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Free Pascal => General => Topic started by: fcu on June 13, 2019, 11:11:03 am

Title: evaluate constant expression at compile time
Post by: fcu on June 13, 2019, 11:11:03 am

Hi
are there a compiler switches to evaluate constant expression at compile time ?
for example

Code: Pascal  [Select][+][-]

1. var
2.  a : array of longint;
3.  i : longint;
4. begin
5.  setlength(a,9);
6.  for i:= 0 to length(a)-1 do
7.   a[i] := i;
8. end.
9.  

assembler for this code :

Code: Pascal  [Select][+][-]

1. # [13] for i:= 0 to length(a)-1 do
2.         movl    U_$P$TEST_$$_A,%eax
3.         testl   %eax,%eax
4.         je      .Lj19
5.         movl    -4(%eax),%eax
6.         addl    $1,%eax
7. .Lj19:
8.         subl    $1,%eax
9. # Var i located in register edx
10.         movl    $0,%edx
11.         cmpl    %edx,%eax
12.         jl      .Lj21
13.         subl    $1,%edx
14.         .balign 4,0x90
15. .Lj22:
16.         addl    $1,%edx
17. # [14] a[i] := i;
18.         movl    U_$P$TEST_$$_A,%ecx
19.         movl    %edx,(%ecx,%edx,4)
20.         cmpl    %edx,%eax
21.         jg      .Lj22
22.  

but if you replace length(a)-1 with 8 the assembler would be much simpler

Code: Pascal  [Select][+][-]

1. # [13] for i:= 0 to 8 do
2.         movl    $0,%eax
3.         subl    $1,%eax
4.         .balign 4,0x90
5. .Lj19:
6.         addl    $1,%eax
7. # [14] a[i] := i;
8.         movl    U_$P$TEST_$$_A,%edx
9.         movl    %eax,(%edx,%eax,4)
10.         cmpl    $8,%eax
11.         jl      .Lj19
12.  

Title: Re: evaluate constant expression at compile time
Post by: fcu on June 13, 2019, 11:15:34 am

seems fpc knew that length(a)-1 is 8 at compile time , but the asm generated is not the same as when i replace length(a)-1 with 8

Title: Re: evaluate constant expression at compile time
Post by: Thaddy on June 13, 2019, 11:49:06 am

It also depends on optimization mode. -O-  to -O4

Title: Re: evaluate constant expression at compile time
Post by: fcu on June 13, 2019, 11:51:19 am

it is compiled with -O3

Title: Re: evaluate constant expression at compile time
Post by: PascalDragon on June 14, 2019, 09:21:33 am

Quote from: fcu on June 13, 2019, 11:11:03 am

are there a compiler switches to evaluate constant expression at compile time ?

The compiler automatically does constant propagation depending on the general optimization switches. However dynamic arrays (for example) are not part of these optimizations.

Title: Re: evaluate constant expression at compile time
Post by: ASerge on June 14, 2019, 04:05:45 pm

Quote from: fcu on June 13, 2019, 11:11:03 am

Code: Pascal  [Select][+][-]

1. var
2.  a : array of longint;
3.  i : longint;
4. begin
5.  setlength(a,9);
6.  for i:= 0 to length(a)-1 do
7.   a[i] := i;
8. end.

Code: Pascal  [Select][+][-]

1. const
2.   L = 9;
3. var
4.   a: array of LongInt;
5.   i: LongInt;
6. begin
7.   SetLength(a, L);
8.   for i := 0 to L - 1 do
9.     a[i] := i;
10. end.

:)

Title: Re: evaluate constant expression at compile time
Post by: Leledumbo on June 17, 2019, 07:42:19 am

Quote from: fcu on June 13, 2019, 11:11:03 am

Hi
are there a compiler switches to evaluate constant expression at compile time ?
for example

Code: Pascal  [Select][+][-]

1. var
2.  a : array of longint;
3.  i : longint;
4. begin
5.  setlength(a,9);
6.  for i:= 0 to length(a)-1 do
7.   a[i] := i;
8. end.
9.  

Let's change that a bit:

Code: Pascal  [Select][+][-]

1. var
2.  a : array [0 .. 8] of longint;
3.  i : longint;
4. begin
5.  for i:= 0 to length(a)-1 do
6.   a[i] := i;
7. end.
8.  

Code: [Select]

# [5] for i:= 0 to length(a)-1 do
movl $-1,U_$P$PROGRAM_$$_I
.p2align 4,,10
.p2align 3
.Lj3:
movl U_$P$PROGRAM_$$_I,%eax
leal 1(%eax),%eax
movl %eax,U_$P$PROGRAM_$$_I
# [6] a[i] := i;
movl U_$P$PROGRAM_$$_I,%eax
movl U_$P$PROGRAM_$$_I,%edx
movl %edx,U_$P$PROGRAM_$$_A(,%rax,4)
cmpl $8,U_$P$PROGRAM_$$_I
jge .Lj5
jmp .Lj3
Same as you wish? Optimization level 0, btw :) (O3 will activate loop unrolling, totally removing the loop).
Length() is a compiler intrinsic, its implementation depends on type of given data. In static arrays, its replaced directly by a constant. In dynamic arrays, it's located at negative offset of the array (4 bytes to the left to be precise), much like all dynamic strings, hence requires a little computation that can't be replaced by a constant. Check this short programmer's guide (https://www.freepascal.org/docs-html/prog/progsu164.html) page.

Quote from: ASerge on June 14, 2019, 04:05:45 pm

Code: Pascal  [Select][+][-]

1. const
2.   L = 9;
3. var
4.   a: array of LongInt;
5.   i: LongInt;
6. begin
7.   SetLength(a, L);
8.   for i := 0 to L - 1 do
9.     a[i] := i;
10. end.

:)

Wirthian optimization :)

Title: Re: evaluate constant expression at compile time
Post by: PascalDragon on June 17, 2019, 09:00:49 am

Quote from: Leledumbo on June 17, 2019, 07:42:19 am

Length() is a compiler intrinsic, its implementation depends on type of given data. In static arrays, its replaced directly by a constant. In dynamic arrays, it's located at negative offset of the array (4 bytes to the left to be precise), much like all dynamic strings, hence requires a little computation that can't be replaced by a constant. Check this short programmer's guide (https://www.freepascal.org/docs-html/prog/progsu164.html) page.

Well, for local dynamic arrays the compiler could in theory keep the length of the array in a register as well (as long as the array isn't passed to some other function) and thus generate more optimized code.