[llvm-dev] Building GC on statepoint-example

Thu Nov 8 09:06:31 PST 2018

Yes, that's more clear (you duplicated the word specified, minor typo).

The other part that threw me off was that the section name was only identified "for Darwin." I was able to find the section via assembly inspection, but that threw me off a bit.
________________________________
From: Philip Reames <listmail at philipreames.com>
Sent: Thursday, November 8, 2018 7:21 AM
To: Jordan Rudd; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Building GC on statepoint-example

Jordon,

I just submitted a change to the docs to hopefully address your confusion.  Can you take a look and let me know if you have further questions?  Start with the revised Stack Map Format section.

Philip

p.s. I just submitted this and it'll take a few minutes to refresh.  In the meantime, you could look at the diff for 346405.

On 11/2/18 2:52 PM, Jordan Rudd via llvm-dev wrote:
Note: These instructions seem to be targeted towards people creating a JIT. My compiler compiles to native. I suppose I just need a reference to the stack location of the stack map, wherever that is.
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org><mailto:llvm-dev-bounces at lists.llvm.org> on behalf of Jordan Rudd via llvm-dev <llvm-dev at lists.llvm.org><mailto:llvm-dev at lists.llvm.org>
Sent: Thursday, November 1, 2018 2:06 PM
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Building GC on statepoint-example

Hi,

I'm trying to build a gc using the statepoint-example strategy. However, after hours of searching, reading documentation and trying to find code examples online, I cannot figure out how to access the Stack Map.

The documentation for statepoint-example says:

The stack map format generated by this GC strategy can be found in the Stack Map Section<https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fllvm.org%2Fdocs%2FStackMaps.html%23stackmap-section&data=02%7C01%7C%7C0d564a3c7b9a42962e2608d6458de2c0%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636772873036011590&sdata=5TKPXTyU5xCrGx67PXW%2FfOHUxhmBJxLAFh1Qckj7jMk%3D&reserved=0> using a format documented here<https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fllvm.org%2Fdocs%2FStatepoints.html%23statepoint-stackmap-format&data=02%7C01%7C%7C0d564a3c7b9a42962e2608d6458de2c0%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636772873036011590&sdata=HXlqIsOn9UXV8kCW508RVYs4UaGk6sDG8VfjmzCHiOc%3D&reserved=0>. This format is intended to be the standard format supported by LLVM going forward.

Ok, great, I see the format. But... how do I access the Stack Map from my runtime? Where is it?

The link seems to send me to this instruction:

A JIT compiler can easily access this section by providing its own memory manager via the LLVM C APILLVMCreateSimpleMCJITMemoryManager(). When creating the memory manager, the JIT provides a callback:LLVMMemoryManagerAllocateDataSectionCallback(). When LLVM creates this section, it invokes the callback and passes the section name.

None of these API's are well-documented, so I'm kind of trying to guess what to do here. As a proof of concept, I tried building a do-nothing memory manager that calls APILLVMCreateSimpleMCJITMemoryManager on static initialization and passes the callbacks to it. I link the necessary llvm libs into my runtime and fire it up.

I can confirm that my safepoint poll code is getting hit. I can confirm that creating the "memory manager" is called on runtime boot and that it's succeeding. But the callbacks never get hit. Even if they were, it's unclear how I would use the parameters of those callbacks to locate the stack map.

What am I missing?

Thanks!
Jordan

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