Yk IR serialiser: Simplify the value lowering map.

Currently the value lowering map stores basic block index and
instruction index pair values.

Because basic block indices are the same in LLVM IR and Yk AOT IR,
there's no need to store a basic block index in the value lowering map.
It can be found elsewhere.

No functional change. Just simplifies the code and saves memory.

Author: vext01

llvm/lib/YkIR/YkIRWriter.cpp

@@ -135,13 +135,20 @@ template <class C, class E> size_t getIndex(C *Container, E *FindElement) {
   return Idx;
 }
 
-// A <BBIdx, InstrIdx> pair that Uniquely identifies an Yk IR instruction within
-// a function.
-using InstrLoc = std::tuple<size_t, size_t>;
+// An instruction index that uniquely identifies an Yk instruction within
+// a basic block.
+//
+// FIXME: At some point it may be worth making type-safe index types (for
+// instruction, block and function indices) and using them throughout
+// the serialiser.
+using InstIdx = size_t;
 
-// Maps an LLVM instruction that generates a value to the corresponding Yk IR
-// instruction.
-using ValueLoweringMap = map<Instruction *, InstrLoc>;
+// Maps an LLVM local (the instruction that creates it) to the correspoinding Yk
+// instruction index in its parent basic block.
+//
+// Note: The Yk basic block index is not stored because it's the same as
+// the LLVM IR block index, which can be found elsewhere (see `getIndex()`).
+using ValueLoweringMap = map<Instruction *, InstIdx>;
 
 // Function lowering context.
 //
@@ -169,18 +176,18 @@ class FuncLowerCtxt {
   void patchUpInstIdxs(MCStreamer &OutStreamer) {
     MCContext &MCtxt = OutStreamer.getContext();
     for (auto &[Inst, Sym] : InstIdxPatchUps) {
-      auto [_, InstIdx] = VLMap.at(Inst);
+      InstIdx InstIdx = VLMap.at(Inst);
       OutStreamer.emitAssignment(Sym, MCConstantExpr::create(InstIdx, MCtxt));
     }
   }
 
   // Add/update an entry in the value lowering map.
-  void updateVLMap(Instruction *I, InstrLoc L) { VLMap[I] = L; }
+  void updateVLMap(Instruction *I, InstIdx L) { VLMap[I] = L; }
 
   // Get the entry for `I` in the value lowering map.
   //
   // Raises `std::out_of_range` if not present.
-  InstrLoc lookupInVLMap(Instruction *I) { return VLMap.at(I); }
+  InstIdx lookupInVLMap(Instruction *I) { return VLMap.at(I); }
 
   // Determines if there's an entry for `I` in the value lowering map.
   bool vlMapContains(Instruction *I) { return VLMap.count(I) == 1; }
@@ -289,12 +296,16 @@ class YkIRWriter {
   }
 
   void serialiseLocalVariableOperand(Instruction *I, FuncLowerCtxt &FLCtxt) {
+    // operand kind:
     serialiseOperandKind(OperandKindLocal);
+    // func_idx:
     OutStreamer.emitSizeT(getIndex(&M, I->getFunction()));
+    // bb_idx:
+    OutStreamer.emitSizeT(getIndex(I->getFunction(), I->getParent()));
 
+    // inst_idx:
     if (FLCtxt.vlMapContains(I)) {
-      auto [BBIdx, InstIdx] = FLCtxt.lookupInVLMap(I);
-      OutStreamer.emitSizeT(BBIdx);
+      InstIdx InstIdx = FLCtxt.lookupInVLMap(I);
       OutStreamer.emitSizeT(InstIdx);
     } else {
       // It's a local variable generated by an instruction that we haven't
@@ -303,16 +314,7 @@ class YkIRWriter {
       //
       // To work around this, we emit a dummy instruction index
       // and patch it up later once it becomes known.
-      //
-      // The basic block index can be immediately known, since the indices are
-      // the same in the LLVM IR and our AOT IR.
-      //
-      // FIXME: In light of the above, there's no need to store basic block
-      // indices in the VLMap?
-      OutStreamer.emitSizeT(getIndex(I->getFunction(), I->getParent()));
-
-      MCContext &MCtxt = OutStreamer.getContext();
-      MCSymbol *PatchUpSym = MCtxt.createTempSymbol();
+      MCSymbol *PatchUpSym = OutStreamer.getContext().createTempSymbol();
       OutStreamer.emitSymbolValue(PatchUpSym, sizeof(size_t));
       FLCtxt.deferInstIdx(I, PatchUpSym);
     }
@@ -375,7 +377,7 @@ class YkIRWriter {
     // right-hand side:
     serialiseOperand(I, FLCtxt, I->getOperand(1));
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -457,7 +459,7 @@ class YkIRWriter {
     // XXX guard cast
     OutStreamer.emitSizeT(CI->getZExtValue());
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -540,7 +542,7 @@ class YkIRWriter {
 
     // If the return type is non-void, then this defines a local.
     if (!I->getType()->isVoidTy()) {
-      FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+      FLCtxt.updateVLMap(I, InstIdx);
     }
     InstIdx++;
   }
@@ -585,7 +587,7 @@ class YkIRWriter {
     // type_idx:
     OutStreamer.emitSizeT(typeIndex(I->getType()));
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -619,7 +621,7 @@ class YkIRWriter {
     // offset:
     serialiseOperand(I, FLCtxt, ConstantInt::get(I->getContext(), Offset));
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -676,7 +678,7 @@ class YkIRWriter {
     // rhs:
     serialiseOperand(I, FLCtxt, I->getOperand(1));
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -725,7 +727,7 @@ class YkIRWriter {
     // dest_type_idx:
     OutStreamer.emitSizeT(typeIndex(I->getDestTy()));
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -772,7 +774,7 @@ class YkIRWriter {
       serialiseOperand(I, FLCtxt, I->getIncomingValue(J));
     }
 
-    FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+    FLCtxt.updateVLMap(I, InstIdx);
     InstIdx++;
   }
 
@@ -812,7 +814,7 @@ class YkIRWriter {
     serialiseString(toString(I));
 
     if (!I->getType()->isVoidTy()) {
-      FLCtxt.updateVLMap(I, {BBIdx, InstIdx});
+      FLCtxt.updateVLMap(I, InstIdx);
     }
     InstIdx++;
   }