Re: modifying current execution line/opcode

[Rici Lake <lua@...>]

Nothing like crossed posts :)

On 15-Dec-06, at 9:21 AM, Mike Pall wrote:

> BTW: Adding arbitrary gotos may also lead to irreducible control
> flow graphs. Which violates one of the nicest properties of Lua,
> namely that all CFGs are reducible. This makes it much easier to
> analyze and/or compile Lua programs.

Indeed. However, it's possible to handle 'break' by annotating the VM 
code with scope identifiers. The rule for a JMP is that the target must 
have the same scope-id or a parent scope-id; in the latter case, the 
JMP must be a BREAK if some local has been between the target and the 
source has been captured. Given the annotations, this is easy to verify 
in the byte-code loader, for example.

The annotations do add somewhat to the size of the compiled bytecode; 
in the implementation I did they add two bytes per opcode plus four 
bytes per scope (base, length, parent)