Suppress version control revision info (Git revision id) is appended

Each time I build a new toolchain via tc-build the current Git revision id is appended and changes when llvm-project Git updates.

--- /boot/config-5.7.0-rc1-5-amd64-clang
+++ .config

-# Compiler: ClangBuiltLinux clang version 10.0.1 (https://github.com/llvm/llvm-project 61b6007157600d8080b87361397bb61ffbcfb196)
+# Compiler: ClangBuiltLinux clang version 10.0.1 (https://github.com/llvm/llvm-project edbe962459da6e3b7b4168118f93a77847b54e02)

The CCACHE benefit is gone as Linux build-system detects the change of compiler.

There exists LLVM_APPEND_VC_REV (default: ON) cmake-option to suppress this:

LLVM_APPEND_VC_REV:BOOL
    Embed version control revision info (Git revision id). The version info is provided by the LLVM_REVISION macro in llvm/include/llvm/Support/VCSRevision.h. Developers using git who don’t need revision info can disable this option to avoid re-linking most binaries after a branch switch. Defaults to ON.

What do you think of integrating an option for this in tc-build?

[1] https://llvm.org/docs/CMake.html#llvm-specific-variables

[dileks]

@msfjarvis Thanks for the patch. I will try it and report.

The comment is not precise enough.

-        # Don't append versioning info to the compiler version
+        # Don't append version control revision info (Git revision id) to the compiler version
+        'LLVM_APPEND_VC_REV': 'OFF',

This can be changed later.

[nathanchance]

I do not think that the revision info in the compiler string matters for the sake of ccache, unless you have changed something.

According to the ccache docs (https://ccache.dev/manual/latest.html#_configuration_settings):

By default, ccache includes the modification time (“mtime”) and size of the compiler in the hash to ensure that results retrieved from the cache are accurate.

If you build a new compiler, the mtime will change (and probably the size in some small way) and ccache will be invalidated, regardless of the version string in the compiler.