V4: Peephole optimize LoadReg/MoveReg

The following sequence:

    StoreReg rX
    LoadReg rX

Can be optimized by dropping the LoadReg, as the value is still in the
accumulator. Also, the sequence:

    StoreReg rX
    MoveReg rY, rX

Can be optimized to:

    StoreReg rX
    StoreReg rY

This last optimization prevents one load from the JS stack (reading rX).

Both cases are only valid if there is no label on the second
instruction.

Change-Id: Ibd4543459e1eab4da55e92248eba544c707c5456
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 31 insertions, 1 deletions

diff --git a/src/qml/compiler/qv4bytecodegenerator.cpp b/src/qml/compiler/qv4bytecodegenerator.cpp
index 4d50654d27..1d1e98ea58 100644
--- a/src/qml/compiler/qv4bytecodegenerator.cpp
+++ b/src/qml/compiler/qv4bytecodegenerator.cpp
@@ -185,6 +185,25 @@ void BytecodeGenerator::finalize(Compiler::Context *context)
 }
 
 int BytecodeGenerator::addInstructionHelper(Instr::Type type, const Instr &i, int offsetOfOffset) {
+    if (lastInstrType == int(Instr::Type::StoreReg)) {
+        if (type == Instr::Type::LoadReg) {
+            if (i.LoadReg.reg == lastInstr.StoreReg.reg) {
+                // value is already in the accumulator
+                return -1;
+            }
+        }
+        if (type == Instr::Type::MoveReg) {
+            if (i.MoveReg.srcReg == lastInstr.StoreReg.reg) {
+                Instruction::StoreReg store;
+                store.reg = i.MoveReg.destReg;
+                addInstruction(store);
+                return -1;
+            }
+        }
+    }
+    lastInstrType = int(type);
+    lastInstr = i;
+
 #if QT_CONFIG(qml_debug)
     if (debugMode && type != Instr::Type::Debug) {
 QT_WARNING_PUSH
diff --git a/src/qml/compiler/qv4bytecodegenerator_p.h b/src/qml/compiler/qv4bytecodegenerator_p.h
index 0033acf3c7..98951b610c 100644
--- a/src/qml/compiler/qv4bytecodegenerator_p.h
+++ b/src/qml/compiler/qv4bytecodegenerator_p.h
@@ -77,7 +77,9 @@ public:
         Label(BytecodeGenerator *generator, LinkMode mode = LinkNow)
             : generator(generator),
               index(generator->labels.size()) {
-            generator->labels.append(mode == LinkNow ? generator->instructions.size() : -1);
+            generator->labels.append(-1);
+            if (mode == LinkNow)
+                link();
         }
         static Label returnLabel() {
             Label l;
@@ -92,6 +94,7 @@ public:
             Q_ASSERT(index >= 0);
             Q_ASSERT(generator->labels[index] == -1);
             generator->labels[index] = generator->instructions.size();
+            generator->clearLastInstruction();
         }
 
         BytecodeGenerator *generator = nullptr;
@@ -261,6 +264,11 @@ public:
             addJumpInstruction(Instruction::JumpTrue()).link(*trueLabel);
     }
 
+    void clearLastInstruction()
+    {
+        lastInstrType = -1;
+    }
+
 private:
     friend struct Jump;
     friend struct Label;
@@ -292,6 +300,9 @@ private:
     int startLine = 0;
     int currentLine = 0;
     bool debugMode = false;
+
+    int lastInstrType = -1;
+    Moth::Instr lastInstr;
 };
 
 }