[BUG]: cannot use LLVM command-line arguments starting with "-l" about compiler-explorer HOT 6 CLOSED

The issue seems a bit wider: any option starting with "-l".

Presumably this is to strip link options, but it prevents to use "-mllvm -loop-something" options such as these:
https://github.com/llvm/llvm-project/blob/f4c0c40f388fff0975ecada4997683cef3cb1fae/llvm/lib/Transforms/Scalar/LoopFlatten.cpp#L85-L103

Also when using LLVM tools inside Godbolt directly (for example, switch language to "LLVM IR" and select a version of the "opt" tool) any option starting with "-l" is removed. Here, you should be able to specify the LLVM options directly, without "-mllvm" prefix, so directly "-loop-something". This does not work.

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At least for GCC and clang, -l gets passed to the linker as a library to link against. I can run this on my local clang:

% clang -loop-some-invalid-option -c /tmp/moo.cc
clang: warning: -loop-some-invalid-option: 'linker' input unused [-Wunused-command-line-argument]
%

no error, that's just a warning. We don't strip anything but hand all the arguments (plus a few more) to the compiler itself. In the default mode we don't link so the -l option is not processed by anything. If you enable the linker: https://godbolt.org/z/xjzETcr8G you get the error you'd expect.

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I agree the -l option does seem to somehow get droppedf by the opt tool though,. so we still have something to investigate! :)

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mayyybe we do filter -l stuff? argh :) we will loko. thanks again for this

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Thanks, Matt. I should have reported my problem more directly: I cannot use LLVM command-line arguments starting with -loop, or even starting with -l. They are either ignored, or they trigger an error.

This is with clang:
https://godbolt.org/z/r15bGsjnr

This is with opt:
https://godbolt.org/z/8YdWPP3e6

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probable culprit: #6297

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