<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Nov 4, 2014 at 12:51 PM, Arnaud A. de Grandmaison <span dir="ltr"><<a href="mailto:arnaud.degrandmaison@arm.com" target="_blank">arnaud.degrandmaison@arm.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Author: aadg<br>
Date: Tue Nov 4 14:51:24 2014<br>
New Revision: 221292<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=221292&view=rev" target="_blank">http://llvm.org/viewvc/llvm-project?rev=221292&view=rev</a><br>
Log:<br>
[PBQP] Tweak spill costs and coalescing benefits<br></blockquote><div><br></div><div>Is this intended to change the observable behavior of the allocator (coalescing more or less often, etc?) - a test would be nice for that.<br><br>If it isn't, it's helpful (well, to me, at least) to describe why something isn't tested in the commit message (just refactoring to lay the foundation for a future change, etc).</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
This patch improves how the different costs (register, interference, spill<br>
and coalescing) relates together. The assumption is now that:<br>
- coalescing (or any other "side effect" of reg alloc) is negative, and<br>
instead of being derived from a spill cost, they use the block<br>
frequency info.<br>
- spill costs are in the [MinSpillCost:+inf( range<br>
- register or interference costs are in [0.0:MinSpillCost( or +inf<br>
<br>
The current MinSpillCost is set to 10.0, which is a random value high<br>
enough that the current constraint builders do not need to worry about<br>
when settings costs. It would however be worth adding a normalization<br>
step for register and interference costs as the last step in the<br>
constraint builder chain to ensure they are not greater than SpillMinCost<br>
(unless this has some sense for some architectures). This would work well<br>
with the current builder pipeline, where all costs are tweaked relatively<br>
to each others, but could grow above MinSpillCost if the pipeline is<br>
deep enough.<br>
<br>
The current heuristic is tuned to depend rather on the number of uses of<br>
a live interval rather than a density of uses, as used by the greedy<br>
allocator. This heuristic provides a few percent improvement on a number<br>
of benchmarks (eembc, spec, ...) and will definitely need to change once<br>
spill placement is implemented: the current spill placement is really<br>
ineficient, so making the cost proportionnal to the number of use is a<br>
clear win.<br>
<br>
Modified:<br>
llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h<br>
llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp<br>
llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp<br>
llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp<br>
<br>
Modified: llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h?rev=221292&r1=221291&r2=221292&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h?rev=221292&r1=221291&r2=221292&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h (original)<br>
+++ llvm/trunk/include/llvm/CodeGen/CalcSpillWeights.h Tue Nov 4 14:51:24 2014<br>
@@ -30,8 +30,10 @@ namespace llvm {<br>
/// @param UseDefFreq Expected number of executed use and def instructions<br>
/// per function call. Derived from block frequencies.<br>
/// @param Size Size of live interval as returnexd by getSize()<br>
+ /// @param NumInstr Number of instructions using this live interval<br>
///<br>
- static inline float normalizeSpillWeight(float UseDefFreq, unsigned Size) {<br>
+ static inline float normalizeSpillWeight(float UseDefFreq, unsigned Size,<br>
+ unsigned NumInstr) {<br>
// The constant 25 instructions is added to avoid depending too much on<br>
// accidental SlotIndex gaps for small intervals. The effect is that small<br>
// intervals have a spill weight that is mostly proportional to the number<br>
@@ -44,7 +46,7 @@ namespace llvm {<br>
/// spill weight and allocation hint.<br>
class VirtRegAuxInfo {<br>
public:<br>
- typedef float (*NormalizingFn)(float, unsigned);<br>
+ typedef float (*NormalizingFn)(float, unsigned, unsigned);<br>
<br>
private:<br>
MachineFunction &MF;<br>
<br>
Modified: llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp?rev=221292&r1=221291&r2=221292&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp?rev=221292&r1=221291&r2=221292&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp (original)<br>
+++ llvm/trunk/lib/CodeGen/CalcSpillWeights.cpp Tue Nov 4 14:51:24 2014<br>
@@ -100,6 +100,7 @@ VirtRegAuxInfo::calculateSpillWeightAndH<br>
MachineLoop *loop = nullptr;<br>
bool isExiting = false;<br>
float totalWeight = 0;<br>
+ unsigned numInstr = 0; // Number of instructions using li<br>
SmallPtrSet<MachineInstr*, 8> visited;<br>
<br>
// Find the best physreg hint and the best virtreg hint.<br>
@@ -116,6 +117,7 @@ VirtRegAuxInfo::calculateSpillWeightAndH<br>
I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();<br>
I != E; ) {<br>
MachineInstr *mi = &*(I++);<br>
+ numInstr++;<br>
if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())<br>
continue;<br>
if (!visited.insert(mi))<br>
@@ -189,5 +191,5 @@ VirtRegAuxInfo::calculateSpillWeightAndH<br>
if (isRematerializable(li, LIS, *MF.getSubtarget().getInstrInfo()))<br>
totalWeight *= 0.5F;<br>
<br>
- li.weight = normalize(totalWeight, li.getSize());<br>
+ li.weight = normalize(totalWeight, li.getSize(), numInstr);<br>
}<br>
<br>
Modified: llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp?rev=221292&r1=221291&r2=221292&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp?rev=221292&r1=221291&r2=221292&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp (original)<br>
+++ llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp Tue Nov 4 14:51:24 2014<br>
@@ -1789,9 +1789,11 @@ unsigned RAGreedy::tryLocalSplit(LiveInt<br>
// instructions.<br>
//<br>
// Try to guess the size of the new interval.<br>
- const float EstWeight = normalizeSpillWeight(blockFreq * (NewGaps + 1),<br>
- Uses[SplitBefore].distance(Uses[SplitAfter]) +<br>
- (LiveBefore + LiveAfter)*SlotIndex::InstrDist);<br>
+ const float EstWeight = normalizeSpillWeight(<br>
+ blockFreq * (NewGaps + 1),<br>
+ Uses[SplitBefore].distance(Uses[SplitAfter]) +<br>
+ (LiveBefore + LiveAfter) * SlotIndex::InstrDist,<br>
+ 1);<br>
// Would this split be possible to allocate?<br>
// Never allocate all gaps, we wouldn't be making progress.<br>
DEBUG(dbgs() << " w=" << EstWeight);<br>
<br>
Modified: llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp?rev=221292&r1=221291&r2=221292&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp?rev=221292&r1=221291&r2=221292&view=diff</a><br>
==============================================================================<br>
--- llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp (original)<br>
+++ llvm/trunk/lib/CodeGen/RegAllocPBQP.cpp Tue Nov 4 14:51:24 2014<br>
@@ -150,11 +150,17 @@ public:<br>
void apply(PBQPRAGraph &G) override {<br>
LiveIntervals &LIS = G.getMetadata().LIS;<br>
<br>
+ // A minimum spill costs, so that register constraints can can be set<br>
+ // without normalization in the [0.0:MinSpillCost( interval.<br>
+ const PBQP::PBQPNum MinSpillCost = 10.0;<br>
+<br>
for (auto NId : G.nodeIds()) {<br>
PBQP::PBQPNum SpillCost =<br>
LIS.getInterval(G.getNodeMetadata(NId).getVReg()).weight;<br>
if (SpillCost == 0.0)<br>
SpillCost = std::numeric_limits<PBQP::PBQPNum>::min();<br>
+ else<br>
+ SpillCost += MinSpillCost;<br>
PBQPRAGraph::RawVector NodeCosts(G.getNodeCosts(NId));<br>
NodeCosts[PBQP::RegAlloc::getSpillOptionIdx()] = SpillCost;<br>
G.setNodeCosts(NId, std::move(NodeCosts));<br>
@@ -350,11 +356,8 @@ public:<br>
unsigned DstReg = CP.getDstReg();<br>
unsigned SrcReg = CP.getSrcReg();<br>
<br>
- const float CopyFactor = 0.5; // Cost of copy relative to load. Current<br>
- // value plucked randomly out of the air.<br>
-<br>
- PBQP::PBQPNum CBenefit =<br>
- CopyFactor * LiveIntervals::getSpillWeight(false, true, &MBFI, &MI);<br>
+ const float Scale = 1.0f / MBFI.getEntryFreq();<br>
+ PBQP::PBQPNum CBenefit = MBFI.getBlockFreq(&MBB).getFrequency() * Scale;<br>
<br>
if (CP.isPhys()) {<br>
if (!MF.getRegInfo().isAllocatable(DstReg))<br>
@@ -607,12 +610,20 @@ void RegAllocPBQP::finalizeAlloc(Machine<br>
}<br>
}<br>
<br>
+static inline float normalizePBQPSpillWeight(float UseDefFreq, unsigned Size,<br>
+ unsigned NumInstr) {<br>
+ // All intervals have a spill weight that is mostly proportional to the number<br>
+ // of uses, with uses in loops having a bigger weight.<br>
+ return NumInstr * normalizeSpillWeight(UseDefFreq, Size, 1);<br>
+}<br>
+<br>
bool RegAllocPBQP::runOnMachineFunction(MachineFunction &MF) {<br>
LiveIntervals &LIS = getAnalysis<LiveIntervals>();<br>
MachineBlockFrequencyInfo &MBFI =<br>
getAnalysis<MachineBlockFrequencyInfo>();<br>
<br>
- calculateSpillWeightsAndHints(LIS, MF, getAnalysis<MachineLoopInfo>(), MBFI);<br>
+ calculateSpillWeightsAndHints(LIS, MF, getAnalysis<MachineLoopInfo>(), MBFI,<br>
+ normalizePBQPSpillWeight);<br>
<br>
VirtRegMap &VRM = getAnalysis<VirtRegMap>();<br>
<br>
<br>
<br>
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</blockquote></div><br></div></div>