[llvm-bugs] [Bug 46363] New: WebAssembly hangs on switch with __builtin_unreachable in default.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46363

            Bug ID: 46363
           Summary: WebAssembly hangs on switch with __builtin_unreachable
                    in default.
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bungeman at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

With

$ emsdk/upstream/bin/clang-11 --version
clang version 11.0.0
(/b/s/w/ir/cache/git/chromium.googlesource.com-external-github.com-llvm-llvm--project
613c4a87ba9bb39d1927402f4dd4c1ef1f9a02f7)

$cat > repro.cpp <<EOF
int CToI(char c) {
    switch (c) {
        case 'r': return 0;
        case 'g': return 1;
        case 'b': return 2;
        case 'a': return 3;
        case '0': return 4;
        case '1': return 5;
        default:
            __builtin_unreachable();
    }
}
EOF

$ emsdk/upstream/bin/clang-11 -cc1 -triple wasm32-unknown-emscripten -emit-obj
-disable-free -main-file-name repro.cpp -mrelocation-model static
-mframe-pointer=none -fno-rounding-math -mconstructor-aliases -target-cpu
generic -fvisibility hidden -dwarf-column-info -debugger-tuning=gdb -O3
-std=c++17 -nostdsysteminc -mllvm -combiner-global-alias-analysis=false -mllvm
-enable-emscripten-sjlj -mllvm -disable-lsr -o repro.o -x c++ repro.cpp

the clang-11 invocation will hang at any optimization level other than -O0.
This can also currently be reproduced on CompilerExplorer with the above code
with the "WebAssembly clang (trunk)" compiler and any optimization level over
-O0.

This seems to be a fairly recent regression as this was known to work at some
point in the not too distant past (will attempt to bisect). Removing any of the
'case' statements seems to make it work, this seems to require at least six
cases to reproduce.

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