[llvm-bugs] [Bug 52583] New: -m option is not ignored when is immediately preceded by -o option

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52583

            Bug ID: 52583
           Summary: -m option is not ignored when is immediately preceded
                    by -o option
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: egor.suvorov at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Here is an issue that one of my students encountered today with GCC, but Clang
is also affected:
https://stackoverflow.com/questions/70069809/unrecognized-emulation-mode-ain-when-compiling-with-gcc-on-ubuntu/70069810

Consider any simple C++ program in a file named `main.cpp`. I compile it with
`clang++ main.cpp -o main`, but I accidentally add a dash before `main`:

clang++ main.cpp -o -main

Expected behavior: the program is compiled to a file named `-main`.

Real behavior:
/usr/bin/ld: unrecognised emulation mode: ain
Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu elf_l1om
elf_k1om i386pep i386pe
clang: error: linker command failed with exit code 1 (use -v to see invocation)

My understanding: Clang treats `-main` as both `-m` option (targeting some
unexisting `ain` architecture) AND name for the output file.

For example, on my 64-bit Ubuntu I can write

clang++ main.cpp -o -melf_x86_64

and it will produce a binary named `-melf_x86_64`. However, the following
command fails because I don't have 32-bit standard library installed:

clang++ main.cpp -o -melf_i386

yields:

/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/libstdc++.so when searching for
-lstdc++
/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/libstdc++.a when searching for -lstdc++
/usr/bin/ld: cannot find -lstdc++
/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libm.so when
searching for -lm
/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libm.a when
searching for -lm
/usr/bin/ld: skipping incompatible /lib/x86_64-linux-gnu/libm.so when searching
for -lm
/usr/bin/ld: skipping incompatible /lib/x86_64-linux-gnu/libm.a when searching
for -lm
/usr/bin/ld: skipping incompatible /usr/lib/x86_64-linux-gnu/libm.so when
searching for -lm
/usr/bin/ld: skipping incompatible /usr/lib/x86_64-linux-gnu/libm.a when
searching for -lm
/usr/bin/ld: skipping incompatible /usr/lib/x86_64-linux-gnu/libm.so when
searching for -lm
/usr/bin/ld: skipping incompatible /usr/lib/x86_64-linux-gnu/libm.a when
searching for -lm
/usr/bin/ld: cannot find -lm
/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libgcc_s.so.1
when searching for libgcc_s.so.1
/usr/bin/ld: skipping incompatible /lib/x86_64-linux-gnu/libgcc_s.so.1 when
searching for libgcc_s.so.1
/usr/bin/ld: skipping incompatible /usr/lib/x86_64-linux-gnu/libgcc_s.so.1 when
searching for libgcc_s.so.1
/usr/bin/ld: skipping incompatible
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/libgcc.a when searching for -lgcc
/usr/bin/ld: cannot find -lgcc
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I suspect some other options may be be affected as well. `-m` here is special
only because `main` start with `m`.

Being able to read the option in two different ways simultaneously looks like a
bug in my world.

I can reproduce the behavior on a wide range of OSes: LLVM on Windows 10
(clang++ version 13.0.0 Target: x86_64-w64-windows-gnu by msys2 project),
clang++ 10.0.0-4ubuntu1 on 64-bit Ubuntu 20.04.3 LTS, as well as trunk version
on Godbolt.

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