[fpc-pascal] -OoASMCSE

[Bernd]

2011/12/31 Jonas Maebe <jonas.maebe at elis.ugent.be>:

> * compiler/i386/daopt386.pas: data flow analysis (DFA, collects information about what each register holds)
> * compiler/i386/csopt386.pas: common subexpression elimination based on information collected by DFA
> * compiler/i386/rropt386.pas: register renaming based on the information collected by DFA


Ok, this is a lot of code.
I have removed it all ;-)

I'm now playing around with this, trying to understand how the data
looks like. I have replaced the original CSE() function with the
following:

function RemoveReload(asml: TAsmList; first, last: tai; pass: longint): boolean;
var
  p : taicpu;
begin
  if pass > 0 then exit;

  writeln;
  writeln('block:');
  p := taicpu(first);
  repeat
    case p.typ of
      ait_label:
      begin
        writeln('label');
      end;

      ait_instruction:
      begin
        writeln(p.GetString);
      end;
    end;

    if p <> last then
      p := taicpu(p.Next);
  until p = last;

  Result := False;
end;

This does nothing (I hope), only print something to the console:

Compiling i386/testus.pas

block:
label
[sub ???,???]
label
[mov ???,???]
[mov ???,???]
[mov ???,???]
[mov ???,???]
[dec ???]
label
[inc ???]
[movsx ???,???]
[mov ???,???]
[sar ???,???]
[mov ???,???]
[mov ???,???]
[movsx ???,???]
[mov ???,???]
[sar ???,???]
[mov ???,???]
[mov ???,???]
[mov ???,???]
[mov ???,???]
[add ???,???]
[adc ???,???]
[mov ???,???]
[mov ???,???]
label
[cmp ???,???]
[j ???]
label
label
[mov ???,???]
[mov ???,???]
[mov ???,???]
[add ???,???]
[ret]

This looks good already ;-) I can even recognize my short test program
(the for loop, the qword addition) even with all the question marks.
This leads me to the first question: why are there question marks? Is
it not yet known which registers will be used at this stage of
compilation or am I just using the wrong method to dump it to the
console?

I have the following simple algorithm in mind:
start at the beginning of the block, one instruction after the other,
maintain a list of instructions, its empty at the beginning.

repeat
* if i come across a mov instruction and the exact same instruction is
in my list
already then remove it from the code.

* if there comes a label then clear the list

* if an instruction changes something (register or memory) that any of
the instructions in my list depend upon then remove the affected
instruction from the list.

next instruction
until end of block

Would something like this work already?

Now I need to find out the following things or how do I get them out
of the properties of the taicpu class:
* which registers does an instruction write to (if any)
* which memory does it write to (if any)
* which registers does it read from
* which memory does it read from

and:
how can I for debugging purposes dump the instructions to the console,
so that its easier for me so see what I have, are there any helper
functions?

more questions certainly to come...

Bernd