Am 03.12.2011 21:41, schrieb Bernd: > Hello, I have observed something I do not understand. The following is > a series of for-loops and the generated code looks strange to me. > There always appears an inc followed by a dec
Leaving the inc away would require an additional jump to do the abort check at the loop entry. > and what is even more > bizarre is the code in between it: sometimes it inserts a mov > %esi,%esi, sometimes lea 0x0(%esi),%esi and sometimes just a simple > nop, Alignment of jmp target. > although the loop looks exactly the same. > > This is FPC from the 2_6 branch (RC1), platform is linux i386 and the > following was produced when compiling my program with -O2 and no > further optimizations. If I use -O3 and -Or then it still looks the > same and if I also use -Os then it will remove the mov %esi,%esi, > the lea and the nop but the inc and dec won't go away: > > Is there a reason why this must be compiled like this? Is it > intentionally trying to waste time because of pipelining or something > or is this a bug or a design flaw.? > > for I := 255 downto 128 do MS1BTABLE[I] := 7; > 0x80662b0 mov $0xff,%eax > 0x80662b5 inc %eax > 0x80662b6 mov %esi,%esi > 0x80662b8 dec %eax > 0x80662b9 movl $0x7,0x8097140(,%eax,4) > 0x80662c4 cmp $0x80,%eax > 0x80662c9 jg 0x80662b8 <INITLOOKUPTABLES+8> > for I := 127 downto 64 do MS1BTABLE[I] := 6; > 0x80662cb mov $0x7f,%eax > 0x80662d0 inc %eax > 0x80662d1 lea 0x0(%esi),%esi > 0x80662d4 dec %eax > 0x80662d5 movl $0x6,0x8097140(,%eax,4) > 0x80662e0 cmp $0x40,%eax > 0x80662e3 jg 0x80662d4 <INITLOOKUPTABLES+36> > for I := 63 downto 32 do MS1BTABLE[I] := 5; > 0x80662e5 mov $0x3f,%eax > 0x80662ea inc %eax > 0x80662eb nop > 0x80662ec dec %eax > 0x80662ed movl $0x5,0x8097140(,%eax,4) > 0x80662f8 cmp $0x20,%eax > 0x80662fb jg 0x80662ec <INITLOOKUPTABLES+60> > for I := 31 downto 16 do MS1BTABLE[I] := 4; > 0x80662fd mov $0x1f,%eax > 0x8066302 inc %eax > 0x8066303 nop > 0x8066304 dec %eax > 0x8066305 movl $0x4,0x8097140(,%eax,4) > 0x8066310 cmp $0x10,%eax > 0x8066313 jg 0x8066304 <INITLOOKUPTABLES+84> > for I := 15 downto 8 do MS1BTABLE[I] := 3; > 0x8066315 mov $0xf,%eax > 0x806631a inc %eax > 0x806631b nop > 0x806631c dec %eax > 0x806631d movl $0x3,0x8097140(,%eax,4) > 0x8066328 cmp $0x8,%eax > 0x806632b jg 0x806631c <INITLOOKUPTABLES+108> > for I := 7 downto 4 do MS1BTABLE[I] := 2; > 0x806632d mov $0x7,%eax > 0x8066332 inc %eax > 0x8066333 nop > 0x8066334 dec %eax > 0x8066335 movl $0x2,0x8097140(,%eax,4) > 0x8066340 cmp $0x4,%eax > 0x8066343 jg 0x8066334 <INITLOOKUPTABLES+132> > _______________________________________________ > fpc-pascal maillist - fpc-pascal@lists.freepascal.org > http://lists.freepascal.org/mailman/listinfo/fpc-pascal > _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
