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

Bruce Hoult via llvm-dev llvm-dev at lists.llvm.org
Fri Dec 18 03:27:17 PST 2015

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.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> --
> Rob Lyerly
> Graduate Research Assistant, Systems Software Research Group
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