Fix multiple locations in stackmaps

When fixing a bug in our control point pass (which split blocks at the wrong
instruction) this shook out another bug in stackmaps. So far we were only able
to store one additional location per register. However, there can be cases
were we need to store 2 additional locations. We get away with this fix for now
but may have to revisit this in the future if even more locations are needed
and implement a more general approach.

Author: ptersilie

llvm/include/llvm/CodeGen/StackMaps.h

@@ -195,10 +195,11 @@ class StackMaps {
     unsigned Size = 0;
     unsigned Reg = 0;
     int64_t Offset = 0;
+    unsigned ExtraReg = 0;
 
     Location() = default;
-    Location(LocationType Type, unsigned Size, unsigned Reg, int64_t Offset)
-        : Type(Type), Size(Size), Reg(Reg), Offset(Offset) {}
+    Location(LocationType Type, unsigned Size, unsigned Reg, int64_t Offset, unsigned ExtraReg = 0)
+        : Type(Type), Size(Size), Reg(Reg), Offset(Offset), ExtraReg(ExtraReg) {}
   };
 
   struct LiveOutReg {

llvm/lib/CodeGen/StackMaps.cpp

@@ -286,21 +286,46 @@ StackMaps::parseOperand(MachineInstr::const_mop_iterator MOI,
 
 
     signed Offset = 0;
-    // Check if there are any mappings for this register in the spillmap. If so,
-    // encode the additional location in the offset field of the stackmap record
-    // (which is unused for register locations). Note that this assumes that
-    // there can only be one additional location for each value, which may turn
-    // out to be false.
+    int RHS = 0;
+    // Check for any additional mappings in the spillmap and add them to his
+    // location to be later encoded into stackmaps. A typical case where we need
+    // to record multiple sources is the following (annotated with SpillMap
+    // info):
+    //
+    // %rcx = load %rbp, -8             // SpillMap[rcx] = -8
+    // %rbx = %rcx                      // SpillMap[rbx] = rcx
+    // STACKMAP(%rbx) or STACKMAP(%rcx)
+    //
+    // First a value is loaded from the stack into %rcx and then immediately
+    // moved into %rbx. This means there are three sources for the same value,
+    // and during deoptimisation we need to make sure we write the value back to
+    // each one of them. Note, that the stackmap may track either of %rbx or
+    // %rcx, resulting in different ways below to retrieve the mappings.
+    int ExtraReg = 0;
     if (MOI->isReg()) {
       Register R = MOI->getReg();
       if (SpillOffsets.count(R) > 0) {
-        Offset = SpillOffsets[R];
+        RHS = SpillOffsets[R];
         assert(SpillOffsets[R] != 0);
-        if (Offset > 0) {
-          // If the additional location is another register encode its DWARF id.
-          // Also temporarily add 1 since 0 is used to mean there is no
-          // additional location.
-          Offset = getDwarfRegNum(Offset, TRI) + 1;
+        if (RHS > 0) {
+          // The additional location is another register: encode its DWARF id.
+          // Also temporarily add 1 since 0 means there is no additional
+          // location.
+          ExtraReg = getDwarfRegNum(RHS, TRI) + 1;
+          // Check if the other register also has a mapping and add it.
+          if (SpillOffsets.count(RHS) > 0) {
+            Offset = SpillOffsets[RHS];
+          }
+        }  else {
+          // The other location is an offset.
+          Offset = RHS;
+          // Find any additional mappings where the current register appears on
+          // the right hand side.
+          for (auto I = SpillOffsets.begin(); I != SpillOffsets.end(); I++) {
+            if (I->second == R) {
+              ExtraReg = getDwarfRegNum(I->first, TRI) + 1;
+            }
+          }
         }
       }
     }
@@ -312,7 +337,7 @@ StackMaps::parseOperand(MachineInstr::const_mop_iterator MOI,
       Offset = TRI->getSubRegIdxOffset(SubRegIdx);
 
     Locs.emplace_back(Location::Register, TRI->getSpillSize(*RC), DwarfRegNum,
-                      Offset);
+                      Offset, ExtraReg);
     return ++MOI;
   }
 
@@ -813,7 +838,7 @@ void StackMaps::emitCallsiteEntries(MCStreamer &OS) {
         OS.emitIntValue(0, 1); // Reserved
         OS.emitInt16(Loc.Size);
         OS.emitInt16(Loc.Reg);
-        OS.emitInt16(0); // Reserved
+        OS.emitInt16(Loc.ExtraReg); // Reserved
         OS.emitInt32(Loc.Offset);
       }
       LiveIdx++;

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -97,14 +97,7 @@ void processInstructions(
       const MachineOperand Rhs = Instr.getOperand(1);
       assert(Lhs.isReg() && "Is register.");
       assert(Rhs.isReg() && "Is register.");
-      if (SpillMap.count(Rhs.getReg()) > 0) {
-        // If the RHS has a mapping, apply the same mapping to the new register.
-        // This means, that stack spills moved into one register and then into
-        // another will continued to be tracked.
-        SpillMap[Lhs.getReg()] = SpillMap[Rhs.getReg()];
-      } else {
-        SpillMap[Lhs.getReg()] = Rhs.getReg();
-      }
+      SpillMap[Lhs.getReg()] = Rhs.getReg();
       // YKFIXME: If the `mov` instruction has a killed-flag, remove the
       // register from the map.
 

llvm/lib/Transforms/Yk/ControlPoint.cpp

@@ -231,13 +231,13 @@ class YkControlPoint : public ModulePass {
     Builder.CreateCall(YKFR, {NewCtrlPointCallInst});
     Builder.CreateUnreachable();
 
-    // To do so we need to first split up the current block and then
-    // insert a conditional branch that either continues or returns.
-
+    // Split up the current block and then insert a conditional branch that
+    // either continues after the control point or invokes frame reconstruction.
     BasicBlock *BB = NewCtrlPointCallInst->getParent();
-    BasicBlock *ContBB = BB->splitBasicBlock(New);
-
+    BasicBlock *ContBB = BB->splitBasicBlock(New->getNextNonDebugInstruction());
     Instruction &OldBr = BB->back();
+    assert(OldBr.getPrevNonDebugInstruction() == New &&
+           "Split block at the wrong spot.");
     OldBr.eraseFromParent();
     Builder.SetInsertPoint(BB);
     // The return value of the control point tells us whether a guard has failed