V4: Undo rotation of the do-while loop

Previously the generated code looked like this:

    Jump loop-body
  loop-condition:
    acc = condition
    JumpFalse loop-end
  loop-body:
    ...
    Jump loop-condition
  loop end:
    ...

This is a problem for anything that analyzes the bytecode in 1 pass from
top to bottom: at the point of the loop-condition the state of the
interpreter isn't known yet: that state will only be known after
analyzing the loop body. The new generated code does not have that
problem:

  loop-body:
    ...
    acc = condition
    JumpTrue loop-body
  loop-end:
    ...

Change-Id: I16977e9ce1f9377e7bf4d09afc16136ba0111463
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 14 insertions, 10 deletions

diff --git a/src/qml/compiler/qv4codegen.cpp b/src/qml/compiler/qv4codegen.cpp
index 56ff15a78c..fcd93bb600 100644
--- a/src/qml/compiler/qv4codegen.cpp
+++ b/src/qml/compiler/qv4codegen.cpp
@@ -3246,22 +3246,26 @@ bool Codegen::visit(DoWhileStatement *ast)
     BytecodeGenerator::Label end = bytecodeGenerator->newLabel();
 
     ControlFlowLoop flow(this, &end, &cond);
-    bytecodeGenerator->jump().link(body);
 
-    cond.link();
-    bytecodeGenerator->addLoopStart(cond);
-
-    if (!AST::cast<TrueLiteral *>(ast->expression)) {
-        TailCallBlocker blockTailCalls(this);
-        condition(ast->expression, &body, &end, true);
-    }
+    // special case that is not a loop:
+    //   do {...} while (false)
+    if (!AST::cast<FalseLiteral *>(ast->expression))
+        bytecodeGenerator->addLoopStart(body);
 
     body.link();
     statement(ast->statement);
     setJumpOutLocation(bytecodeGenerator, ast->statement, ast->semicolonToken);
 
-    if (!AST::cast<FalseLiteral *>(ast->expression))
-        bytecodeGenerator->jump().link(cond);
+    cond.link();
+    if (AST::cast<TrueLiteral *>(ast->expression)) {
+        // do {} while (true) -> just jump back to the loop body, no need to generate a condition
+        bytecodeGenerator->jump().link(body);
+    } else if (AST::cast<FalseLiteral *>(ast->expression)) {
+        // do {} while (false) -> fall through, no need to generate a condition
+    } else {
+        TailCallBlocker blockTailCalls(this);
+        condition(ast->expression, &body, &end, false);
+    }
 
     end.link();