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[docs][statepoints] add a section spelling out simplifications for non-relocating GCs 

llvm-svn: 346447
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Philip Reames 2018-11-08 23:07:04 +00:00
parent 78b46457fb
commit e777f013ac
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llvm/docs/Statepoints.rst
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@ -254,6 +254,28 @@ following command.
opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps
Simplifications for Non-Relocating GCs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Some of the complexity in the previous example is unnecessary for a
non-relocating collector. While a non-relocating collector still needs the
information about which location contain live references, it doesn't need to
represent explicit relocations. As such, the previously described explicit
lowering can be simplified to remove all of the ``gc.relocate`` intrinsic
calls and leave uses in terms of the original reference value.
Here's the explicit lowering for the previous example for a non-relocating
collector:
.. code-block:: llvm
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
ret i8 addrspace(1)* %obj
}
Base & Derived Pointers
^^^^^^^^^^^^^^^^^^^^^^^