except.txt

来自「开放源码的编译器open watcom 1.6.0版的源代码」· 文本 代码 · 共 90 行

- module containing a try block with catch statements

    - generate single byte COMDEF with a unique name that represents
      the type
      e.g., __XC	for class C
            __X_1	for basic types (char, short, etc.)
    - generate code to wind and unwind try-blocks, and test whether
      exceptions have been thrown
      e.g.,
	struct try_stack {
	    struct try_stack *next;
	    void (*handler)( void );
	} __tmp;
	__tmp.next = __CatchStack;
	__tmp.handler = L1;
	__CatchStack = &__tmp;
	...
	__CatchStack = __CatchStack->next;
	...
	ret
    L1: cmp	__XC,0
        je 	L2
	__XC contains index into __ThrowInfo table of addresses
	...
	jmp	after end of this try block
    L2: cmp	__X_1,0
	je	L3
	...
	jmp	after end of this try block
    L3  jmp	__MoveOnToNextLevelOfCatch
	...
	
- module containing a continue throw

	jmp	__MoveOnToNextLevelOfCatch
	
- module containing a new throw

    - throwing a scalar will always have a long, float, and double
      conversions
    - zap COMDEF area with function call if a new throw
    - alloca enough for the throw object and initialize
    - alloca enough for the following structure:
	base address of throw object
	table of offsets past base for all possible conversions
    - set __CurrentThrow to point to the structure above
    - set all possible exceptions that can be caught (COMDEFs)
    e.g.,
	mov __XC,index
	mov __X_1,index
	
    for indices larger > 256, we will use Intel coding and increase the
    size of the COMDEF area for that exception
    
Implementation details:
    
- The COMDEF areas must go into a read-write data area but this
  doesn't have to be in DGROUP.  It also needs a begin/end mechanism
  so that it can be zapped to all zeros at any time.
- The scalar types should have separate storage for IBM 370/M68K style
  architectures.
- Large objects may not be able to be sliced off the stack so a threshold
  option may be necessary to move to a statically allocated throw object.
- Resumable exceptions should be possible in a future enhancement!
- throw data (objects + table) should be COMDEFs (the segments should be
  arranged so that the throw data is part of or under the stack).
- catch lists can register the active catches in their variables
  we could have a special pre-defined function __will_be_caught( type )
  that generates an expression like ( __XC != 0 ).  Once a throw is
  executed, the check area has to be zeroed and all pending catches updated.
  This may be a governing factor in using tables describing catch blocks.
  This stuff has to be optional.

Optimization headaches:

    - chaining together auto destructors as the objects are created
    - off[SP] addressing on the 386 shouldn't be used because SP may
      be far away during catch code execution

Optimization opportunities:

    - option for "I promise not to use exceptions"
      easy -- any exception will generate a unexpected() because there will
      be no catch blocks
    - option for no conversion of exceptions
    - option for don't care about my auto destructors
      (#pragma on a per-function basis, too)
    - compiler should identify times where a compiler generated constructor
      is used for an object (and all its subobjects!) and ignore these