Start scaffolding for a MachineTraceMetrics analysis pass.

This is still a work in progress.

Out-of-order CPUs usually execute instructions from multiple basic
blocks simultaneously, so it is necessary to look at longer traces when
estimating the performance effects of code transformations.

The MachineTraceMetrics analysis will pick a typical trace through a
given basic block and provide performance metrics for the trace. Metrics
will include:

- Instruction count through the trace.
- Issue count per functional unit.
- Critical path length, and per-instruction 'slack'.

These metrics can be used to determine the performance limiting factor
when executing the trace, and how it will be affected by a code
transformation.

Initially, this will be used by the early if-conversion pass.

llvm-svn: 160796

1 files changed, 30 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/EarlyIfConversion.cpp b/llvm/lib/CodeGen/EarlyIfConversion.cpp
index 9840a402804..cfe3e9d7e1e 100644
--- a/llvm/lib/CodeGen/EarlyIfConversion.cpp
+++ b/llvm/lib/CodeGen/EarlyIfConversion.cpp
@@ -17,6 +17,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "early-ifcvt"
+#include "MachineTraceMetrics.h"
 #include "llvm/Function.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/PostOrderIterator.h"
@@ -515,6 +516,8 @@ class EarlyIfConverter : public MachineFunctionPass {
   MachineRegisterInfo *MRI;
   MachineDominatorTree *DomTree;
   MachineLoopInfo *Loops;
+  MachineTraceMetrics *Traces;
+  MachineTraceMetrics::Ensemble *MinInstr;
   SSAIfConv IfConv;
 
 public:
@@ -527,6 +530,8 @@ private:
   bool tryConvertIf(MachineBasicBlock*);
   void updateDomTree(ArrayRef<MachineBasicBlock*> Removed);
   void updateLoops(ArrayRef<MachineBasicBlock*> Removed);
+  void invalidateTraces();
+  bool shouldConvertIf();
 };
 } // end anonymous namespace
 
@@ -537,6 +542,7 @@ INITIALIZE_PASS_BEGIN(EarlyIfConverter,
                       "early-ifcvt", "Early If Converter", false, false)
 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
 INITIALIZE_PASS_END(EarlyIfConverter,
                       "early-ifcvt", "Early If Converter", false, false)
 
@@ -546,6 +552,8 @@ void EarlyIfConverter::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addPreserved<MachineDominatorTree>();
   AU.addRequired<MachineLoopInfo>();
   AU.addPreserved<MachineLoopInfo>();
+  AU.addRequired<MachineTraceMetrics>();
+  AU.addPreserved<MachineTraceMetrics>();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
@@ -576,12 +584,31 @@ void EarlyIfConverter::updateLoops(ArrayRef<MachineBasicBlock*> Removed) {
     Loops->removeBlock(Removed[i]);
 }
 
+/// Invalidate MachineTraceMetrics before if-conversion.
+void EarlyIfConverter::invalidateTraces() {
+  Traces->invalidate(IfConv.Head);
+  Traces->invalidate(IfConv.Tail);
+  Traces->invalidate(IfConv.TBB);
+  Traces->invalidate(IfConv.FBB);
+}
+
+/// Apply cost model and heuristics to the if-conversion in IfConv.
+/// Return true if the conversion is a good idea.
+///
+bool EarlyIfConverter::shouldConvertIf() {
+  if (!MinInstr)
+    MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
+  DEBUG(dbgs() << MinInstr->getTrace(IfConv.Head));
+  return true;
+}
+
 /// Attempt repeated if-conversion on MBB, return true if successful.
 ///
 bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) {
   bool Changed = false;
-  while (IfConv.canConvertIf(MBB)) {
+  while (IfConv.canConvertIf(MBB) && shouldConvertIf()) {
     // If-convert MBB and update analyses.
+    invalidateTraces();
     SmallVector<MachineBasicBlock*, 4> RemovedBlocks;
     IfConv.convertIf(RemovedBlocks);
     Changed = true;
@@ -600,6 +627,8 @@ bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) {
   MRI = &MF.getRegInfo();
   DomTree = &getAnalysis<MachineDominatorTree>();
   Loops = getAnalysisIfAvailable<MachineLoopInfo>();
+  Traces = &getAnalysis<MachineTraceMetrics>();
+  MinInstr = 0;
 
   bool Changed = false;
   IfConv.runOnMachineFunction(MF);