Errors in TDPL

[bearophile]

Adam Ruppe:
> foreach(i; retro(iota(0, 10))) { }

Oh, right! Or even just:

foreach (i; retro(iota(10))) {}

But abstraction has a cost, see below. I have written three test programs.

----------------

// test1
import std.c.stdio: printf;
void main() {
    enum int N = 100_000_000;
    int count;
    for (int i; i < N; i++)
        count++;
    printf("%d\n", count);
}

Asm of the inner code of the test1, opt version:

5:		inc	ECX
		inc	EAX
		cmp	EAX,05F5E100h
		jb	L5

----------------

// test2
import std.c.stdio: printf;
void main() {
    enum int N = 100_000_000;
    int count;
    foreach_reverse (i; 0 .. N)
        count++;
    printf("%d\n", count);
}

Asm of the inner code of the test2, opt version:

LF:		dec	ECX
		inc	EDX
		mov	EAX,ECX
		inc	EAX
		jg	LF

----------------

// test3
import std.c.stdio: printf;
import std.range: iota, retro;
void main() {
    enum int N = 100_000_000;
    int count;
    foreach (i; retro(iota(N)))
        count++;
    printf("%d\n", count);
}

Asm of the inner code of the test3, opt version:

L6A:		inc	EBX
		lea	EAX,010h[ESP]
		call	near ptr _D3std5range145__T4TakeTS3std5range112__T8SequenceVAyaa27_612e6669656c645b305d202b206e202a20612eD0677AACB4B6BC826B92E7DBB9E1E359
		cmp	dword ptr 020h[ESP],0
		jne	L6A


_D3std5range145__T4TakeTS3std5range112__T8SequenceVAyaa27_612e6669656c645b305d202b206e202a20612eD0677AACB4B6BC826B92E7DBB9E1E359	comdat
L0:		sub	ESP,01Ch
		mov	ECX,offset FLAT:_D3std5range145__T4TakeTS3std5range112__T8SequenceVAyaa27_612e6669656c645b305d202b206e202a20612e0B5C6D6E0C89B8D48DF56A414048DB6F
		push	EBX
		push	ESI
		mov	ESI,EAX
		mov	EDX,010h[ESI]
		mov	0Ch[ESP],EAX
		neg	EDX
		sbb	EDX,EDX
		neg	EDX
		xor	DL,1
		mov	010h[ESP],ECX
		je	L46
		mov	EDX,ECX
		push	dword ptr FLAT:_DATA[064h]
		push	dword ptr FLAT:_DATA[060h]
		push	051Fh
		mov	EAX,018h[ESP]
		mov	EBX,018h[ESP]
		call	EDX
		push	EDX
		push	EAX
		call	near ptr _D3std9contracts7bailOutFAyaixAaZv
L46:		dec	dword ptr 010h[ESI]
		pop	ESI
		pop	EBX
		add	ESP,01Ch
		ret


_D3std9contracts7bailOutFAyaixAaZv is extern.

------------------

Running time, best of 3, seconds:
  test1:     0.31
  test1 opt: 0.07
  test2:     0.31
  test2 opt: 0.12
  test3:     6.38
  test3 opt: 0.52

not opt version = dmd (no other option)
opt version = dmd -O -release -inline

Compile times opt version, seconds:
  test1: 0.05
  test2: 0.05
  test3: 0.28

So with the current dmd compiler in the inner loop in a performance-critical routine you can't use foreach(retro(iota(N))).

In future a D compiler can recognize and optimize such usage of foreach(iota()) and foreach(retro(iota())) as ShedSkin does with usage of xrange()/range() in a loop.

Bye,
bearophile