Recognize in the fast-mode code generator when a subexpression is a

constant known at compile time.  Do not ever use the stack to
materialize (non-function-argument) constants.  Currently, constants
are only the non-materialized, non-function literals in the AST.

It is a known issue that there is no test coverage for the cases of
assigning a non-literal to a variable and returning a literal.  Those
code paths are unreachable and tests will be added when they become
reachable.

For the code '.result = true', we had previously on ia32:

27  push 0xf5c28161             ;; object: 0xf5c28161 <true>
32  pop [ebp+0xf4]

Now:

27  mov eax,0xf5c26161          ;; object: 0xf5c26161 <true>
32  mov [ebp+0xf4],eax

======== We had previously on x64:

25  movq r10,0x7fb8c2f78199    ;; object: 0x7fb8c2f78199 <true>
35  push r10
37  pop [rbp-0x18]

Now:

25  movq r10,0x7fb131386199    ;; object: 0x7fb131386199 <true>
35  movq [rbp-0x18],r10

The generated code for ARM did not include the extra memory traffic.
It was already eliminated by the ARM assembler's push/pop elimination.

Review URL: http://codereview.chromium.org/300003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3088 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/fast-codegen-arm.cc

@@ -114,8 +114,19 @@ void FastCodeGenerator::VisitExpressionStatement(ExpressionStatement* stmt) {
 void FastCodeGenerator::VisitReturnStatement(ReturnStatement* stmt) {
   Comment cmnt(masm_, "[ ReturnStatement");
   SetStatementPosition(stmt);
-  Visit(stmt->expression());
-  __ pop(r0);
+  Expression* expr = stmt->expression();
+  Visit(expr);
+
+  // Complete the statement based on the location of the subexpression.
+  Location source = expr->location();
+  ASSERT(!source.is_nowhere());
+  if (source.is_temporary()) {
+    __ pop(r0);
+  } else {
+    ASSERT(source.is_constant());
+    ASSERT(expr->AsLiteral() != NULL);
+    __ mov(r0, Operand(expr->AsLiteral()->handle()));
+  }
   __ RecordJSReturn();
   __ mov(sp, fp);
   __ ldm(ia_w, sp, fp.bit() | lr.bit());
@@ -143,33 +154,46 @@ void FastCodeGenerator::VisitVariableProxy(VariableProxy* expr) {
 }
 
 
-void FastCodeGenerator::VisitLiteral(Literal* expr) {
-  Comment cmnt(masm_, "[ Literal");
-  if (expr->location().is_temporary()) {
-    __ mov(ip, Operand(expr->handle()));
-    __ push(ip);
-  } else {
-    ASSERT(expr->location().is_nowhere());
-  }
-}
-
-
 void FastCodeGenerator::VisitAssignment(Assignment* expr) {
   Comment cmnt(masm_, "[ Assignment");
   ASSERT(expr->op() == Token::ASSIGN || expr->op() == Token::INIT_VAR);
+  Expression* rhs = expr->value();
+  Visit(rhs);
 
-  Visit(expr->value());
-
+  // Left-hand side is always a (parameter or local) slot.
   Variable* var = expr->target()->AsVariableProxy()->AsVariable();
   ASSERT(var != NULL && var->slot() != NULL);
 
-  if (expr->location().is_temporary()) {
-    __ ldr(ip, MemOperand(sp));
+  // Complete the assignment based on the location of the right-hand-side
+  // value and the desired location of the assignment value.
+  Location destination = expr->location();
+  Location source = rhs->location();
+  ASSERT(!destination.is_constant());
+  ASSERT(!source.is_nowhere());
+
+  if (source.is_temporary()) {
+    if (destination.is_temporary()) {
+      // Case 'temp1 <- (var = temp0)'.  Preserve right-hand-side temporary
+      // on the stack.
+      __ ldr(ip, MemOperand(sp));
+    } else {
+      ASSERT(destination.is_nowhere());
+      // Case 'var = temp'.  Discard right-hand-side temporary.
+      __ pop(ip);
+    }
+    __ str(ip, MemOperand(fp, SlotOffset(var->slot())));
   } else {
-    ASSERT(expr->location().is_nowhere());
-    __ pop(ip);
+    ASSERT(source.is_constant());
+    ASSERT(rhs->AsLiteral() != NULL);
+    // Two cases: 'temp <- (var = constant)', or 'var = constant' with a
+    // discarded result.  Always perform the assignment.
+    __ mov(ip, Operand(rhs->AsLiteral()->handle()));
+    __ str(ip, MemOperand(fp, SlotOffset(var->slot())));
+    if (destination.is_temporary()) {
+      // Case 'temp <- (var = constant)'.  Save result.
+      __ push(ip);
+    }
   }
-  __ str(ip, MemOperand(fp, SlotOffset(var->slot())));
 }
 
 

src/compiler.cc

@@ -614,6 +614,7 @@ void CodeGenSelector::VisitVariableProxy(VariableProxy* expr) {
 
 void CodeGenSelector::VisitLiteral(Literal* expr) {
   // All literals are supported.
+  expr->set_location(Location::Constant());
 }
 
 

src/fast-codegen.cc

@@ -196,6 +196,11 @@ void FastCodeGenerator::VisitSlot(Slot* expr) {
 }
 
 
+void FastCodeGenerator::VisitLiteral(Literal* expr) {
+  // No code is emitted (here) for simple literals.
+}
+
+
 void FastCodeGenerator::VisitRegExpLiteral(RegExpLiteral* expr) {
   UNREACHABLE();
 }

src/ia32/fast-codegen-ia32.cc

@@ -104,8 +104,19 @@ void FastCodeGenerator::VisitExpressionStatement(ExpressionStatement* stmt) {
 void FastCodeGenerator::VisitReturnStatement(ReturnStatement* stmt) {
   Comment cmnt(masm_, "[ ReturnStatement");
   SetStatementPosition(stmt);
-  Visit(stmt->expression());
-  __ pop(eax);
+  Expression* expr = stmt->expression();
+  Visit(expr);
+
+  // Complete the statement based on the location of the subexpression.
+  Location source = expr->location();
+  ASSERT(!source.is_nowhere());
+  if (source.is_temporary()) {
+    __ pop(eax);
+  } else {
+    ASSERT(source.is_constant());
+    ASSERT(expr->AsLiteral() != NULL);
+    __ mov(eax, expr->AsLiteral()->handle());
+  }
   __ RecordJSReturn();
   // Do not use the leave instruction here because it is too short to
   // patch with the code required by the debugger.
@@ -132,30 +143,45 @@ void FastCodeGenerator::VisitVariableProxy(VariableProxy* expr) {
 }
 
 
-void FastCodeGenerator::VisitLiteral(Literal* expr) {
-  Comment cmnt(masm_, "[ Literal");
-  if (expr->location().is_temporary()) {
-    __ push(Immediate(expr->handle()));
-  } else {
-    ASSERT(expr->location().is_nowhere());
-  }
-}
-
-
 void FastCodeGenerator::VisitAssignment(Assignment* expr) {
   Comment cmnt(masm_, "[ Assignment");
   ASSERT(expr->op() == Token::ASSIGN || expr->op() == Token::INIT_VAR);
-  Visit(expr->value());
+  Expression* rhs = expr->value();
+  Visit(rhs);
 
+  // Left-hand side is always a (parameter or local) slot.
   Variable* var = expr->target()->AsVariableProxy()->AsVariable();
   ASSERT(var != NULL && var->slot() != NULL);
 
-  if (expr->location().is_temporary()) {
-    __ mov(eax, Operand(esp, 0));
-    __ mov(Operand(ebp, SlotOffset(var->slot())), eax);
+  // Complete the assignment based on the location of the right-hand-side
+  // value and the desired location of the assignment value.
+  Location destination = expr->location();
+  Location source = rhs->location();
+  ASSERT(!destination.is_constant());
+  ASSERT(!source.is_nowhere());
+
+  if (source.is_temporary()) {
+    if (destination.is_temporary()) {
+      // Case 'temp1 <- (var = temp0)'.  Preserve right-hand-side temporary
+      // on the stack.
+      __ mov(eax, Operand(esp, 0));
+      __ mov(Operand(ebp, SlotOffset(var->slot())), eax);
+    } else {
+      ASSERT(destination.is_nowhere());
+      // Case 'var = temp'.  Discard right-hand-side temporary.
+      __ pop(Operand(ebp, SlotOffset(var->slot())));
+    }
   } else {
-    ASSERT(expr->location().is_nowhere());
-    __ pop(Operand(ebp, SlotOffset(var->slot())));
+    ASSERT(source.is_constant());
+    ASSERT(rhs->AsLiteral() != NULL);
+    // Two cases: 'temp <- (var = constant)', or 'var = constant' with a
+    // discarded result.  Always perform the assignment.
+    __ mov(eax, rhs->AsLiteral()->handle());
+    __ mov(Operand(ebp, SlotOffset(var->slot())), eax);
+    if (destination.is_temporary()) {
+      // Case 'temp <- (var = constant)'.  Save result.
+      __ push(eax);
+    }
   }
 }
 

src/location.h

@@ -41,6 +41,7 @@ class Location BASE_EMBEDDED {
 
   bool is_temporary() { return type_ == TEMP; }
   bool is_nowhere() { return type_ == NOWHERE; }
+  bool is_constant() { return type_ == CONSTANT; }
 
  private:
   enum Type { TEMP, NOWHERE, CONSTANT };

src/x64/fast-codegen-x64.cc

@@ -112,8 +112,19 @@ void FastCodeGenerator::VisitExpressionStatement(ExpressionStatement* stmt) {
 void FastCodeGenerator::VisitReturnStatement(ReturnStatement* stmt) {
   Comment cmnt(masm_, "[ ReturnStatement");
   SetStatementPosition(stmt);
-  Visit(stmt->expression());
-  __ pop(rax);
+  Expression* expr = stmt->expression();
+  Visit(expr);
+
+  // Complete the statement based on the location of the subexpression.
+  Location source = expr->location();
+  ASSERT(!source.is_nowhere());
+  if (source.is_temporary()) {
+    __ pop(rax);
+  } else {
+    ASSERT(source.is_constant());
+    ASSERT(expr->AsLiteral() != NULL);
+    __ Move(rax, expr->AsLiteral()->handle());
+  }
   __ RecordJSReturn();
   // Do not use the leave instruction here because it is too short to
   // patch with the code required by the debugger.
@@ -149,31 +160,45 @@ void FastCodeGenerator::VisitVariableProxy(VariableProxy* expr) {
 }
 
 
-void FastCodeGenerator::VisitLiteral(Literal* expr) {
-  Comment cmnt(masm_, "[ Literal");
-  if (expr->location().is_temporary()) {
-    __ Push(expr->handle());
-  } else {
-    ASSERT(expr->location().is_nowhere());
-  }
-}
-
-
 void FastCodeGenerator::VisitAssignment(Assignment* expr) {
   Comment cmnt(masm_, "[ Assignment");
   ASSERT(expr->op() == Token::ASSIGN || expr->op() == Token::INIT_VAR);
+  Expression* rhs = expr->value();
+  Visit(rhs);
 
-  Visit(expr->value());
-
+  // Left-hand side is always a (parameter or local) slot.
   Variable* var = expr->target()->AsVariableProxy()->AsVariable();
   ASSERT(var != NULL && var->slot() != NULL);
 
-  if (expr->location().is_temporary()) {
-    __ movq(rax, Operand(rsp, 0));
-    __ movq(Operand(rbp, SlotOffset(var->slot())), rax);
+  // Complete the assignment based on the location of the right-hand-side
+  // value and the desired location of the assignment value.
+  Location destination = expr->location();
+  Location source = rhs->location();
+  ASSERT(!destination.is_constant());
+  ASSERT(!source.is_nowhere());
+
+  if (source.is_temporary()) {
+    if (destination.is_temporary()) {
+      // Case 'temp1 <- (var = temp0)'.  Preserve right-hand-side temporary
+      // on the stack.
+      __ movq(kScratchRegister, Operand(rsp, 0));
+      __ movq(Operand(rbp, SlotOffset(var->slot())), kScratchRegister);
+    } else {
+      ASSERT(destination.is_nowhere());
+      // Case 'var = temp'.  Discard right-hand-side temporary.
+      __ pop(Operand(rbp, SlotOffset(var->slot())));
+    }
   } else {
-    ASSERT(expr->location().is_nowhere());
-    __ pop(Operand(rbp, SlotOffset(var->slot())));
+    ASSERT(source.is_constant());
+    ASSERT(rhs->AsLiteral() != NULL);
+    // Two cases: 'temp <- (var = constant)', or 'var = constant' with a
+    // discarded result.  Always perform the assignment.
+    __ Move(kScratchRegister, rhs->AsLiteral()->handle());
+    __ movq(Operand(rbp, SlotOffset(var->slot())), kScratchRegister);
+    if (destination.is_temporary()) {
+      // Case 'temp <- (var = constant)'.  Save result.
+      __ push(kScratchRegister);
+    }
   }
 }