----- Reply to message ----- Subject: [fpc-devel] Producing assembly with less branches? From: Stefan Glienke <sglie...@dsharp.org> To: <fpc-devel@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 _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel