WASM_X86: Add comments about emit_exit2() and X86Visitor stack working

Author: ubaidsk

src/libasr/codegen/wasm_to_x86.cpp

@@ -11,6 +11,22 @@ namespace LFortran {
 
 namespace wasm {
 
+/*
+
+This X86Visitor uses stack to pass arguments and return values from functions.
+Since in X86, instructions operate on registers (and not on stack),
+for every instruction we pop elements from top of stack and store them into
+registers. After operating on the registers, the result value is then
+pushed back onto the stack.
+
+One of the reasons to use stack to pass function arguments is that,
+it allows us to define and call functions with any number of parameters.
+As registers are limited in number, if we use them to pass function arugments,
+the number of arguments we could pass to a function would get limited by
+the number of registers available with the CPU.
+
+*/
+
 class X86Visitor : public WASMDecoder<X86Visitor>,
                    public WASM_INSTS_VISITOR::BaseWASMVisitor<X86Visitor> {
    public:

src/libasr/codegen/x86_assembler.h

@@ -815,7 +815,14 @@ void emit_elf32_footer(X86Assembler &a);
 
 void emit_exit(X86Assembler &a, const std::string &name,
     uint32_t exit_code);
+
+// this is similar to emit_exit() but takes the argument (i.e. exit code)
+// from top of stack. To call this exit2, one needs to jump to it
+// instead of call it. (Because calling pushes the instruction address and
+// base pointer value (ebp) of previous function and thus makes the
+// exit code parameter less reachable)
 void emit_exit2(X86Assembler &a, const std::string &name);
+
 void emit_data_string(X86Assembler &a, const std::string &label,
     const std::string &s);
 void emit_print(X86Assembler &a, const std::string &msg_label,