[RS4GC] Introduce intrinsics to get base ptr and offset
There can be a need for some optimizations to get (base, offset)
for any GC pointer. The base can be calculated by generating
needed instructions as it is done by the
RewriteStatepointsForGC::findBasePointer() function. The offset
can be calculated in the same way. Though to not expose the base
calculation and to make the offset calculation as simple as
ptrtoint(derived_ptr) - ptrtoint(base_ptr), which is illegal
outside RS4GC, this patch introduces 2 intrinsics:
@llvm.experimental.gc.get.pointer.base(%derived_ptr)
@llvm.experimental.gc.get.pointer.offset(%derived_ptr)
These intrinsics are inlined by RS4GC along with generation of
statepoint sequences.
With these new intrinsics the GC parseable lowering for atomic
memcpy intrinsics (6ec2c5e402a724ba99bce82a9cac7a3006d660f4)
could be implemented as a separate pass.
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D100445
diff --git a/llvm/docs/Statepoints.rst b/llvm/docs/Statepoints.rst
index 765384b..e13f5d9 100644
--- a/llvm/docs/Statepoints.rst
+++ b/llvm/docs/Statepoints.rst
@@ -430,6 +430,13 @@
as single no-op before and after the call instruction. These no-ops are often
removed by the backend during dead machine instruction elimination.
+Before the abstract machine model is lowered to the explicit statepoint model
+of relocations by the :ref:`RewriteStatepointsForGC` pass it is possible for
+any derived pointer to get its base pointer and offset from the base pointer
+by using the ``gc.get.pointer.base`` and the ``gc.get.pointer.offset``
+intrinsics respectively. These intrinsics are inlined by the
+:ref:`RewriteStatepointsForGC` pass and must not be used after this pass.
+
.. _statepoint-stackmap-format:
@@ -620,12 +627,16 @@
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
As a part of lowering to the explicit model of relocations
-RewriteStatepointsForGC performs GC specific lowering for
-'``llvm.memcpy.element.unordered.atomic.*``',
-'``llvm.memmove.element.unordered.atomic.*``' intrinsics.
+RewriteStatepointsForGC performs GC specific lowering for the following
+intrinsics:
-There are two possible lowerings for these copy operations: GC leaf lowering
-and GC parseable lowering. If a call is explicitly marked with
+* ``gc.get.pointer.base``
+* ``gc.get.pointer.offset``
+* ``llvm.memcpy.element.unordered.atomic.*``
+* ``llvm.memmove.element.unordered.atomic.*``
+
+There are two possible lowerings for the memcpy and memmove operations:
+GC leaf lowering and GC parseable lowering. If a call is explicitly marked with
"gc-leaf-function" attribute the call is lowered to a GC leaf call to
'``__llvm_memcpy_element_unordered_atomic_*``' or
'``__llvm_memmove_element_unordered_atomic_*``' symbol. Such a call can not
