[llvm-dev] FYI: Relocating vector of pointers
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 14 15:46:26 PST 2016
TLDR. For anyone who is using the RewriteStatepointsForGC utility pass,
there is a recent change you should know about which may require you to
make some small changes to your stackmap parsing.
I have landed a small series of patches which change how we're handling
vector of pointers when reporting live pointers for the GC at
safepoints. Previously, the RS4GC pass was attempting to scalarize any
such vectors which were live over a safepoint so that it could insert
explicit relocations for each element of the vector. In the near
future, this scalarization code will be removed and we will report the
vector of pointers directly in the stack map for the associated safepoints.
This will require each consumer to the GC stackmap recorded in
LLVM_StackMap to make a small change to your parsing. In particular,
you could previously assume that all operands in the GC section were
pointer sized. With the new code, your parsing must be ready to
encounter a spill slot which is a multiple of the pointer size. The
interpretation of such a slot is as a collection of pointer slots, one
for each pointer-size bytes in the reported spill slot. (i.e. a spill
slot for a 4 element wide pointer vector on x86_64 will be 32 bytes wide
and contain 4 slots representing one pointer each.)
If adding the additional handling in your VM is problematic, please let
me know. Joseph Tremoulet pointed out to me that we could do the
conversion before actually writing the stack map record (i.e. report 4
distinct pointer slots instead of 1 4-element vector slot) and that this
might be helpful for handling first-class aggregates in the future. At
the moment, the changes to convert before writing the stack map records
appear to be a bit more work than is justified, but if anyone has a
reason why they need these changes, they could be done.
At the moment, the new functionality is hidden behind a hidden opt
flag. You can specify "-rs4gc-split-vector-values=0" to enable the new
code for testing purposes. Unless someone objects, I plan to flip the
default and delete the old code within the next week or so.
The changes are:
256352: [Statepoints] Use Indirect operands for spill slots
<http://reviews.llvm.org/rL256352>
257022: [Statepoints] Initial support for relocating vectors of pointers
<http://reviews.llvm.org/rL257022>
257244: [rs4gc] Optionally directly relocated vector of pointers
<http://reviews.llvm.org/rL257244>
Philip
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