[fpc-devel] Producing assembly with less branches?

[Stefan Glienke]

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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

Still kinda disappointing compared to what it could be - while this is 
some simple code a modern compiler should try to eliminate conditional 
jumps even with the incredibly powerful branch predictors nowadays.

clang and gcc emit this - I would guess they detect quite some common 
patterns like this.

   xor ecx, ecx
   cmp eax, edx
   mov eax, -1
   setg cl
   cmovge eax, ecx
   ret

   cmp     eax, edx
   mov     edx, -1
   setg    al
   movzx   eax, al
   cmovl   eax, edx
   ret


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