/*The moral of this story is simple inst level optimizations don't matter much on a modern Intel CPU because they convert complex insts to a stream of RISC insts. I learned this the hard way when I thought I was greatly improving my compiler by cutting code by a third. No significant speed-up. Depressing. */ #define SAMPLES (8*10000000+1) asm { LIMIT:: DU64 SAMPLES; //Memory reference should be bad, right? _BADLY_UNOPTIMIZED:: MOV RAX,0 MOV RCX,1 @@05: MOV RDX,RCX INC RCX //if no dependencies, Free! ADD RAX,RDX MOV RDX,LIMIT-16 //added 16 displacement to make it worse CMP RCX,U64 16[RDX] JB @@05 RET _WELL_OPTIMIZED1:: XOR RAX,RAX MOV RCX,SAMPLES-1 @@05: ADD RAX,RCX DEC RCX JNZ @@05 RET _WELL_OPTIMIZED2:: //Unrolled XOR RAX,RAX MOV RCX,SAMPLES-1 @@05: ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX ADD RAX,RCX DEC RCX JNZ @@05 RET _WELL_OPTIMIZED3:: XOR RAX,RAX MOV RCX,SAMPLES-1 @@05: ADD RAX,RCX LOOP @@05 //Inst has slow speed, but saves code size. RET } _extern _BADLY_UNOPTIMIZED I64 Loop1(); _extern _WELL_OPTIMIZED1 I64 Loop2(); _extern _WELL_OPTIMIZED2 I64 Loop3(); _extern _WELL_OPTIMIZED3 I64 Loop4(); I64 i; F64 t0; CPURep; "Bad Code\n"; t0=tS; i=Loop1; "Res:%d Time:%9.6f\n" ,i,tS-t0; "Good Code #1\n"; t0=tS; i=Loop2; "Res:%d Time:%9.6f\n" ,i,tS-t0; "Good Code #2\n"; t0=tS; i=Loop3; "Res:%d Time:%9.6f\n" ,i,tS-t0; "Good Code #3\n"; t0=tS; i=Loop4; "Res:%d Time:%9.6f\n" ,i,tS-t0; /* Program Output 8 Cores 2.660GHz Bad Code Res:3200000040000000 Time: 0.069966 Good Code #1 Res:3200000040000000 Time: 0.062567 Good Code #2 Res:3200000040000000 Time: 0.062907 Good Code #3 Res:3200000040000000 Time: 0.156359 */