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This patch is the third step to extend the current multilib system to support the selection of library variants which do not correspond to existing command-line options. Proposal can be found in https://discourse.llvm.org/t/rfc-multilib-custom-flags/81058 The multilib mechanism supports libraries that target code generation or language options such as --target, -mcpu, -mfpu, -mbranch-protection. However, some library variants are particular to features that do not correspond to any command-line options. Examples include variants for multithreading and semihosting. This work introduces a way to instruct the multilib system to consider these features in library selection. This particular patch is comprised of the core processing of these flags. - Custom flags in the command-line are read and forwarded to the multilib system. If multiple flag values are present for the same flag declaration, the last one wins. Default flag values are inserted for flag declarations for which no value was given. - Feed `MacroDefines` back into the driver. Each item `<string>` in the `MacroDefines` list is formatted as `-D<string>`. Library variants should list their requirement on one or more custom flags like they do for any other flag. The new command-line option is passed as-is to the multilib system, therefore it should be listed in the format `-fmultilib-flag=<str>`. Moreover, a variant that does not specify a requirement on any particular flag can be matched against any value of that flag. If the user specifies `-fmultilib-flag=<name>` with a name that is invalid, but close enough to any valid flag value name in terms of edit distance, a suggesting error is shown: ``` error: unsupported option '-fmultilib-flag=invalidname'; did you mean '-fmultilib-flag=validname'? ``` The candidate with the smallest edit distance is chosen for the suggestion, up to a certain maximum value (implementation detail), after which a non-suggesting error is shown instead: ``` error: unsupported option '-fmultilib-flag=invalidname' ```
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