Why does JIT order affect performance?
Why does the order in which C# methods in .NET 4.0 are just-in-time compiled affect how quickly they execute? For example, consider two equivalent methods:
p
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Please note that I do not trust the "Suppress JIT optimization on module load" option, I spawn the process without debugging and attach my debugger after the JIT has run.
In the version where single-line runs faster, this is
Main:SingleLineTest(); 00000000 push ebp 00000001 mov ebp,esp 00000003 call dword ptr ds:[0019380Ch] MultiLineTest(); 00000009 call dword ptr ds:[00193818h] SingleLineTest(); 0000000f call dword ptr ds:[0019380Ch] MultiLineTest(); 00000015 call dword ptr ds:[00193818h] SingleLineTest(); 0000001b call dword ptr ds:[0019380Ch] MultiLineTest(); 00000021 call dword ptr ds:[00193818h] 00000027 pop ebp } 00000028 retNote that
MultiLineTesthas been placed on an 8 byte boundary, andSingleLineTeston a 4 byte boundary.Here's
Mainfor the version where both run at the same speed:MultiLineTest(); 00000000 push ebp 00000001 mov ebp,esp 00000003 call dword ptr ds:[00153818h] SingleLineTest(); 00000009 call dword ptr ds:[0015380Ch] MultiLineTest(); 0000000f call dword ptr ds:[00153818h] SingleLineTest(); 00000015 call dword ptr ds:[0015380Ch] MultiLineTest(); 0000001b call dword ptr ds:[00153818h] SingleLineTest(); 00000021 call dword ptr ds:[0015380Ch] MultiLineTest(); 00000027 call dword ptr ds:[00153818h] 0000002d pop ebp } 0000002e retAmazingly, the addresses chosen by the JIT are identical in the last 4 digits, even though it allegedly processed them in the opposite order. Not sure I believe that any more.
More digging is necessary. I think it was mentioned that the code before the loop wasn't exactly the same in both versions? Going to investigate.
Here's the "slow" version of
SingleLineTest(and I checked, the last digits of the function address haven't changed).Stopwatch stopwatch = new Stopwatch(); 00000000 push ebp 00000001 mov ebp,esp 00000003 push edi 00000004 push esi 00000005 push ebx 00000006 mov ecx,7A5A2C68h 0000000b call FFF91EA0 00000010 mov esi,eax 00000012 mov dword ptr [esi+4],0 00000019 mov dword ptr [esi+8],0 00000020 mov byte ptr [esi+14h],0 00000024 mov dword ptr [esi+0Ch],0 0000002b mov dword ptr [esi+10h],0 stopwatch.Start(); 00000032 cmp byte ptr [esi+14h],0 00000036 jne 00000047 00000038 call 7A22B314 0000003d mov dword ptr [esi+0Ch],eax 00000040 mov dword ptr [esi+10h],edx 00000043 mov byte ptr [esi+14h],1 int count = 0; 00000047 xor edi,edi for (uint i = 0; i < 1000000000; ++i) { 00000049 xor edx,edx count += i % 16 == 0 ? 1 : 0; 0000004b mov eax,edx 0000004d and eax,0Fh 00000050 test eax,eax 00000052 je 00000058 00000054 xor eax,eax 00000056 jmp 0000005D 00000058 mov eax,1 0000005d add edi,eax for (uint i = 0; i < 1000000000; ++i) { 0000005f inc edx 00000060 cmp edx,3B9ACA00h 00000066 jb 0000004B } stopwatch.Stop(); 00000068 mov ecx,esi 0000006a call 7A23F2C0 Console.WriteLine("Single-line test --> Count: {0}, Time: {1}", count, stopwatch.ElapsedMilliseconds); 0000006f mov ecx,797C29B4h 00000074 call FFF91EA0 00000079 mov ecx,eax 0000007b mov dword ptr [ecx+4],edi 0000007e mov ebx,ecx 00000080 mov ecx,797BA240h 00000085 call FFF91EA0 0000008a mov edi,eax 0000008c mov ecx,esi 0000008e call 7A23ABE8 00000093 push edx 00000094 push eax 00000095 push 0 00000097 push 2710h 0000009c call 783247EC 000000a1 mov dword ptr [edi+4],eax 000000a4 mov dword ptr [edi+8],edx 000000a7 mov esi,edi 000000a9 call 793C6F40 000000ae push ebx 000000af push esi 000000b0 mov ecx,eax 000000b2 mov edx,dword ptr ds:[03392034h] 000000b8 mov eax,dword ptr [ecx] 000000ba mov eax,dword ptr [eax+3Ch] 000000bd call dword ptr [eax+1Ch] 000000c0 pop ebx } 000000c1 pop esi 000000c2 pop edi 000000c3 pop ebp 000000c4 retAnd the "fast" version:
Stopwatch stopwatch = new Stopwatch(); 00000000 push ebp 00000001 mov ebp,esp 00000003 push edi 00000004 push esi 00000005 push ebx 00000006 mov ecx,7A5A2C68h 0000000b call FFE11F70 00000010 mov esi,eax 00000012 mov ecx,esi 00000014 call 7A1068BC stopwatch.Start(); 00000019 cmp byte ptr [esi+14h],0 0000001d jne 0000002E 0000001f call 7A12B3E4 00000024 mov dword ptr [esi+0Ch],eax 00000027 mov dword ptr [esi+10h],edx 0000002a mov byte ptr [esi+14h],1 int count = 0; 0000002e xor edi,edi for (uint i = 0; i < 1000000000; ++i) { 00000030 xor edx,edx count += i % 16 == 0 ? 1 : 0; 00000032 mov eax,edx 00000034 and eax,0Fh 00000037 test eax,eax 00000039 je 0000003F 0000003b xor eax,eax 0000003d jmp 00000044 0000003f mov eax,1 00000044 add edi,eax for (uint i = 0; i < 1000000000; ++i) { 00000046 inc edx 00000047 cmp edx,3B9ACA00h 0000004d jb 00000032 } stopwatch.Stop(); 0000004f mov ecx,esi 00000051 call 7A13F390 Console.WriteLine("Single-line test --> Count: {0}, Time: {1}", count, stopwatch.ElapsedMilliseconds); 00000056 mov ecx,797C29B4h 0000005b call FFE11F70 00000060 mov ecx,eax 00000062 mov dword ptr [ecx+4],edi 00000065 mov ebx,ecx 00000067 mov ecx,797BA240h 0000006c call FFE11F70 00000071 mov edi,eax 00000073 mov ecx,esi 00000075 call 7A13ACB8 0000007a push edx 0000007b push eax 0000007c push 0 0000007e push 2710h 00000083 call 782248BC 00000088 mov dword ptr [edi+4],eax 0000008b mov dword ptr [edi+8],edx 0000008e mov esi,edi 00000090 call 792C7010 00000095 push ebx 00000096 push esi 00000097 mov ecx,eax 00000099 mov edx,dword ptr ds:[03562030h] 0000009f mov eax,dword ptr [ecx] 000000a1 mov eax,dword ptr [eax+3Ch] 000000a4 call dword ptr [eax+1Ch] 000000a7 pop ebx } 000000a8 pop esi 000000a9 pop edi 000000aa pop ebp 000000ab retJust the loops, fast on the left, slow on the right:
00000030 xor edx,edx 00000049 xor edx,edx 00000032 mov eax,edx 0000004b mov eax,edx 00000034 and eax,0Fh 0000004d and eax,0Fh 00000037 test eax,eax 00000050 test eax,eax 00000039 je 0000003F 00000052 je 00000058 0000003b xor eax,eax 00000054 xor eax,eax 0000003d jmp 00000044 00000056 jmp 0000005D 0000003f mov eax,1 00000058 mov eax,1 00000044 add edi,eax 0000005d add edi,eax 00000046 inc edx 0000005f inc edx 00000047 cmp edx,3B9ACA00h 00000060 cmp edx,3B9ACA00h 0000004d jb 00000032 00000066 jb 0000004BThe instructions are identical (being relative jumps, the machine code is identical even though the disassembly shows different addresses), but the alignment is different. There are three jumps. the
jeloading a constant1is aligned in the slow version and not in the fast version, but it hardly matters, since that jump is only taken 1/16 of the time. The other two jumps (jmpafter loading a constant zero, andjbrepeating the entire loop) are taken millions more times, and are aligned in the "fast" version.I think this is the smoking gun.
讨论(0) -
My time is 2400 and 2600 on i5-2410M 2,3Ghz 4GB ram 64bit Win 7.
Here is my output: Single first
After starting the process and then attaching the debugger
SingleLineTest(); MultiLineTest(); SingleLineTest(); MultiLineTest(); SingleLineTest(); MultiLineTest(); -------------------------------- SingleLineTest() Stopwatch stopwatch = new Stopwatch(); 00000000 push ebp 00000001 mov ebp,esp 00000003 push edi 00000004 push esi 00000005 push ebx 00000006 mov ecx,685D2C68h 0000000b call FFD91F70 00000010 mov esi,eax 00000012 mov ecx,esi 00000014 call 681D68BC stopwatch.Start(); 00000019 cmp byte ptr [esi+14h],0 0000001d jne 0000002E 0000001f call 681FB3E4 00000024 mov dword ptr [esi+0Ch],eax 00000027 mov dword ptr [esi+10h],edx 0000002a mov byte ptr [esi+14h],1 int count = 0; 0000002e xor edi,edi for (int i = 0; i < 1000000000; ++i) 00000030 xor edx,edx { count += i % 16 == 0 ? 1 : 0; 00000032 mov eax,edx 00000034 and eax,8000000Fh 00000039 jns 00000040 0000003b dec eax 0000003c or eax,0FFFFFFF0h 0000003f inc eax 00000040 test eax,eax 00000042 je 00000048 00000044 xor eax,eax 00000046 jmp 0000004D 00000048 mov eax,1 0000004d add edi,eax for (int i = 0; i < 1000000000; ++i) 0000004f inc edx 00000050 cmp edx,3B9ACA00h 00000056 jl 00000032 } stopwatch.Stop(); 00000058 mov ecx,esi 0000005a call 6820F390 Console.WriteLine("Single-line test --> Count: {0}, Time: {1}", count, stopwatch.ElapsedMilliseconds); 0000005f mov ecx,6A8B29B4h 00000064 call FFD91F70 00000069 mov ecx,eax 0000006b mov dword ptr [ecx+4],edi 0000006e mov ebx,ecx 00000070 mov ecx,6A8AA240h 00000075 call FFD91F70 0000007a mov edi,eax 0000007c mov ecx,esi 0000007e call 6820ACB8 00000083 push edx 00000084 push eax 00000085 push 0 00000087 push 2710h 0000008c call 6AFF48BC 00000091 mov dword ptr [edi+4],eax 00000094 mov dword ptr [edi+8],edx 00000097 mov esi,edi 00000099 call 6A457010 0000009e push ebx 0000009f push esi 000000a0 mov ecx,eax 000000a2 mov edx,dword ptr ds:[039F2030h] 000000a8 mov eax,dword ptr [ecx] 000000aa mov eax,dword ptr [eax+3Ch] 000000ad call dword ptr [eax+1Ch] 000000b0 pop ebx } 000000b1 pop esi 000000b2 pop edi 000000b3 pop ebp 000000b4 retMulti first:
MultiLineTest(); SingleLineTest(); MultiLineTest(); SingleLineTest(); MultiLineTest(); SingleLineTest(); MultiLineTest(); -------------------------------- SingleLineTest() Stopwatch stopwatch = new Stopwatch(); 00000000 push ebp 00000001 mov ebp,esp 00000003 push edi 00000004 push esi 00000005 push ebx 00000006 mov ecx,685D2C68h 0000000b call FFF31EA0 00000010 mov esi,eax 00000012 mov dword ptr [esi+4],0 00000019 mov dword ptr [esi+8],0 00000020 mov byte ptr [esi+14h],0 00000024 mov dword ptr [esi+0Ch],0 0000002b mov dword ptr [esi+10h],0 stopwatch.Start(); 00000032 cmp byte ptr [esi+14h],0 00000036 jne 00000047 00000038 call 682AB314 0000003d mov dword ptr [esi+0Ch],eax 00000040 mov dword ptr [esi+10h],edx 00000043 mov byte ptr [esi+14h],1 int count = 0; 00000047 xor edi,edi for (int i = 0; i < 1000000000; ++i) 00000049 xor edx,edx { count += i % 16 == 0 ? 1 : 0; 0000004b mov eax,edx 0000004d and eax,8000000Fh 00000052 jns 00000059 00000054 dec eax 00000055 or eax,0FFFFFFF0h 00000058 inc eax 00000059 test eax,eax 0000005b je 00000061 0000005d xor eax,eax 0000005f jmp 00000066 00000061 mov eax,1 00000066 add edi,eax for (int i = 0; i < 1000000000; ++i) 00000068 inc edx 00000069 cmp edx,3B9ACA00h 0000006f jl 0000004B } stopwatch.Stop(); 00000071 mov ecx,esi 00000073 call 682BF2C0 Console.WriteLine("Single-line test --> Count: {0}, Time: {1}", count, stopwatch.ElapsedMilliseconds); 00000078 mov ecx,6A8B29B4h 0000007d call FFF31EA0 00000082 mov ecx,eax 00000084 mov dword ptr [ecx+4],edi 00000087 mov ebx,ecx 00000089 mov ecx,6A8AA240h 0000008e call FFF31EA0 00000093 mov edi,eax 00000095 mov ecx,esi 00000097 call 682BABE8 0000009c push edx 0000009d push eax 0000009e push 0 000000a0 push 2710h 000000a5 call 6B0A47EC 000000aa mov dword ptr [edi+4],eax 000000ad mov dword ptr [edi+8],edx 000000b0 mov esi,edi 000000b2 call 6A506F40 000000b7 push ebx 000000b8 push esi 000000b9 mov ecx,eax 000000bb mov edx,dword ptr ds:[038E2034h] 000000c1 mov eax,dword ptr [ecx] 000000c3 mov eax,dword ptr [eax+3Ch] 000000c6 call dword ptr [eax+1Ch] 000000c9 pop ebx } 000000ca pop esi 000000cb pop edi 000000cc pop ebp 000000cd ret讨论(0) -
So for a definitive answer... I suspect we would need to dig into the dis-assembly.
However, I have a guess. The compiler for the
SingleLineTest()stores each result of the equation on the stack and pops each value as needed. However, theMultiLineTest()may be storing values and having to access them from there. This could cause a few clock cycles to be missed. Where as grabbing the values off the stack will keep it in a register.Interestingly, changing the order of the function compilation may be adjusting the garbage collector's actions. Because
isMultipleOf16is defined within the loop, it may be be handled funny. You may want to move the definition outside of the loop and see what that changes...讨论(0)
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