RewriteStatepointsForGC and Invoke

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1

Hi all,

I have been trying to use LLVM’s GC infrastructure (Statepoints) and ran into an issue when using exception handling.
The problem happens when I use an invoke instruction to support exception handling, and try to apply the pass RewriteStatepointsForGC to produce stackmaps.
I was able to create a minimal example using the example in the documentation of RewriteStatepointsForGC:

declare void ()* @foo()
declare i8* @__gxx_personality_v0()
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc “statepoint-example” personality i8* ()* @__gxx_personality_v0 {
blk_entry:
invoke void ()* @foo() to label %blk_normal unwind label %blk_exc
blk_normal:
ret i8 addrspace(1)* %obj
blk_exc:
%res1 = landingpad { i8*, i32 }
catch i8* null
ret i8 addrspace(1)* %obj
}

The pass is suppose to rewrite the invoke call into a call to llvm.experimental.gc.statepoint, producing a token, and the token can be used in llvm.experimental.gc.relocate, to get the relocated pointer. For a normal call, the IR after the pass should look like:


```
blk_entry:
  %statepoint_token = call token (i64, i32, void ()* ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_p0f_isVoidff(i64 2882400000, i32 0, void ()* ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
  %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %statepoint_token, i32 7, i32 7) ; (%obj, %obj)
  br label %blk_normal

blk_normal:                                       ; preds = %blk_entry
  ret i8 addrspace(1)* %obj.relocated
```

However, since I’m using an invoke, I do not have access to the token returned by the call to llvm.experimental.gc.statepoint in blk_exc, making it not possible to relocate %obj when an exception is thrown. When running the pass on the initial example I get:

Call parameter type does not match function signature!

%res1 = landingpad { i8*, i32 }

catch i8* null

token %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8({ i8*, i32 } %res1, i32 7, i32 7) ; (%obj, %obj)

LLVM ERROR: Broken function found, compilation aborted!

I assume the pass does not consider the invoke scenario, but even then, I would not know how to fix this issue myself, since I cannot access %statepoint_token in the exception block.

Is this a known issue? Is there any way to work around it that I might not be seeing?

Thanks in advance,

2

There is a warning in llvm docs about your problem (https://llvm.org/docs/Statepoints.html#id33) :

Relocations along exceptional paths are currently broken in ToT. In particular, there is current
no way to represent a rethrow on a path which also has relocations. See this llvm-dev discussion
for more detail.

(What's ToT? I don't know)
And it points to this topic: Redirecting to Google Groups

Basically it works for cleanup landing pads only and should looks like this:

     blk_exc:
        %res1 = landingpad token
        cleanup
        ret i8 addrspace(1)* %obj