[llvm-bugs] [Bug 41570] New: Trunk LTO gives internal error if object is compiled separately with clang -flto -c with a .bc file extension

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41570

            Bug ID: 41570
           Summary: Trunk LTO gives internal error if object is compiled
                    separately with clang -flto -c with a .bc file
                    extension
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: lto
          Assignee: unassignedbugs at nondot.org
          Reporter: peter.smith at linaro.org
                CC: llvm-bugs at lists.llvm.org

With a clang built this morning:
clang --version
clang version 9.0.0 (trunk 358996)
I get an error when I compile with -flto separately with a .bc suffix and then
attempt to invoke the code-generator. The reason for doing this is to work
around LTO not supporting -Os and -Oz
(https://bugs.llvm.org/show_bug.cgi?id=41568). 

I couldn't reproduce with clang 7.0.0 but I could with clang 8.0.0 

cat main.c
int main(void) {
    return 0;
}

clang -flto -Os -c main.c -o main.bc
clang -flto main.bc -o main

module flag identifiers must be unique (or of 'require' type)
!"EnableSplitLTOUnit"
fatal error: error in backend: Broken module found, compilation aborted!
clang-6: error: clang frontend command failed with exit code 70 (use -v to see
invocation)
clang version 9.0.0 (trunk 358996)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /linaro/upstream/buildclang/bin
clang-9: note: diagnostic msg: PLEASE submit a bug report to
http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and
associated run script.
clang-9: note: diagnostic msg: Error generating preprocessed source(s) - no
preprocessable inputs.

This seems to only occur when the .bc file extension is used, it doesn't when
.o is used so this maybe expected behaviour, it did have me confused for some
time though.

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