<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
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.<br>
<br>
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.<br>
<br>
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.)<br>
<br>
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. <br>
<br>
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.<br>
<br>
The changes are:<br>
<a href="http://reviews.llvm.org/rL256352" class="phui-handle">256352:
[Statepoints] Use Indirect operands for spill slots</a> <br>
<a href="http://reviews.llvm.org/rL257022" class="phui-handle">257022:
[Statepoints] Initial support for relocating vectors of pointers</a><br>
<a href="http://reviews.llvm.org/rL257244" class="phui-handle">257244:
[rs4gc] Optionally directly relocated vector of pointers</a> <br>
<br>
Philip<br>
</body>
</html>