[generators] Add some explanation on forcing context allocation.

R=mstarzinger@chromium.org
TBR=rossberg
BUG=

Review URL: https://codereview.chromium.org/1806293002

Cr-Commit-Position: refs/heads/master@{#34864}

src/full-codegen/arm/full-codegen-arm.cc

@@ -1908,7 +1908,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   __ ldr(r2, FieldMemOperand(r1, JSGeneratorObject::kReceiverOffset));
   __ push(r2);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ ldr(r3, FieldMemOperand(r4, JSFunction::kSharedFunctionInfoOffset));
   __ ldr(r3,
          FieldMemOperand(r3, SharedFunctionInfo::kFormalParameterCountOffset));

src/full-codegen/arm64/full-codegen-arm64.cc

@@ -3844,7 +3844,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
                               JSGeneratorObject::kReceiverOffset));
   __ Push(x10);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ Ldr(x10, FieldMemOperand(function, JSFunction::kSharedFunctionInfoOffset));
 
   // The number of arguments is stored as an int32_t, and -1 is a marker

src/full-codegen/ia32/full-codegen-ia32.cc

@@ -1823,7 +1823,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   // Push receiver.
   __ push(FieldOperand(ebx, JSGeneratorObject::kReceiverOffset));
 
-  // Push holes for arguments to generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ mov(edx, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
   __ mov(edx,
          FieldOperand(edx, SharedFunctionInfo::kFormalParameterCountOffset));

src/full-codegen/mips/full-codegen-mips.cc

@@ -1903,7 +1903,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   __ lw(a2, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
   __ push(a2);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ lw(a3, FieldMemOperand(t0, JSFunction::kSharedFunctionInfoOffset));
   __ lw(a3,
         FieldMemOperand(a3, SharedFunctionInfo::kFormalParameterCountOffset));

src/full-codegen/mips64/full-codegen-mips64.cc

@@ -1905,7 +1905,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   __ ld(a2, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
   __ push(a2);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ ld(a3, FieldMemOperand(a4, JSFunction::kSharedFunctionInfoOffset));
   // The argument count is stored as int32_t on 64-bit platforms.
   // TODO(plind): Smi on 32-bit platforms.

src/full-codegen/ppc/full-codegen-ppc.cc

@@ -1870,7 +1870,10 @@ void FullCodeGenerator::EmitGeneratorResume(
   __ LoadP(r5, FieldMemOperand(r4, JSGeneratorObject::kReceiverOffset));
   __ push(r5);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ LoadP(r6, FieldMemOperand(r7, JSFunction::kSharedFunctionInfoOffset));
   __ LoadWordArith(
       r6, FieldMemOperand(r6, SharedFunctionInfo::kFormalParameterCountOffset));

src/full-codegen/s390/full-codegen-s390.cc

@@ -1825,7 +1825,10 @@ void FullCodeGenerator::EmitGeneratorResume(
   __ LoadP(r4, FieldMemOperand(r3, JSGeneratorObject::kReceiverOffset));
   __ push(r4);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ LoadP(r5, FieldMemOperand(r6, JSFunction::kSharedFunctionInfoOffset));
   __ LoadW(
       r5, FieldMemOperand(r5, SharedFunctionInfo::kFormalParameterCountOffset));

src/full-codegen/x64/full-codegen-x64.cc

@@ -1848,7 +1848,10 @@ void FullCodeGenerator::EmitGeneratorResume(
   // Push receiver.
   __ Push(FieldOperand(rbx, JSGeneratorObject::kReceiverOffset));
 
-  // Push holes for arguments to generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ movp(rdx, FieldOperand(rdi, JSFunction::kSharedFunctionInfoOffset));
   __ LoadSharedFunctionInfoSpecialField(rdx, rdx,
       SharedFunctionInfo::kFormalParameterCountOffset);

src/full-codegen/x87/full-codegen-x87.cc

@@ -1815,7 +1815,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   // Push receiver.
   __ push(FieldOperand(ebx, JSGeneratorObject::kReceiverOffset));
 
-  // Push holes for arguments to generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ mov(edx, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
   __ mov(edx,
          FieldOperand(edx, SharedFunctionInfo::kFormalParameterCountOffset));

src/parsing/parser.cc

@@ -4103,7 +4103,8 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
     if (is_generator) {
       // For generators, allocating variables in contexts is currently a win
       // because it minimizes the work needed to suspend and resume an
-      // activation.
+      // activation.  The machine code produced for generators (by full-codegen)
+      // relies on this forced context allocation, but not in an essential way.
       scope_->ForceContextAllocation();
 
       // Calling a generator returns a generator object.  That object is stored