[fpc-pascal] i386: strange code generated in simple for loop

[Florian Klämpfl]

Am 03.12.2011 21:41, schrieb Bernd:
> Hello, I have observed something I do not understand. The following is
> a series of for-loops and the generated code looks strange to me.
> There always appears an inc followed by a dec 

Leaving the inc away would require an additional jump to do the abort
check at the loop entry.

> and what is even more
> bizarre is the code in between it: sometimes it inserts a mov
> %esi,%esi, sometimes lea    0x0(%esi),%esi and sometimes just a simple
> nop, 

Alignment of jmp target.

> although the loop looks exactly the same.
> 
> This is FPC from the 2_6 branch (RC1), platform is linux i386 and the
> following was produced when compiling my program with -O2 and no
> further optimizations. If I use -O3 and  -Or then it still looks the
> same and if I also use -Os then it will remove the mov    %esi,%esi,
> the lea and the nop but the inc and dec won't go away:
> 
> Is there a reason why this must be compiled like this? Is it
> intentionally trying to waste time because of pipelining or something
> or is this a bug or a design flaw.?
> 
>   for I := 255 downto 128 do MS1BTABLE[I] := 7;
> 0x80662b0 mov    $0xff,%eax
> 0x80662b5 inc    %eax
> 0x80662b6 mov    %esi,%esi
> 0x80662b8 dec    %eax
> 0x80662b9 movl   $0x7,0x8097140(,%eax,4)
> 0x80662c4 cmp    $0x80,%eax
> 0x80662c9 jg     0x80662b8 <INITLOOKUPTABLES+8>
>   for I := 127 downto  64 do MS1BTABLE[I] := 6;
> 0x80662cb mov    $0x7f,%eax
> 0x80662d0 inc    %eax
> 0x80662d1 lea    0x0(%esi),%esi
> 0x80662d4 dec    %eax
> 0x80662d5 movl   $0x6,0x8097140(,%eax,4)
> 0x80662e0 cmp    $0x40,%eax
> 0x80662e3 jg     0x80662d4 <INITLOOKUPTABLES+36>
>   for I :=  63 downto  32 do MS1BTABLE[I] := 5;
> 0x80662e5 mov    $0x3f,%eax
> 0x80662ea inc    %eax
> 0x80662eb nop
> 0x80662ec dec    %eax
> 0x80662ed movl   $0x5,0x8097140(,%eax,4)
> 0x80662f8 cmp    $0x20,%eax
> 0x80662fb jg     0x80662ec <INITLOOKUPTABLES+60>
>   for I :=  31 downto  16 do MS1BTABLE[I] := 4;
> 0x80662fd mov    $0x1f,%eax
> 0x8066302 inc    %eax
> 0x8066303 nop
> 0x8066304 dec    %eax
> 0x8066305 movl   $0x4,0x8097140(,%eax,4)
> 0x8066310 cmp    $0x10,%eax
> 0x8066313 jg     0x8066304 <INITLOOKUPTABLES+84>
>   for I :=  15 downto   8 do MS1BTABLE[I] := 3;
> 0x8066315 mov    $0xf,%eax
> 0x806631a inc    %eax
> 0x806631b nop
> 0x806631c dec    %eax
> 0x806631d movl   $0x3,0x8097140(,%eax,4)
> 0x8066328 cmp    $0x8,%eax
> 0x806632b jg     0x806631c <INITLOOKUPTABLES+108>
>   for I :=   7 downto   4 do MS1BTABLE[I] := 2;
> 0x806632d mov    $0x7,%eax
> 0x8066332 inc    %eax
> 0x8066333 nop
> 0x8066334 dec    %eax
> 0x8066335 movl   $0x2,0x8097140(,%eax,4)
> 0x8066340 cmp    $0x4,%eax
> 0x8066343 jg     0x8066334 <INITLOOKUPTABLES+132>
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