Hello!
Ingmar Tulva wrote 9 lines
Anyway, has someone actually analyzed how benefitial register calling
convention is? Sure it provides huge speed boost in case of a function
which adds two arguments together and returns the result - or is it so
sure? In fact, I imagine that in most cases, register convention will
eventually be detrimental to both speed and code size.
Again, no complain intended, just curiousity.
I'm not a developper of free-pascal, so the following statements are just general considerations.
To me, register convention is a great step forward, concerning the economic usage of available resources.
Register convention saves opcode space in the called function, because within the instruction opcode, registers are encoded with small bit fields, whereas offsets into a stack frame are encoded as (8-, 16-, or 32-bit) words, unless the processor supports something like short offsets. Actually, I don't know if the i386 does support such short offsets. AFAIK, the 68000 does not.
Every register parameter also saves at least two memory (stack) accesses, which are considerably slower compared to register accesses and increase bus use.
When writing low-level functions in 680x0 assembler, I always pass parameters in registers (there are 15 of them, which is quite a lot). In an embedded system, I sometimes even use "global" register variables, e. g. register D7 contains, throughout the whole program, the current user ID.
A bit off-topic: I also return boolean result in a processor flag, to allow fast testing:
pascal:
begin
if my_boolean_function then begin
bla
assembler
jsr my_boolean_function
bcs.s yes
..
yes: bla
Implementations I know of, pass boolean result in D0, which is slower, but still faster compared to stack parameters.
In 180 degree contradiction to your opinion, I think that register convention will, in most cases, save both memory and execution time :)
As an example, consider the call
function main;
var
a: integer;
b: integer;
begin
my_function(a, b);
end;
With conventional calling:
move a, -(a7) // 2 memory accesses
move b, -(a7) // 2 memory accesses
jsr my_function
WIth register calling:
move a, d0 // 1 memory access
move b, d1 // 1 memory access
jsr my_function
This example still does not include the benefits within procedure my_function, namely the saving of instruction extension words with stack offsets. In that case, 2 memory accesses (to the instruction extension word and to the stack data) are saved by register convention.
A further benefit would result from optimization between the registers passing parameters to my_function, and the registers used within function main for variables a and b, by allocating identical registers, thus removing instructions of the form "move d5, d5". That's an optimization technique easy in assembler, but difficult for a compiler, especially when maintaining register allocation through serveral levels of function calls.
If anyone does some performance testing, comparing the two conventions, please post it to the list.
Anton.
