[cfe-dev] Macros for revision number and commit hash

[Chris Lattner via cfe-dev]

> On Mar 15, 2019, at 2:13 PM, JF Bastien via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Hello macro enthusiasts!
> 
> By default, clang defines a lot of macros that allow developers to figure out things about the compiler that’s building them. I’m proposing that we add two more:
> __clang_revision_number__ : an unsigned integer, representing the current branch’s revision number.
> __clang_commit_hash__ : a string containing a hexadecimal hash, representing the current checkout’s hash.
> These would be set by cmake when building LLVM, and make the most sense in a monorepo on git. There’s been discussion of clang revision number in the context of git migration, captured on the git migration page <https://llvm.org/docs/Proposals/GitHubMove.html#on-managing-revision-numbers-with-git>.

I’m generally +1 on exposing such metadata.  The only downside I see is that a git pull could cause a relink of the entire world (even if otherwise unnecessary) just to update these fields.  I suspect that this is going to be more painful in a monorepo world for people who work downstream of clang (e.g. LLDB developers).

Is there an implementation approach that allows us to get the benefit of this extra metadata without reducing dev productivity?

-Chris


> 
> Here are examples of similar numbers we currently offer:
> 
> auto major     = __clang_major__;      // 7
> auto minor     = __clang_minor__;      // 0
> auto patch     = __clang_patchlevel__; // 0
> auto version   = __clang_version__;    // "7.0.0 (tags/RELEASE_700/final 342594)”
> auto VERSION   = __VERSION__;          // "4.2.1 Compatible Clang 7.0.0 (tags/RELEASE_700/final 342594)”
> auto gnu_major = __GNUC__;             // 4
> auto gnu_minor = __GNUC_MINOR__;       // 2
> auto gnu_patch = __GNUC_PATCHLEVEL__;  // 1
> auto gxx_abi   = __GXX_ABI_VERSION;    // 1002
> 
> The two new values will be generally useful to tools that automatically handle revisions. For example, we currently have bots that parse clang -v, and that output is unfortunately aimed at humans (bots aren’t human). That’s broken in the past, and having these two values would help reduce the likelihood of breakage because macros have a pretty stable format and are easy to test.
> 
> Let us know what you think!
> 
> JF
> 
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