llvm dev - Apr 2021 - [RFC] [RewriteStatepointsForGC] Introduce intrinsics to get base ptr and offset

Hi LLVM developers.

Working with GC pointers
(https://llvm.org/docs/LangRef.html#accurate-garbage-collection-intrinsics)
there can be a need for some optimizations to get pointer base and/or offset
from the base. 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
RewriteStatepointsForGC, 2 intrinsics are proposed:

    declare i8 addrspace(1)* @llvm.experimental.gc.get.pointer.base(i8
addrspace(1)* readnone nocapture %base_ptr) nounwind readnone willreturn
"gc-leaf-function"
    declare i64 @llvm.experimental.gc.get.pointer.offset(i8 addrspace(1)*
readnone nocapture %base_ptr) nounwind readnone willreturn
"gc-leaf-function"

These intrinsics are inlined by RewriteStatepointsForGC along with generation of
statepoint sequences.

For example, with these new intrinsics the GC parseable lowering for atomic
memcpy intrinsics (https://reviews.llvm.org/D88861) could be implemented as a
separate pass.

Here is a patch that implements the proposal.
https://reviews.llvm.org/D100445 [RS4GC] Introduce intrinsics to get base ptr
and offset

Thanks.
-Yevgeny Rouban

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