[fpc-devel] Producing assembly with less branches?

[Stefan Glienke]

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.


-- 
Diese E-Mail wurde von Avast Antivirus-Software auf Viren geprüft.
https://www.avast.com/antivirus