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 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, 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
