[llvm-dev] How to find real heap references using LLVM Stack Map

Kavindu Gimhan Zoysa via llvm-dev llvm-dev at lists.llvm.org
Thu May 20 10:53:00 PDT 2021

Hi Philip,

could you please explain what do you mean by `return PC on the stack`, is
it the value in the rip (instruction pointer) after I call foo() from

Kavindu Gimhan Zoysa,
BSc(Hons) | ENTC | UoM,

GitHub <https://github.com/KavinduZoysa> LinkedIn
<https://www.linkedin.com/in/kavindu-gimhan-zoysa-85939a122/> Medium

On Thu, 20 May 2021 at 22:27, Philip Reames <listmail at philipreames.com>

> This is out of scope for LLVM, but let me give you a high level summary.
> Your GC needs to be able to walk all stack frames on the stack at point of
> suspension.  This can be done by using libunwind, manually writing your own
> stack crawler, etc..
> Once you have the ability to walk the stack, each return PC on the stack
> will correspond to a stack map entry in the stack map section.  In this
> case, the return PC for the foo call will correspond to an entry in that
> section.  All of the offsets in that entry refer to the frame corresponding
> to main.  Your stack walker must be able to turn those into actual
> addresses.
> For callee saved registers - which I *think* is the case you're actually
> asking about - , you can either a) disable that by tweaking your copy of
> LLVM, or b) use the callee saved register information generated for
> eh_frame to translate CSRs into their spill locations.  If you want to
> avoid dealing with this, look at the "no_callee_saved_registers" function
> attribute.  It's a very blunt hammer, but you can come back later and
> refine.
> This is a pretty standard GC implementation technique.  Any off the shelf
> GC you use should already be able to do this.
> Philip
> On 5/20/21 7:50 AM, Kavindu Gimhan Zoysa via llvm-dev wrote:
> Hi all,
> Let's say I have an example like this
>> *fn foo() {      heapRef2 = getHeapRef()      readStackMap() } fn main()
>> {     heapRef1 = getHeapRef()     foo(); }*
> When we start to read stack map in foo(), our stack has two heap
> references (heapRef1 and heapRef2). From the LLVM stack map we can get the
> location of *heapRef2*, using let's say rsp + offset.
> But, When we consider about *heapRef1, *its' offset is given with respect
> to its' function(main) rsp or rbp. But at this moment those registers are
> pointed to foo()'s stack frame locations. There fore how do we get the
> location of *heapRef1?*
> Highly appreciate your input on this.
> Thank you,
> Kavindu
> Kavindu Gimhan Zoysa,
> BSc(Hons) | ENTC | UoM,
> SSE | WSO2
> GitHub <https://github.com/KavinduZoysa> LinkedIn
> <https://www.linkedin.com/in/kavindu-gimhan-zoysa-85939a122/> Medium
> <https://medium.com/@kavindugimhanzoysa>
> _______________________________________________
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