https://llvm.org/bugs/show_bug.cgi?id=26223
Bug ID: 26223
Summary: [GC] Effective rematerialization at non-entry polls
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: listm...@philipreames.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
(This bug is specific to the implementation of rewrite-statepoints-for-gc.)
If we have a bit of code like this:
%addr = gep %o, 8
loop {
if (poll) {
safepoint();
}
load %addr
}
We currently end up rewriting this as:
%addr = gep %o, 8
loop {
%addr1 = phi (%addr, %addr2)
if (poll) {
safepoint();
%remat = gep %o.relocated, 8
}
%addr2 = phi (%addr1, %remat)
load %addr2
}
This ends up forcing us to rematerialize the address explicitly and likely will
cause us to spill/fill the address if register constrained. This creates a
bunch of dependent loads (fill from stack, load from result) which show up as
hot in a couple of benchmarks.
A much better result would be:
%addr = gep %o, 8
loop {
if (poll) {
safepoint();
}
%remat = gep %o.relocated, 8
load %remat
}
This version allows the GEP to be folded directly into x86's native addressing
modes.
(Note: For conciseness, I'm not writing the phis for relocating %o, assume
they're all there.)
I can see a couple of ways of approaching this:
- Rematerialize not at relocations, but at uses. This would require either a
post rewriting CSE to clean up, or a bit of smarts to avoid naive placement.
This might be overly x86 specific.
- Push rematerializations into unconditional successors. This would have the
effect of eliminating PHIs (because we know the remat is also legal there).
- Add a post processing pass which tries to pull remat values through their
only use. This could be done as either a step in RS4GC, or possibly just an
instcombine rule. Given a single use gep of a gc-relocate, look to see if the
single use is a phi of the original value. If so, replace the PHI with a gep
of the PHI producing the fixed up base.
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