[llvm-dev] RFC MachineLICM to hoist invariant stores
Zaara Syeda via llvm-dev
llvm-dev at lists.llvm.org
Wed Nov 15 14:17:44 PST 2017
I’m looking at MachineLICM and running into a limitation of not hoisting
instructions that store a constant value to a constant spot on the stack.
I see this example in the case of indirect calls for PowerPC. Indirect
calls on PPC must be surrounded by instructions that save and restore the
TOC (similar to the GOT) pointer. This pointer is in register X2 and will
not change throughout the function.
%vreg12<def> = COPY %X2; G8RC:%vreg12
STD %vreg12, 24, %X1; mem:ST8[Stack+24] G8RC:%vreg12
This currently can't be hoisted because:
1. It is a store instruction
2. It uses a physical register (the stack pointer register - X1)
3. The copy that defines the physical register being stored isn't
profitable to hoist
As a proof of concept, I’ve relaxed these conditions to catch these loop
invariant stores by using the fact that registers X1/X2 satisfy
isCallerPreservedPhysReg(). With these relaxations I get the following
results in SPEC:
Benchmark | Improvement | # of Hoists
dealII |0.10 % |1203
h264ref |11 % |347
Gcc |1.42 % |135
xalancbmk |0.86 % |3886
Astar | -2.39% | 1
Astar here is an outlier where we see a degradation. In this case, the
store was hoisted out of an if statement that is executed infrequently
compared to the outer code. This results in the store to r2 being executed
more times dynamically.
I would like to add TargetInstrInfo hooks like
isInvariantStore(MachineInstr &) and
isCopyFeedingInvariantStore(MachineInstr &) that would tell MachineLICM
that the two instructions are safe and profitable to hoist.
Please advise if adding the target hooks is a good solution and if other
targets would be interested in such hooks, or whether I should be take a
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