[clang] `-fembed-bitcode` is incompatible with some compilation flags and their negations

[romainthomas]

Hello there,

The commit 51313bc authored by @compnerd in 2018 introduced errors when using -fembed-bitcode with a set of flags.

I can understand why we should prevent using some of these flags with the embedded bitcode but
I don't understand why in some cases we prevent a flag and its negation:

For instance:

# Error 
$ clang -fembed-bitcode -ffunction-sections

# Error 
$ clang -fembed-bitcode -fno-function-sections

# Ok
$ clang -fembed-bitcode 

Shouldn't we relax the conditions on the -fno- flags?

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Romain Thomas (romainthomas)

Hello there,

The commit 51313bc authored by @compnerd in 2018 introduced errors when using -fembed-bitcode with a set of flags.

I can understand why we should prevent using some of these flags with the embedded bitcode but
I don't understand why in some cases we prevent a flag and its negation:

For instance:

# Error 
$ clang -fembed-bitcode -ffunction-sections

# Error 
$ clang -fembed-bitcode -fno-function-sections

# Ok
$ clang -fembed-bitcode 

Shouldn't we relax the conditions on the -fno- flags?

[compnerd]

I'm not sure about support for the negation here. Particularly for the emission of Bitcode, the use would require an application uniformly across the module and so function sections can be elided uniformly as well. It seems that special casing the negative form is of low/no value? What use case are you trying to support?

[romainthomas]

The use case is that -ffunction-sections/-fdata-sections could be set ahead of the user flags control. For instance this happens in CMake: Utilities/cmlibarchive/CMakeLists.txt,
as well as on Android: ndk-r28b/build/cmake/flags.cmake.

So we might want to override the previous definition (which is out of control of the user) with -fno-function-sections:

$ clang -DANDROID -fdata-sections -funwind-tables  -fno-data-sections -fembed-bitcode
        |                                       |  |                                |
        +---------------------------------------+  +--------------------------------+
        From CMake                                 From the user                     

For sure we can workaround the default -fdata-sections in CMake but I was wondering if it would make sense to allow the negation of these flags in LLVM. If so, I can create a PR for that.