[Oberon] Intermediate scopes in Oberon-07

[Andreas Pirklbauer]

  > 3) Procedure variables. In the compiler, this comes for free. 


ADDENDUM: Actually, on RISC (used in PO2013), the run-time overhead
of calling a procedure VARIABLE p versus a procedure P is as follows:

Calling a procedure P directly (assume P declared local to module M): 

  BL -8        ; BL (i.e. branch using a 24bit offset as dest addr, modifier u = 0)

Calling a procedure variable p:

  LDR SB R0 8  ; get static base (will be fixed up to 'LDR SB MT mno' by module loader)
  LDR R0 SB 0  ; R0 := M[SB] = value stored in procedure variable p
  BLEQ MT      ; generate TRAP if it is zero
  BL R0        ; BLR (i.e. branch using a REGISTER as dest addr, modifier u = 1)

For procedures whose body is large, this extra overhead may be
viewed as negligible, but for short procedures it may be significant
relative to the execution time of the (rest of the) procedure body.

This would suggest that KEEPING procedure forward references in the
language may have been worth considering (as their procedure calls
are of course compiled into the same single BL instruction as for P).

But they are inherently nasty to deal with in a compiler (which must
check equality of headers, etc) which is why they have been eliminated.

Andreas



Test program:
=============

MODULE M;
  VAR p: PROCEDURE;
  PROCEDURE P; BEGIN END P;
BEGIN P; p
END M.

ORP.Compile M.Mod ~
ORTool.DecObj M.rsc ~


# code for procedure P (generated by ORP.ProcedureDecl)

   0     4EE90004       SUB SP SP  4
   1     AFE00000       STR LNK SP  0
   2     8FE00000       LDR LNK SP  0
   3     4EE80004       ADD SP SP 4
   4     C700000F       B LNK

# module entry code (generated by ORG.Header)

   5     4EE90004       SUB SP SP  4
   6     AFE00000       STR  LNK SP  0

# code for calling procedure P directly (offset -8, i.e. the BL branches to 0)

   7     F7FFFFF8       BL  -8            ; BL (i.e. branch using a 24bit offset as dest addr, modifier u = 0)

# code for calling procedure *variable* p

   8     8D000008       LDR SB R0  8      ; get static base (will be fixed up to 'LDR SB MT mno' by module loader)
   9     80D00000       LDR R0 SB  0      ; R0 := M[SB] = value stored in procedure variable p
  10     D1004C5C       BLEQ  MT          ; generate TRAP if it is zero
  11     D7000000       BL  R0            ; BLR (i.e. branch using a REGISTER as dest addr, modifier u = 1)

# module exit code (generated by ORG.Close)

  12     8FE00000       LDR LNK SP  0
  13     4EE80004       ADD SP SP  4
  14     C700000F       B LNK