May be having case statement problem.

App has case statement with 146 sequential labels. They are in order, from a defined type.
The assembler output is scanning each label, where it seems a jump would be more efficient.

ppc 32, Darwin
/usr/local/bin/ppcppc cape80.pas -Ci -Co -g -gl -O1 -vr -a -al -FEbuild/cape80.build/cape80.build -ocape80

Also tried -O2 and 3

Result:

# [735] case l3o3.FormType of

  // Select 3o3 processor
	lwz	r2,160(r1)
	cmplwi	cr0,r2,0
	beq	cr0,L1051
	cmplwi	cr0,r2,1
	beq	cr0,L1052
	cmplwi	cr0,r2,2
	beq	cr0,L1053
	cmplwi	cr0,r2,3
	beq	cr0,L1054

etc.

Any hints?

P Davidson

I don't understand well what you have:

case L3o3.Formtype of
1,2,3,....,146 : begin <somecode>
                 end;
or
case L3o3.FormType of
1 : begin <somecode1>
      end
2 : begin <somecode2>
      end

and so on.....

For the second case the assemmbler code seems ok to me, also L1051 L1052 L1053 looks like are different destinations. For the first, well, it can be optimized "a bit" by hand, but note that it's difficult to do in "automatic mode" by a compiler.

lwz r2,160(r1) // Load in r2 the value to compare subi r3,r2,145 // Change 145 with the top value. Don't need to record in cr0, so subi is used instead of subi. (with point) cmplwi cr3,r3,0 // Change 0 with the low value. So if 145-r2 is greater or equal 0, it's between them or is one of them bge cr3,L1051 // branch to L1051. Use bge+ cr3,L1051 if you are sure that branch will be taken often

Note that cr3, is a condition register field and can be used by the compiler for other pourposes. This code only works with integer values 0 < r2 < 65535, perhaps also works with chars. If you know that the condition is true (r2 is between 0 and 145 often), you can use the '+' format of the branch.

The code may not work, i'm writing it by memory.

Not this

This :)

Was under the impression that doing a jump, offset by label number, would be faster. Specially for such long case statements.

P Davidson
http://CoraxNetworks.com

Jump by offset label number? I don't understand it.

Sorry Eduardo, am not knowledgeable of compiler internals. There is (?) an optimization in FPC that uses a jump table(?) instead of scanning each label value of a case statement. Hoping that this is the case, as it seems to be with i386. This would mean faster execution of the case. Yes?

P Davidson
http://CoraxNetworks.com

Use 2.1.1

Peter

version 2.1.1 [2005/11/08] for powerpc
Same result, Peter :|

P Davidson
http://CoraxNetworks.com

Don't know how fpc ppc works internally. Surely this optimization is not done, it's uncommon, i think.

For now, the only optimization to the compiler output maybe use other crs for comparations and separate the branch from the comparation to prevent stalls. Here an example.

Original code

# [735] case l3o3.FormType of

  // Select 3o3 processor
         lwz     r2,160(r1)
         cmplwi  cr0,r2,0
         beq     cr0,L1051
         cmplwi  cr0,r2,1
         beq     cr0,L1052
         cmplwi  cr0,r2,2
         beq     cr0,L1053
         cmplwi  cr0,r2,3
         beq     cr0,L1054

Modified code

# [735] case l3o3.FormType of

  // Select 3o3 processor
         lwz     r3,160(r1)     // r3, has the value to check
         cmplwi  cr0,r3,0
         cmplwi  cr2,r3,1        // cr1 is used for fpu condition and

cannot be used for integer
cmplwi cr3,r3,2
cmplwi cr4,r3,3
beq cr0,L1051 // separate branch from comp (4 lines up) can make the branch be pre-calculated and executed in zero cycles cmplwi cr5,r3,4
beq cr2,L1052
cmplwi cr0,r3,5
beq cr3,L1053
cmplwi cr2,r3,6
beq cr4,L1054

and so on...

Perhaps this code can do the trick, but don't know if is easily integrable in fpc ppc codegenerator, you can adapt it a bit if you want

** IN r3 Value to check r29 branch lookup table r5 top/end r6 low/begin r4 where to branch

SecuentialCase: lwz r3,VALUE // VALUE from TOC or BSS or other. subi r29,r29,4 // Adjust
value for access
caseloop: lwzu r4,4(r29) // in address pointed by r29 begins the jump look-up table (L1051,L1052) mtctr r4 // Move
address in r4 to CTR special register. CTR has now the destiny address lwz r5,TOP // last value
lwz r6,LOW // low value
cmplwi cr2,r3,r6 // compare if
value with r6
bcctr 12,10 // BO = 12
Branch if condition True. BI = 10 Condition flag Equal of cr2.

                          addi       r6,r6,1              // r6 := r6 +1
                          cmplwi   cr3,r5,r6           // is r6 > r5 ?
                          ble        cr3,caseloop     // Branch to

caseloop if FALSE
otherwise statement or branch to it

** OUT r29, r6 originals values are destroyed.

The 2.1.1 compiler contains code to generate jumptables for ppc, see powerpc/nppcset.pas. You'll have to debug the compiler to see why it doesn't use them.

Peter

You could use an array of procedures. It's members are initialized once,

var my_table: array [0 .. 146] of procedure;
my_table[0] := @my_proc_0;
my_tyble[1] := @my_proc_1;
...

and the 'case' statement is simply replaced with

begin
  ..
  my_table[x];
  ..
end;

which is fast, because it performs no compare, but loads the procedure address from the array, indexed with x.

Anton.

Haven't really debugged, but there is an cs_optimize in aktmodeswitches somewhere near where this code is called.

Paul: do you enable some -O parameter? Play with it etc.

I think -O1r is the best optimization available for ppc-darwin, isn't it ?

Regards,

Adriaan van Os

As far as I can see from the source, try -O1gr

both optimization and "for size" seem to be needed.

Thanks Adriaan, Marco.
All is well :)

Marco van de Voort wrote

-O1gr

P Davidson
Corax Networks Inc.
http://CoraxNetworks.com

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