enable the printf: this doesn't change the behavior
related to the bug, but it does violate the demand for test cases to have no
external dependencies, so I used the return value of main() for the submitted
version.



GCC was configured with:
/home/mth/opendingux-buildroot/output/toolchain/gcc-4.5.1/configure
--prefix=/opt/opendingux-toolchain/usr --build=i686-pc-linux-gnu
--host=i686-pc-linux-gnu --target=mipsel-unknown-linux-uclibc
--enable-languages=c,c++ --with-sysroot=/opt/opendingux-toolchain
--with-build-time-tools=/opt/opendingux-toolchain/usr/mipsel-unknown-linux-uclibc/bin
--disable-__cxa_atexit --enable-target-optspace --with-gnu-ld --disable-libssp
--disable-multilib --disable-tls --enable-shared
--with-gmp=/home/mth/opendingux-buildroot/output/toolchain/gmp
--with-mpfr=/home/mth/opendingux-buildroot/output/toolchain/mpfr
--with-mpc=/home/mth/opendingux-buildroot/output/toolchain/mpc --disable-nls
--enable-threads --disable-decimal-float --with-float=soft --with-abi=32
--with-tune=mips32 --with-pkgversion='Buildroot 2010.11-g76b414b'
--with-bugurl=http://bugs.buildroot.net/