Sparse Conditional Constant Propagation

Author: dibyendumajumdar

optvm/src/main/java/com/compilerprogramming/ezlang/compiler/BasicBlock.java

@@ -96,6 +96,7 @@ public BasicBlock(int bid, BasicBlock... preds) {
     }
     public void add(Instruction instruction) {
         instructions.add(instruction);
+        instruction.block = this;
     }
     public void addSuccessor(BasicBlock successor) {
         successors.add(successor);
@@ -120,6 +121,7 @@ public void insertPhiFor(Register var) {
             inputs.add(var);
         Instruction.Phi phi = new Instruction.Phi(var, inputs);
         instructions.add(0, phi);
+        phi.block = this;
     }
     public List<Instruction.Phi> phis() {
         List<Instruction.Phi> list = new ArrayList<>();

optvm/src/main/java/com/compilerprogramming/ezlang/compiler/CompiledFunction.java

@@ -127,6 +127,7 @@ else if (virtualStack.size() > 1)
 
     public void code(Instruction instruction) {
         currentBlock.add(instruction);
+        instruction.block = currentBlock;
     }
 
     private void compileStatement(AST.Stmt statement) {

optvm/src/main/java/com/compilerprogramming/ezlang/compiler/Instruction.java

@@ -29,6 +29,7 @@ public abstract class Instruction {
     public final int opcode;
     public Operand.RegisterOperand def;
     public Operand[] uses;
+    public BasicBlock block;
 
     public Instruction(int opcode, Operand... uses) {
         this.opcode = opcode;

optvm/src/main/java/com/compilerprogramming/ezlang/compiler/Optimizer.java

@@ -4,6 +4,7 @@ public class Optimizer {
 
     public void optimize(CompiledFunction function) {
         new EnterSSA(function);
+        new SparseConditionalConstantPropagation().constantPropagation(function);
         new ExitSSA(function);
         new ChaitinGraphColoringRegisterAllocator().assignRegisters(function, 64);
     }

optvm/src/main/java/com/compilerprogramming/ezlang/compiler/SSAEdges.java

@@ -0,0 +1,80 @@
+package com.compilerprogramming.ezlang.compiler;
+
+import com.compilerprogramming.ezlang.exceptions.CompilerException;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * SSA Edges are connections from the unique point where a variable is
+ * given a value to a use of that variable. SSA edges are essentially
+ * def-use chains in the SSA program.
+ */
+public class SSAEdges {
+
+    public static final class SSADef {
+        public final Register register;
+        public final Instruction instruction;
+        public final List<Instruction> useList;
+
+        public SSADef(Register register, Instruction instruction) {
+            this.register = register;
+            this.instruction = instruction;
+            this.useList = new ArrayList<>();
+        }
+    }
+
+    public static Map<Integer, SSADef> buildDefUseChains(CompiledFunction function) {
+
+        if (!function.isSSA) throw new CompilerException("Function must be in SSA form");
+
+        Map<Integer, SSADef> defUseChains = new HashMap<>();
+        recordDefs(function, defUseChains);
+        recordUses(function, defUseChains);
+
+        return defUseChains;
+    }
+
+    private static void recordDefs(CompiledFunction function, Map<Integer, SSADef> defUseChains) {
+        for (BasicBlock block : function.getBlocks()) {
+            for (Instruction instruction : block.instructions) {
+                if (instruction instanceof Instruction.Phi phi) {
+                    recordDef(defUseChains, phi.value(), instruction);
+                }
+                else if (instruction.definesVar()) {
+                    recordDef(defUseChains, instruction.def(), instruction);
+                }
+            }
+        }
+    }
+
+    private static void recordUses(CompiledFunction function, Map<Integer, SSADef> defUseChains) {
+        for (BasicBlock block : function.getBlocks()) {
+            for (Instruction instruction : block.instructions) {
+                if (instruction instanceof Instruction.Phi phi) {
+                    recordUses(defUseChains, phi.inputs, block, instruction);
+                }
+                else {
+                    List<Register> uses = instruction.uses();
+                    if (!uses.isEmpty())
+                        recordUses(defUseChains, uses.toArray(new Register[uses.size()]), block, instruction);
+                }
+            }
+        }
+    }
+
+    private static void recordUses(Map<Integer, SSADef> defUseChains, Register[] inputs, BasicBlock block, Instruction instruction) {
+        for (Register register : inputs) {
+            SSADef def = defUseChains.get(register.id);
+            def.useList.add(instruction);
+        }
+    }
+
+    private static void recordDef(Map<Integer, SSADef> defUseChains, Register value, Instruction instruction) {
+        if (defUseChains.containsKey(value.id))
+            throw new CompilerException("Register already defined, invalid multiple definition in SSA");
+        defUseChains.put(value.id, new SSADef(value, instruction));
+    }
+}