Poor register allocation (constants causing spilling)

[rlougher]

Bugzilla Link	24139
Version	trunk
OS	All
Attachments	Test case
CC	@adibiagio,@gregbedwell,@hfinkel,@RKSimon,@eschnett,@simonpcook

Extended Description

While investigating a performance issue with an internal codebase I came across what looks to be poor register allocation. I have constructed a small(ish) reproducible which demonstrates the issue (see test.ll attached).

I have spent some time going through the register allocator to understand what is happening. I have also experimented with some small changes to try and improve the allocation. Unfortunately, the full report is fairly long and detailed. However, in short, I found that not splitting rematerializable live-ranges lead to significantly better register allocation, and an overall performance improvement of 3%.

*** The Problem

Compile the attached testcase as follows:

llc -mcpu=btver2 test.ll

Examining the assembly in test.s we can see a constant is being loaded into %xmm8 (second instruction in foo). Tracing the constant we can see the following:

foo: ... vmovaps .LCPI0_0(%rip), %xmm8 # xmm8 = [6.366197e-01,6.366197e-01,...] ... vmulps %xmm8, %xmm0, %xmm1 # first use of constant vmovaps %xmm8, %xmm9 # move constant into another register ... vmovaps %xmm0, -40(%rsp) # 16-byte Spill vmovaps %xmm9, %xmm0 # move constant into vacated register ... vmulps %xmm0, %xmm3, %xmm2 # second use of constant ... vmulps %xmm0, %xmm5, %xmm3 # last use of constant ...

The constant is used 3 times within a live range of 86 instructions. Although the constant is clearly rematerializable, the allocator has gone to some length to keep it in a register, and it has spilled a value to the stack. It would have been cheaper to simply fold the constant load into the 3 uses.

This is not the only example. Later on we can see this:

vmovaps .LCPI0_1(%rip), %xmm6   # xmm6 = [2147483648,2147483648,...]
vxorps  %xmm6, %xmm2, %xmm3
...
vandps  %xmm6, %xmm5, %xmm2
...
vmovaps %xmm1, -56(%rsp)        # 16-byte Spill
vmovaps %xmm6, %xmm1
...
vmovaps -56(%rsp), %xmm0        # 16-byte Reload
...
vxorps  %xmm1, %xmm3, %xmm4
...

Here, we have a spill and reload to keep the constant in a register for a single use!

*** Live-Range Splitting

Looking at the first constant, we can see that the copies are inserted by the greedy register allocator when it splits the live range. The main entry point is selectOrSplit. In brief:

1) The constant is assigned and evicted several times. Eventually selectOrSplit decides to split the live-range. This splits the interval in two, creating two new intervals (A and B). At the boundary of the intervals a copy is inserted that copies between the virtual registers.

2) The virtual register for the first interval (A) is assigned to XMM8.

3) The virtual register for the second interval (B) is assigned and evicted several times before selectOrSplit decides to split again (C and D).

4) The virtual register for interval C is assigned to XMM9.

5) The virtual register for interval D is assigned to XMM0.

At the boundary between A and C we have a copy from XMM8 to XMM9, and at the boundary between C and D we have a copy from XMM9 to XMM0.

The spill is the result of several evictions. To assign interval D to XMM0 another virtual register was evicted. This virtual register had previously caused an eviction, which was subsequently spilled.

*** Spill Weights

The main question is why does the register allocator decide to split the constant live range and evict other non-rematerializable registers? This comes down to the calculation of the spill weight which is used to determine whether one interval should evict another. If the live interval is rematerializable the spill weight is halved (multiplied by 0.5). From what I can see, this calculation is the only place in the greedy register allocator (as opposed to the spiller) where rematerializability is considered.

So rematerializable intervals should be less important than non-rematerializable intervals, assuming similar number of uses, size of range, etc. However, there appears to be a flaw:

• A rematerializable interval once split is no longer rematerializable *

The isRematerializable check in CalcSpillWeights.cpp uses the target instruction info to check that the machine instruction for the live interval definition is trivially rematerializable. In the case of interval A above, the definition is a load from the constant-pool. This is trivially rematerializable and the spill weight is halved. However, in the remainder intervals B, C and D the definition is a copy. This is not trivially rematerializable, and the spill weight is unadjusted. This means these ranges are considered as expensive as non-rematerializable ranges.

*** Experiment One

In constrast to the spill weight calculation, the inline spiller can trace rematerializability back through the copies resulting from live range splitting (what it calls sibling values). The first experiment modified the greedy register allocator so that it would not split a rematerializable interval more than once (i.e. instead of splitting interval B into C and D it is spilled). The purpose of this experiment was to confirm that the inline spiller was able to follow the copy and rematerialize the load from the constant-pool. In this case, the load was folded into the two remaining uses (in fact, the use in interval A was also folded as it was the single remaining use).

*** Experiment Two

This experiment modified the greedy register allocator so that the spill weights for split rematerializable ranges were adjusted correctly (a hack rather than a proper fix). The hope was that with the "correct" spill weight the relevant priorities of the live ranges would result in better allocation (see end for full statistics).

Unmodified:

Spilled 3 values to the stack 124 instructions in foo

Modified:

Spilled 1 value to the stack 113 instructions in foo

Unfortunately, the performance showed only minimal improvement.

*** Experiment Three

The code gives no explanation for the spill weight adjustment value of 0.5. This experiment was designed to determine what adjustment factor was needed to obtain no spills. This point was reached with an adjustment factor for rematerializable intervals of 0.3.

Modified:

No spills to the stack 111 instructions in foo

Again, the performance showed only minimal improvement.

*** Experiment Four

In the previous experiments splitting was still occurring leading to register copies. In this final experiment the register allocator was modified so that rematerializable ranges are not split at all, but are spilled if they cannot be assigned (obviously, the spiller will rematerialize the values).

Modified:

No spills to the stack 106 instructions in foo

This version contains considerably more folded loads than the previous experiments, and shows a 3% speed improvement.

[qcolombet]

mentioned in issue llvm/llvm-bugzilla-archive#25210

[eschnett]

This is a pretty thorough analysis. I assume it applies mostly to long stretches of almost-uninterrupted code, as would be found in complicated loop kernels. Was there additional communication regarding your findings?

I looked at the code generated for our loop kernels http://einsteintoolkit.org, and found problems similar to the ones you describe -- values are moved around between registers instead of re-loaded from the stack.

Does your patch still apply? Why did you post a patch for experiment 2, while experiment 4 shows the interesting speedup?

[rlougher]

Patch for "experiment 2" posted for review: http://reviews.llvm.org/D11686

[rlougher]

Register Allocator Statistics for the experiments in the description.

Unmodified:

32 regalloc - Number of copies inserted for splitting 2 regalloc - Number of folded loads 3 regalloc - Number of folded stack accesses 19 regalloc - Number of identity moves eliminated after rewriting 18 regalloc - Number of instructions deleted by DCE 158 regalloc - Number of interferences evicted 280 regalloc - Number of new live ranges queued 311 regalloc - Number of registers assigned 173 regalloc - Number of registers unassigned 2 regalloc - Number of reloads inserted 8 regalloc - Number of rematerialized defs for spilling 3 regalloc - Number of rematerialized defs for splitting 7 regalloc - Number of single use loads folded after DCE 3 regalloc - Number of spill slots allocated 13 regalloc - Number of spilled live ranges 3 regalloc - Number of spills inserted 31 regalloc - Number of split local live ranges 31 regalloc - Number of splits finished 29 regalloc - Number of splits that were simple

Experiment Two:

26 regalloc - Number of copies inserted for splitting 9 regalloc - Number of folded loads 10 regalloc - Number of folded stack accesses 12 regalloc - Number of identity moves eliminated after rewriting 21 regalloc - Number of instructions deleted by DCE 88 regalloc - Number of interferences evicted 181 regalloc - Number of new live ranges queued 224 regalloc - Number of registers assigned 102 regalloc - Number of registers unassigned 4 regalloc - Number of rematerialized defs for spilling 8 regalloc - Number of single use loads folded after DCE 1 regalloc - Number of spill slots allocated 10 regalloc - Number of spilled live ranges 4 regalloc - Number of spilled snippets 1 regalloc - Number of spills inserted 26 regalloc - Number of split local live ranges 26 regalloc - Number of splits finished 26 regalloc - Number of splits that were simple

Experiment Three:

26 regalloc - Number of copies inserted for splitting 10 regalloc - Number of folded loads 10 regalloc - Number of folded stack accesses 13 regalloc - Number of identity moves eliminated after rewriting 20 regalloc - Number of instructions deleted by DCE 83 regalloc - Number of interferences evicted 174 regalloc - Number of new live ranges queued 220 regalloc - Number of registers assigned 98 regalloc - Number of registers unassigned 3 regalloc - Number of rematerialized defs for spilling 7 regalloc - Number of single use loads folded after DCE 9 regalloc - Number of spilled live ranges 4 regalloc - Number of spilled snippets 26 regalloc - Number of split local live ranges 26 regalloc - Number of splits finished 26 regalloc - Number of splits that were simple

Experiment Four:

21 regalloc - Number of folded loads 21 regalloc - Number of folded stack accesses 2 regalloc - Number of identity moves eliminated after rewriting 7 regalloc - Number of instructions deleted by DCE 42 regalloc - Number of interferences evicted 49 regalloc - Number of new live ranges queued 148 regalloc - Number of registers assigned 42 regalloc - Number of registers unassigned 7 regalloc - Number of spilled live ranges

[rlougher]

assigned to @rlougher