[fpc-devel] Producing assembly with less branches?

[Marģers .]

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Subject: [fpc-devel] Producing assembly with less branches?
From:  Stefan Glienke <sglienke at dsharp.org>
To:  <fpc-devel at lists.freepascal.org>
> Hi,

> not sure if anything significantly changed in trunk compared to 3.2 wrt
> to optimized code being generated but I am quite disappointed that fpc
> (checked win64 with -O3 and -O4) does not use cmovxx instructions and
> alike for the most basic things and produces terrible code like this:

> unit1.pas:49                              if left < right then
> 000000010002E3C0 39ca                     cmp    edx,ecx
> 000000010002E3C2 7e06                     jle    0x10002e3ca <COMPAREINT+10>
> unit1.pas:50                              Result := -1
> 000000010002E3C4 b8ffffffff               mov    eax,0xffffffff
> 000000010002E3C9 c3                       ret
> unit1.pas:51                              else if left > right then
> 000000010002E3CA 39ca                     cmp    edx,ecx
> 000000010002E3CC 7d06                     jge    0x10002e3d4 <COMPAREINT+20>
> unit1.pas:52                              Result := 1
> 000000010002E3CE b801000000c3             mov    eax,0x1
> unit1.pas:54                              Result := 0;
> 000000010002E3D4 31c0                     xor    eax,eax
> unit1.pas:55                              end;
> 000000010002E3D6 c3                       ret

> Similar for even simpler things:

> unit1.pas:43                              if i < 0 then
> 000000010002E3A1 85c0                     test   eax,eax
> 000000010002E3A3 7d03                     jge    0x10002e3a8
> <BUTTON1CLICK+72>
> unit1.pas:44                              i := 0;
> 000000010002E3A5 31c0                     xor    eax,eax
> 000000010002E3A7 90                       nop

> Imo someone should work at that and make the compiler produce less
> branches. Not sure if that is on your list but it should be looked at.

it's already done in trunk (sadly not in 3.2.0)
to get cmov instruction emitted, has to meet two conditions
1) if statement without else part
2) assign value of variable (not constant).

your code has to look like to benefit from cmov

function cmov2(left, right : longint):longint;
var l1,lf: longint;
     r : longint;
begin
     l1:=1;
     lf:=-1;
     r:=0;
     if left > right then
     begin
          r:=lf;
     end;// else
     if left < right then
     begin
          r:=l1;
     end;// else  r:=0;
     cmov2:=r;
end;
  

00400370    b9 01 00 00 00                mov ecx,00000001h
00400375    ba ff ff ff ff                mov edx,0ffffffffh
0040037a    31 c0                         xor eax,eax
0040037c    39 fe                         cmp esi,edi
0040037e    0f 4c c2                      cmovl eax,edx
00400381    39 fe                         cmp esi,edi
00400383    0f 4f c1                      cmovnle eax,ecx
00400386    c3                            ret