Trace copies when checking for rematerializability in spill weight calculation

PR24139 contains an analysis of poor register allocation. One of the findings
was that when calculating the spill weight, a rematerializable interval once
split is no longer rematerializable. This is because the isRematerializable
check in CalcSpillWeights.cpp does not follow the copies introduced by live
range splitting (after splitting, the live interval register definition is a
copy which is not rematerializable).

Reviewers: qcolombet

Differential Revision: http://reviews.llvm.org/D11686

llvm-svn: 244439

1 files changed, 37 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/CalcSpillWeights.cpp b/llvm/lib/CodeGen/CalcSpillWeights.cpp
index d08fae09323..abc655ac34c 100644
--- a/llvm/lib/CodeGen/CalcSpillWeights.cpp
+++ b/llvm/lib/CodeGen/CalcSpillWeights.cpp
@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/CodeGen/VirtRegMap.h"
 #include "llvm/CodeGen/CalcSpillWeights.h"
 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
@@ -24,6 +25,7 @@ using namespace llvm;
 
 void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
                            MachineFunction &MF,
+                           VirtRegMap *VRM,
                            const MachineLoopInfo &MLI,
                            const MachineBlockFrequencyInfo &MBFI,
                            VirtRegAuxInfo::NormalizingFn norm) {
@@ -31,7 +33,7 @@ void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
                << "********** Function: " << MF.getName() << '\n');
 
   MachineRegisterInfo &MRI = MF.getRegInfo();
-  VirtRegAuxInfo VRAI(MF, LIS, MLI, MBFI, norm);
+  VirtRegAuxInfo VRAI(MF, LIS, VRM, MLI, MBFI, norm);
   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
     if (MRI.reg_nodbg_empty(Reg))
@@ -74,7 +76,10 @@ static unsigned copyHint(const MachineInstr *mi, unsigned reg,
 // Check if all values in LI are rematerializable
 static bool isRematerializable(const LiveInterval &LI,
                                const LiveIntervals &LIS,
+                               VirtRegMap *VRM,
                                const TargetInstrInfo &TII) {
+  unsigned Reg = LI.reg;
+  unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
        I != E; ++I) {
     const VNInfo *VNI = *I;
@@ -86,6 +91,36 @@ static bool isRematerializable(const LiveInterval &LI,
     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
     assert(MI && "Dead valno in interval");
 
+    // Trace copies introduced by live range splitting.  The inline
+    // spiller can rematerialize through these copies, so the spill
+    // weight must reflect this.
+    if (VRM) {
+      while (MI->isFullCopy()) {
+        // The copy destination must match the interval register.
+        if (MI->getOperand(0).getReg() != Reg)
+          return false;
+
+        // Get the source register.
+        Reg = MI->getOperand(1).getReg();
+
+        // If the original (pre-splitting) registers match this
+        // copy came from a split.
+        if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
+            VRM->getOriginal(Reg) != Original)
+          return false;
+
+        // Follow the copy live-in value.
+        const LiveInterval &SrcLI = LIS.getInterval(Reg);
+        LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
+        VNI = SrcQ.valueIn();
+        assert(VNI && "Copy from non-existing value");
+        if (VNI->isPHIDef())
+          return false;
+        MI = LIS.getInstructionFromIndex(VNI->def);
+        assert(MI && "Dead valno in interval");
+      }
+    }
+
     if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
       return false;
   }
@@ -188,7 +223,7 @@ VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
   // it is a preferred candidate for spilling.
   // FIXME: this gets much more complicated once we support non-trivial
   // re-materialization.
-  if (isRematerializable(li, LIS, *MF.getSubtarget().getInstrInfo()))
+  if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
     totalWeight *= 0.5F;
 
   li.weight = normalize(totalWeight, li.getSize(), numInstr);