[llvm-dev] [LTO] -time-passes and libLTO

[James Henderson via llvm-dev]

Hi,

We have been investigating an issue when running LTO with our proprietary
linker, which links against libLTO dynamically. The issue is that when we
pass -time-passes via the lto_codegen_debug_options function in the LTO C
API, no time information is produced during compilation. The reason for
this is that time information is stored in state owned by a ManagedStatic
instance, and is only printed when the state is destroyed. This in turn
only happens when ManagedStatics are cleaned up, via the llvm_shutdown
function. As we do not link against LLVM (except libLTO dynamically), we
have no access to llvm_shutdown, which in turn means we are not able to
clean up ManagedStatic instances and thus no timing information is produced.

We have considered a few options and have come up with the following
suggestions, and would appreciate some feedback:

1)      Add llvm_shutdown (or rather likely some wrapper function that does
the same job) to the C interface of libLTO. This should be called when we
are done with the library.

2)      Add a “full-shutdown” command-line option to LLVM - that can be
passed via lto_codegen_debug_options - which causes ManagedStatic-owned
state to be destroyed on shutdown. This could even be more widely useful
outside the LTO case.

3)      Call llvm_shutdown() immediately after compilation as part of the
compile function.

4)      None of the above, because there’s a better way that I am unaware
of to clean up this state.

I have a marginal preference for 1), but does anybody have any other
preferences or suggestions?

Regards,
James
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