[llvm-dev] Avoiding function addresses in llvm_prf_data when value profiling is disabled

[Petr Hosek via llvm-dev]

I've done some work to enable --gc-sections to work with instr profiling in
https://reviews.llvm.org/D64045, but I still need to write more tests
before that change is ready to land.

On Mon, Dec 2, 2019 at 4:03 PM Xinliang David Li via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> That sounds like a good idea.  Or when VP is disabled, avoid recording the
> function addresses.
>
> David
>
> On Mon, Dec 2, 2019 at 2:53 PM Reid Kleckner <rnk at google.com> wrote:
>
>> Could we use !associated metadata and SHF_LINK_ORDER to make instr
>> profiling work with gc-sections, as has been done for ASan global metadata?
>>
>> On Mon, Dec 2, 2019 at 2:35 PM Pirama Arumuga Nainar via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I encountered an issue when enabling code coverage for Android and would
>>> like to confirm my findings and ask for suggestions.
>>>
>>> Here is my finding:
>>>
>>> When -fprofile-instr-generate, an llvm_prf_data section gets created
>>> that has entries for each function in the TU.  One of the entries is
>>> FunctionAddr which holds the address of the function.  Underneath, it is a
>>> relocation so it gets patched with the runtime, in-memory address of the
>>> function.  The whole llvm_prf_data section, or at least the FunctionAddr
>>> entry, is used only when value profiling is enabled.
>>>
>>> Value profiling is disabled by default and can be enabled with `-mllvm
>>> -enable-value-profiling`.
>>>
>>> My problem is below:
>>>
>>> The FunctionAddr entries create references to all functions in the TU.
>>> This causes an issue with -ffunction-sections and -Wl,--gc-sections (which
>>> Android uses by default).  The reference to functions causes the linker to
>>> retain all of these functions.
>>>
>>> This causes build errors when enabling code coverage for Android because
>>> the build rules are written assuming --gc-sections is passed to the linker
>>> and do not specify all necessary libraries to ensure proper linking of the
>>> discarded sections.
>>>
>>> Question:
>>>
>>> I'm considering setting FunctionAddr to 0 if value profiling is disabled
>>> (or omitting llvm_prf_data altogether if it exists solely for value
>>> profiling).  Does that sound reasonable?
>>>
>>> Thanks,
>>> -Pirama
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
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