[xmlvm-users] pop2

[Arno P. <ar...@pu...>]

Sometimes the compiler will generate a pop2 byte code instruction. The 
documentation says that it will remove either one long or double 
element, or two integers (the spec refers to these as computational 
types 1 and 2). In XMLVM, we have a union called XMLVMElem that 
represents the union of all data types that can be pushed onto the 
stack. The sizeof(XMLVMElem) therefore corresponds to a computational 
type 2. This makes it difficult to implement pop2 since we need to know 
if the top of the stack is of computational type 1 or 2. We don't do a 
data flow analysis and therefore we don't know. As a consequence, we 
cannot implement pop2.

Here are two situations where the compiler can generate the pop2 
instruction:

// Example 1
long foo() { return 42; }

void bar() { foo(); }

Since bar() does not make use of the long return value, the compiler 
will generate the pop2 instruction to clean up the stack.

// Example 2
int foo() { return 42; }

void bar() { foo(); foo(); }

Here a good-optimizing compiler will do one pop2 instruction after the 
second call to foo().

We are currently working on a conversion from stack- to register-based 
instructions which will also take care of this problem. What can you do 
in the meantime to avoid the pop2-problem? Luckily the answer is simple: 
assign the return value to a local variable:

void bar() { long ret = foo(); }

For good optimizing compilers it might notice the unused local variable 
and might still generate the pop2 instruction. In this case make 'ret' 
an instance variable.

Arno