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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
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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