Am 15.04.2019 um 09:06 schrieb Bernd Oppolzer:
Am 15.04.2019 um 08:29 schrieb Tomas Hajny:
On Mon, April 15, 2019 07:52, Bernd Oppolzer wrote:
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On Samstag, 13. April 2019 22:30:55 CEST Alexey Tor. wrote:
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Can FPC optimize it so it only reads s[i] once (to register), not 3
times?
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True for New Stanford Pascal:
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I fail to see how is this related to FPC or the question of the original
poster (who was explicitly asking about FPC). Could we stay on topic,
please?
Tomas
(one of FPC mailing list moderators)
well, I was hoping for an answer out of the FPC community
what FPC does to the different codings, so that the users can
get some guidelines out of it (for example: common subexpressions
in locigal expressions are not eliminated, so it is better to use the
IN syntax -
or maybe the other way round). My mails were meant to motivate this :-)
but so far only suggestions how to optimize at the source code level.
(and of course: because FPC is sort of inspiration for my work,
I would like to see what FPC does in this cases and to compare it
with what I have)
Sorry about that ...
And you can't just test yourself? After all FPC is free and provides a
"-al" option that keeps the assembly code around...
Anyway, take this code:
=== code begin ===
var
c: Char;
begin
c := 'A';
if (c = 'a') or (c = 'b') or (c = 'c') then
c := 'B';
if c in ['a', 'b', 'c'] then
c := 'B';
if c in ['a', 'g', 'l'] then
c := 'B';
end.
=== code end ===
The result on x86_64 with -O- will be this:
=== code begin ===
# [9] c := 'A';
movb $65,U_$P$TMW_$$_C(%rip)
# [10] if (c = 'a') or (c = 'b') or (c = 'c') then
cmpb $97,U_$P$TMW_$$_C(%rip)
je .Lj3
jmp .Lj4
.Lj4:
cmpb $98,U_$P$TMW_$$_C(%rip)
je .Lj3
jmp .Lj5
.Lj5:
cmpb $99,U_$P$TMW_$$_C(%rip)
je .Lj3
jmp .Lj6
.Lj3:
# [11] c := 'B';
movb $66,U_$P$TMW_$$_C(%rip)
.Lj6:
# [12] if c in ['a', 'b', 'c'] then
movzbl U_$P$TMW_$$_C(%rip),%eax
subl $97,%eax
cmpl $3,%eax
jb .Lj7
.Lj7:
jc .Lj8
jmp .Lj9
.Lj8:
# [13] c := 'B';
movb $66,U_$P$TMW_$$_C(%rip)
.Lj9:
# [14] if c in ['a', 'g', 'l'] then
movzbl U_$P$TMW_$$_C(%rip),%eax
cmpl $97,%eax
je .Lj10
cmpl $103,%eax
je .Lj10
cmpl $108,%eax
je .Lj10
.Lj10:
je .Lj11
jmp .Lj12
.Lj11:
# [15] c := 'B';
movb $66,U_$P$TMW_$$_C(%rip)
=== code end ===
With -O2 it will be this:
=== code begin ===
# Var c located in register dl
# [9] c := 'A';
movb $65,%dl
# [10] if (c = 'a') or (c = 'b') or (c = 'c') then
cmpb $97,%dl
je .Lj3
cmpb $98,%dl
je .Lj3
cmpb $99,%dl
jne .Lj6
.Lj3:
# [11] c := 'B';
movb $66,%dl
.Lj6:
# [12] if c in ['a', 'b', 'c'] then
movzbl %dl,%eax
subl $97,%eax
cmpl $3,%eax
jnc .Lj9
# [13] c := 'B';
movb $66,%dl
.Lj9:
# [14] if c in ['a', 'g', 'l'] then
movzbl %dl,%eax
cmpl $97,%eax
je .Lj11
cmpl $103,%eax
je .Lj11
cmpl $108,%eax
je .Lj11
jne .Lj12
.Lj11:
# [15] c := 'B';
movb $66,%dl
=== code end ===
So it optimized the variable access into a register, but the first
if-condition is more like the third if-condition than the second.
Regards,
Sven
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