"For" loops optimizations proposal [feature]

Original Reporter info from Mantis: lagprogramming

• Reporter name:

Description:

Regarding near zero {-1,1} comparisons I think it would be a good idea to consider optimizing the "for" loops.

CASE 1

for signedvariable:={INITIALVALUE} to -1 do {CODE}

act like

signedvariable:={INITIALVALUE};
if signedvariable<0 then
   repeat
   {CODE}
   inc(signedvariable);
   until signedvariable=0;

CASE 2

for signedvariable:={INITIALVALUE} downto 1 do {CODE}

act like

signedvariable:={INITIALVALUE};
if signedvariable>0 then
   repeat
   {CODE}
   dec(signedvariable);
   until signedvariable=0;

CASE 3

for unsignedvariable:={INITIALVALUE} downto 1 do {CODE}

act like

unsignedvariable:={INITIALVALUE};
if unsignedvariable<>0 then
   repeat
   {CODE}
   dec(unsignedvariable);
   until unsignedvariable=0;

{INITIALVALUE} should match the sign of {to -1, downto 1} to optimize these loops.

Results using fpc svn 28896 for the below code

function fordowntoloop(parameter:integer):boolean;
begin
  for parameter:=parameter downto 1 do
    if (parameter and 1)=0 then exit(true);
  result:=false;
end;

.section .text.n_unit1_$$_fordowntoloop$longint$$boolean
	.balign 16,0x90
.globl	UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN
	.type	UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN,@function
UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN:
.Lc1:
.Ll1:
# [unit1.pas]
# [33] begin
	nop
# Var parameter located in register edi
# Var $result located in register al
# Var parameter located in register edi
.Ll2:
# [34] for parameter:=parameter downto 1 do
	cmpl	$1,%edi
	jl	.Lj6
	addl	$1,%edi
	.balign 8,0x90
.Lj7:
	subl	$1,%edi
.Ll3:
# [35] if (parameter and 1)=0 then exit(true);
	movl	%edi,%edx
	andl	$1,%edx
	testl	%edx,%edx
	jne	.Lj9
	movb	$1,%al
	jmp	.Lj3
.Lj9:
.Ll4:
	cmpl	$1,%edi
	jg	.Lj7
.Lj6:
.Ll5:
# [36] result:=false;
	movb	$0,%al
.Lj3:
# PeepHole Optimization,var9
.Ll6:
# [37] end;
	andl	$255,%eax
	nop
	ret
.Lc2:
.Lt1:
.Le0:
	.size	UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN, .Le0 - UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN

function ifrepeatloop(parameter:integer):boolean;
begin
  parameter:=parameter;//ignored by compiler
  if parameter>0 then
   repeat
   if (parameter and 1)=0 then exit(true);
   dec(parameter);
   until parameter=0;
   result:=false;
end;

.section .text.n_unit1_$$_ifrepeatloop$longint$$boolean
	.balign 16,0x90
.globl	UNIT1_$$_IFREPEATLOOP$LONGINT$$BOOLEAN
	.type	UNIT1_$$_IFREPEATLOOP$LONGINT$$BOOLEAN,@function
UNIT1_$$_IFREPEATLOOP$LONGINT$$BOOLEAN:
.Lc3:
.Ll8:
# [40] begin
	nop
# Var parameter located in register edi
# Var $result located in register al
# Var parameter located in register edi
.Ll9:
# [42] if parameter>0 then
	cmpl	$0,%edi
	jng	.Lj19
	.balign 8,0x90
.Lj20:
.Ll10:
# [44] if (parameter and 1)=0 then exit(true);
	movl	%edi,%edx
	andl	$1,%edx
	testl	%edx,%edx
	jne	.Lj24
	movb	$1,%al
	jmp	.Lj14
.Lj24:
.Ll11:
# [45] dec(parameter);
	subl	$1,%edi
.Ll12:
# [46] until parameter=0;
	testl	%edi,%edi
	jne	.Lj20
.Lj19:
.Ll13:
# [47] result:=false;
	movb	$0,%al
.Lj14:
# PeepHole Optimization,var9
.Ll14:
# [48] end;
	andl	$255,%eax
	nop
	ret
.Lc4:
.Lt2:
.Le1:
	.size	UNIT1_$$_IFREPEATLOOP$LONGINT$$BOOLEAN, .Le1 - UNIT1_$$_IFREPEATLOOP$LONGINT$$BOOLEAN

procedure TForm1.Button1Click(Sender: TObject);
var r:integer;
begin
  for r:=-3 to -1 do fordowntoloop(r);
end; 

.section .text.n_unit1$_$tform1_$__$$_button1click$tobject
	.balign 16,0x90
.globl	UNIT1$_$TFORM1_$__$$_BUTTON1CLICK$TOBJECT
	.type	UNIT1$_$TFORM1_$__$$_BUTTON1CLICK$TOBJECT,@function
UNIT1$_$TFORM1_$__$$_BUTTON1CLICK$TOBJECT:
.Lc5:
.Ll16:
# [52] begin
	pushq	%rbx
	nop
.Lc7:
# Var Sender located in register rsi
# Var $self located in register rdi
# Var r located in register ebx
# Var r located in register ebx
.Ll17:
# [53] for r:=-3 to -1 do fordowntoloop(r);
	movl	$-3,%ebx
	subl	$1,%ebx
	.balign 8,0x90
.Lj33:
	addl	$1,%ebx
	movl	%ebx,%edi
	call	UNIT1_$$_FORDOWNTOLOOP$LONGINT$$BOOLEAN
	cmpl	$-1,%ebx
	jl	.Lj33
.Ll18:
# [54] end;
	popq	%rbx
	nop
	ret
.Lc6:
.Lt3:
.Le2:
	.size	UNIT1$_$TFORM1_$__$$_BUTTON1CLICK$TOBJECT, .Le2 - UNIT1$_$TFORM1_$__$$_BUTTON1CLICK$TOBJECT

Mantis conversion info:

• Mantis ID: 26925
• Build: 2.7.1 rev 28896