TempleOSTS_130315.ISO

TempleOS/Demo/Lectures/Optimization.CPP

@@ -0,0 +1,115 @@
+/*
+The moral of this story is simple
+instruction level optimizations
+don't matter much on a modern Intel CPU
+because they convert complex instructions
+to a stream of RISC instructions.
+
+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 suck, 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		//This instruction sucks, speedwise, but it 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=tT;
+i=Loop1;
+"Result:%d Time:%9.6f\n",i,tT-t0;
+
+"Good Code #1\n";
+t0=tT;
+i=Loop2;
+"Result:%d Time:%9.6f\n",i,tT-t0;
+
+"Good Code #2\n";
+t0=tT;
+i=Loop3;
+"Result:%d Time:%9.6f\n",i,tT-t0;
+
+"Good Code #3\n";
+t0=tT;
+i=Loop4;
+"Result:%d Time:%9.6f\n",i,tT-t0;
+
+
+/*  Program Output$HL,0$$WW+H,1$$FD,1$
+
+8 Cores 2.660GHz
+Bad Code
+Result:3200000040000000 Time: 0.069966
+Good Code #1
+Result:3200000040000000 Time: 0.062567
+Good Code #2
+Result:3200000040000000 Time: 0.062907
+Good Code #3
+Result:3200000040000000 Time: 0.156359
+
+$HL,1$*/