[llvm-dev] Status of "llvm.pcmarker" intrinsic?

[Bruce Hoult via llvm-dev]

Wouldn't you get precisely the same numbers (and in the same order) by
dumping the return address of each function just before returning from it?

The mapping between PC and a given IR instruction (let alone source line of
code) is a pretty fuzzy thing in the presence of even simple optimizations.
The return address of a function is much more well-defined than the address
of some arbitrary IR instruction.


On Thu, Dec 17, 2015 at 7:55 PM, Rob Lyerly via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> That's kind of a shame because it did exactly what I needed -- I'll have
> to find another route!
>
> I wanted to use "llvm.pcmarker" to find the address of a given IR
> instruction when generated using different back-ends.  In particular, I
> wanted to be able to map the return address of a specific function call on
> x86-64 with the return address for the *same* function call on Aarch64.  My
> plan was to develop a pass that dumped the pcmarker intrinsic after every
> function call site, so that I could correlate the return addresses between
> function call sites on both architectures.
>
> I'm still a newbie in terms of LLVM internals, so I'm wondering what would
> be the easiest approach to accomplish this.  I've seen elsewhere people
> recommending adding custom intrinsics that get converted into
> pseudo-instructions in the back-end.  Those pseudo instructions would then
> generate a label when CodeGen'd...does this seem sane?  Or is there an
> easier approach to solving this?
>
> On Wed, Dec 16, 2015 at 6:49 PM, Philip Reames <listmail at philipreames.com>
> wrote:
>
>> There seems to be semantic overlap with stackmap, patchpoint, and
>> statepoint as well.
>>
>> I suspect we should remove pcmarker and forward serialize it in bitcode
>> as a nop.
>>
>> Philip
>>
>>
>> On 12/16/2015 02:14 PM, Justin Bogner via llvm-dev wrote:
>>
>>> Rob Lyerly via llvm-dev <llvm-dev at lists.llvm.org> writes:
>>>
>>>> I've seen previous messages about "llvm.pcmarker" not being supported on
>>>> x86 (e.g.
>>>> http://lists.llvm.org/pipermail/llvm-dev/2010-February/029239.html
>>>> and http://lists.llvm.org/pipermail/llvm-dev/2012-June/051104.html).
>>>> However, these messages are several years old -- is the intrinsic still
>>>> not
>>>> implemented?
>>>>
>>> As far as I can tell llvm.pcmarker was only ever implemented for Alpha,
>>> and that backend was removed in 2011. All of the code and documentation
>>> relating to pcmarker has been dead for years, and should probably just
>>> be removed.
>>> _______________________________________________
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>>> llvm-dev at lists.llvm.org
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>>>
>>
>>
>
>
> --
> Rob Lyerly
> Graduate Research Assistant, Systems Software Research Group
>
>
>
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