[llvm-dev] Resurrect Bug18710 (Only generate .ARM.exidx and ARM.extab when needed with EHABI)

[Christian Bruel via llvm-dev]

Hello Everyone,

This is my first attempt to getting used with the submission process. 
Trying to get the "good practice" with the coding standard, tools, 
mailing lists... and already a few questions:

- Is it possible to "link" 2 related entries in Phabricator ? one for 
LLVM and one for CFE ? what's the best way of posting 2 related or 
dependent patches ?

- I'd like to update and follow on the discussion on bugzilla tracker 
that Renato created already but I can't edit (account requested to 
llvm-admin at lists.llvm.org without luck). Could someone help ?

So now the proposal:

The .exidx and .extab unwind sections are needed, when exceptions are 
enabled, for unwinding if the function can throw and to hold the 
EXIDX_CANTUNWIND value if the function does not throw.

The problem was already discussed in various threads and defects, so I 
won't repeat  the details but basically that confusion is that if the 
compiler (or the user) determined that the function cannot throw, we 
have the NOUNWIND attribute which is not orthogonal with either 
exception support. Despite this we should still emit the unwind entry 
for the EXIDX_CANTUNWIND value, which is always done.

Another issue is that canThrow (!NoUnwind) is set by default. Setting 
NoUnwind is an optimisation which is not handled by the ARM Streamer. A 
first approach was to have the backend return needsUnwindTableEntry() 
false if noexception support, but unwind tables can be used without 
exception handler (e.g) for debug infos or C unwind with attribute 
cleanup, and this happened to be quite intrusive...

But simpler, when the exceptions are not enabled, we should be able to 
remove them. Just passing this this bit of information through a new 
disable-arm-cantunwind llvm option makes the uwtable and nounwind table 
orthogonal with exception support.

This option is set in clang when no exception support allowing to have 
the -f[no-]exceptions and -f[no-]unwind-tables match GCC for C and C++.

Thanks a lot for your feedbacks, questions and hints for getting on board !

https://reviews.llvm.org/D31139

https://reviews.llvm.org/D31140


Christian




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