Merge pull request #31 from CompilerProgramming/develop

Add test for simple SSA liveness example, also add comments and reorg…

Author: dibyendumajumdar

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

@@ -67,11 +67,12 @@ public class BasicBlock {
     LiveSet liveOut;
 
     /**
-     * Inputs to successor block's phi function
+     * phiUses(B) is the set of variables used in a phi-operation at entry of a block successor of the block B
+     * That is, inputs to successor block's phi functions.
      */
     LiveSet phiUses;
     /**
-     * Phi definitions in this block
+     * phiDefs(B) the variables defined by phi-operations at entry of the block B
      */
     LiveSet phiDefs;
     /**

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

@@ -9,11 +9,15 @@
  * pages 446-447.
  *
  * It turned out that this dataflow implementation cannot correctly handle
- * phis, because with phis, the inputs are live at the predecessor blocks.
- * We have to look at alternative approaches when input is SSA form.
- * Surprisingly even with this approach, the lost copy and swap problems
+ * phis. Therefore, we have to look at alternative approaches when input is SSA form.
+ * Surprisingly even with the flawed approach, the lost copy and swap problems
  * appeared to work correctly.
  *
+ * Phis need special considerations:
+ *
+ * Phi inputs are live out at the predecessor blocks, but not live in for phi block.
+ * Phi def is live in at phi block but not in live out at predecessor blocks.
+ *
  * The new approach is based on formula described in
  * Computing Liveness Sets for SSA-Form Programs
  * Florian Brandner, Benoit Boissinot, Alain Darte, Benoît Dupont de Dinechin, Fabrice Rastello
@@ -48,25 +52,29 @@ private void initBlocks(RegisterPool regPool, List<BasicBlock> blocks) {
 
     private void init(List<BasicBlock> blocks) {
         for (BasicBlock block : blocks) {
-            // We st up phiDefs first because when we
+            // Any vars created by phi instructions are added to phiDefs
+            // for this block; these vars will be live on entry to block
+            // but not live out from predecessor blocks.
+            //
+            // We set up phiDefs first because when we
             // look at phi uses we need to refer back here
             // see comments on phi cycles below
             for (Instruction instruction : block.instructions) {
                 if (instruction instanceof Instruction.Phi phi) {
                     block.phiDefs.add(phi.value());
                 }
+                // There is a scenario where other instructions can appear
+                // between phi instructions - this happens during the SSA deconstruction
+                // using Brigg's method. But we don't calculate liveness
+                // in the middle of that process, so assuming all phis are together
+                // at the top of the block is okay
                 else break;
             }
             for (Instruction instruction : block.instructions) {
-                for (Register use : instruction.uses()) {
-                    if (!block.varKill.contains(use))
-                        block.UEVar.add(use);
-                }
-                if (instruction.definesVar() && !(instruction instanceof Instruction.Phi)) {
-                    Register def = instruction.def();
-                    block.varKill.add(def);
-                }
                 if (instruction instanceof Instruction.Phi phi) {
+                    // Any uses in a Phi are added to the phiUses of predecessor
+                    // block. These uses will be in live out of predecessor block but
+                    // not live in for current block.
                     for (int i = 0; i < block.predecessors.size(); i++) {
                         BasicBlock pred = block.predecessors.get(i);
                         if (!phi.isRegisterInput(i))
@@ -76,11 +84,24 @@ private void init(List<BasicBlock> blocks) {
                         // if there is loop back and there are cycles
                         // such as e.g. the swap copy problem
                         if (pred == block &&
-                            block.phiDefs.contains(use))
+                                block.phiDefs.contains(use))
                             continue;
                         pred.phiUses.add(use);
                     }
                 }
+                else {
+                    // Non phi instructions follow regular
+                    // logic. Any var that is used before being defined
+                    // is added to upward expose set.
+                    for (Register use : instruction.uses()) {
+                        if (!block.varKill.contains(use))
+                            block.UEVar.add(use);
+                    }
+                    if (instruction.definesVar()) {
+                        Register def = instruction.def();
+                        block.varKill.add(def);
+                    }
+                }
             }
         }
     }
@@ -89,6 +110,7 @@ private void computeLiveness(List<BasicBlock> blocks) {
         boolean changed = true;
         while (changed) {
             changed = false;
+            // TODO we should process in RPO order
             for (BasicBlock block : blocks) {
                 if (recomputeLiveOut(block))
                     changed = true;
@@ -97,8 +119,17 @@ private void computeLiveness(List<BasicBlock> blocks) {
     }
 
     // See 'Computing Liveness Sets for SSA-Form Programs'
+    //
     // LiveIn(B) = PhiDefs(B) U UpwardExposed(B) U (LiveOut(B) \ Defs(B))
-    // LiveOut(B) = U all S  (LiveIn(S) \ PhiDefs(S)) U PhiUses(B)
+    // LiveOut(B) = U all S  (LiveIn(S) \ PhiDefs(S)) U PhiUses
+    //
+    // For a phi-function a_0 = phi(a_1, ..., a_n ) in block B_0, where a_i comes from block B_i, then:
+    // * a_0 is considered to be live-in for B_0, but, with respect to this phi-function, it is
+    //   not live-out for B_i, i > 0.
+    // * a_i, i > 0, is considered to be live-out of B_i , but, with respect to this phi-function,
+    //   it is not live-in for B_0.
+    // This corresponds to placing a copy of a_i to a_0 on each edge from B_i to B_0.
+    //
     private boolean recomputeLiveOut(BasicBlock block) {
         LiveSet oldLiveOut = block.liveOut.dup();
         LiveSet t = block.liveOut.dup().subtract(block.varKill);

optvm/src/test/java/com/compilerprogramming/ezlang/compiler/TestLiveness.java

@@ -444,4 +444,120 @@ func foo(p: Int)->Int
 """, actual);
     }
 
+    @Test
+    public void testSimpleCase() {
+        String src = """
+                func foo()->Int 
+                {
+                    return 1 && 2
+                }
+                """;
+        var typeDict = compileSrc(src);
+        var funcSymbol = typeDict.lookup("foo");
+        CompiledFunction func = (CompiledFunction) ((Symbol.FunctionTypeSymbol) funcSymbol).code();
+        func.livenessAnalysis();
+        StringBuilder result = new StringBuilder();
+        result.append("Pre-SSA\n");
+        func.toStr(result, true);
+        new EnterSSA(func, Options.NONE);
+        func.livenessAnalysis();
+        result.append("Post-SSA\n");
+        func.toStr(result, true);
+        Assert.assertEquals("""
+Pre-SSA
+func foo()->Int
+Reg #0 %t0
+L0:
+    if 1 goto L2 else goto L3
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {}
+    #LIVEIN  = {}
+    #LIVEOUT = {}
+L2:
+    %t0 = 2
+    goto  L4
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {0}
+    #LIVEIN  = {}
+    #LIVEOUT = {0}
+L4:
+    ret %t0
+    goto  L1
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {0}
+    #VARKILL = {}
+    #LIVEIN  = {0}
+    #LIVEOUT = {}
+L1:
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {}
+    #LIVEIN  = {}
+    #LIVEOUT = {}
+L3:
+    %t0 = 0
+    goto  L4
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {0}
+    #LIVEIN  = {}
+    #LIVEOUT = {0}
+Post-SSA
+func foo()->Int
+Reg #0 %t0
+Reg #1 %t0_0
+Reg #2 %t0_1
+Reg #3 %t0_2
+L0:
+    if 1 goto L2 else goto L3
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {}
+    #LIVEIN  = {}
+    #LIVEOUT = {}
+L2:
+    %t0_1 = 2
+    goto  L4
+    #PHIDEFS = {}
+    #PHIUSES = {2}
+    #UEVAR   = {}
+    #VARKILL = {2}
+    #LIVEIN  = {}
+    #LIVEOUT = {2}
+L4:
+    %t0_2 = phi(%t0_1, %t0_0)
+    ret %t0_2
+    goto  L1
+    #PHIDEFS = {3}
+    #PHIUSES = {}
+    #UEVAR   = {3}
+    #VARKILL = {}
+    #LIVEIN  = {3}
+    #LIVEOUT = {}
+L1:
+    #PHIDEFS = {}
+    #PHIUSES = {}
+    #UEVAR   = {}
+    #VARKILL = {}
+    #LIVEIN  = {}
+    #LIVEOUT = {}
+L3:
+    %t0_0 = 0
+    goto  L4
+    #PHIDEFS = {}
+    #PHIUSES = {1}
+    #UEVAR   = {}
+    #VARKILL = {1}
+    #LIVEIN  = {}
+    #LIVEOUT = {1}
+""", result.toString());
+    }
 }