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