下面的代码给出了在Visual Studio内部运行版本和在Visual Studio外部运行版本时不同的输出。我使用的是Visual Studio 2008，目标是。net 3.5。我也尝试过。net 3.5 SP1。
当在Visual Studio之外运行时，JIT应该起作用。要么(a)我忽略了c#中一些微妙的东西，要么(b) JIT实际上是错误的。我怀疑JIT会出问题，但我已经没有其他可能性了……
在Visual Studio中运行时输出:
0 0,
0 1,
1 0,
1 1,
在Visual Studio外部运行release时的输出:
0 2,
0 2,
1 2,
1 2,
原因是什么?
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Test
{
struct IntVec
{
public int x;
public int y;
}
interface IDoSomething
{
void Do(IntVec o);
}
class DoSomething : IDoSomething
{
public void Do(IntVec o)
{
Console.WriteLine(o.x.ToString() + " " + o.y.ToString()+",");
}
}
class Program
{
static void Test(IDoSomething oDoesSomething)
{
IntVec oVec = new IntVec();
for (oVec.x = 0; oVec.x < 2; oVec.x++)
{
for (oVec.y = 0; oVec.y < 2; oVec.y++)
{
oDoesSomething.Do(oVec);
}
}
}
static void Main(string[] args)
{
Test(new DoSomething());
Console.ReadLine();
}
}
}
这是一个JIT优化器错误。它展开了内部循环，但没有更新oVec。Y值正确:
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
0000000a xor esi,esi ; oVec.x = 0
for (oVec.y = 0; oVec.y < 2; oVec.y++) {
0000000c mov edi,2 ; oVec.y = 2, WRONG!
oDoesSomething.Do(oVec);
00000011 push edi
00000012 push esi
00000013 mov ecx,ebx
00000015 call dword ptr ds:[00170210h] ; first unrolled call
0000001b push edi ; WRONG! does not increment oVec.y
0000001c push esi
0000001d mov ecx,ebx
0000001f call dword ptr ds:[00170210h] ; second unrolled call
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
00000025 inc esi
00000026 cmp esi,2
00000029 jl 0000000C
当你让oVec。Y增量为4，展开的调用太多了。
一个解决办法是:
for (int x = 0; x < 2; x++) {
for (int y = 0; y < 2; y++) {
oDoesSomething.Do(new IntVec(x, y));
}
}
更新:在2012年8月重新检查，这个错误在4.0.30319版本被修复。但仍然存在于v2.0.50727抖动中。在这么长时间之后，他们似乎不太可能在旧版本中修复这个问题。
我相信这是一个真正的JIT编译错误。我会向微软报告，看看他们怎么说。有趣的是，我发现x64 JIT没有同样的问题。
以下是我对x86 JIT的解读。
// save context
00000000 push ebp
00000001 mov ebp,esp
00000003 push edi
00000004 push esi
00000005 push ebx
// put oDoesSomething pointer in ebx
00000006 mov ebx,ecx
// zero out edi, this will store oVec.y
00000008 xor edi,edi
// zero out esi, this will store oVec.x
0000000a xor esi,esi
// NOTE: the inner loop is unrolled here.
// set oVec.y to 2
0000000c mov edi,2
// call oDoesSomething.Do(oVec) -- y is always 2!?!
00000011 push edi
00000012 push esi
00000013 mov ecx,ebx
00000015 call dword ptr ds:[002F0010h]
// call oDoesSomething.Do(oVec) -- y is always 2?!?!
0000001b push edi
0000001c push esi
0000001d mov ecx,ebx
0000001f call dword ptr ds:[002F0010h]
// increment oVec.x
00000025 inc esi
// loop back to 0000000C if oVec.x < 2
00000026 cmp esi,2
00000029 jl 0000000C
// restore context and return
0000002b pop ebx
0000002c pop esi
0000002d pop edi
0000002e pop ebp
0000002f ret
对我来说，这看起来像是一个坏的优化……
这是一个JIT优化器错误。它展开了内部循环，但没有更新oVec。Y值正确:
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
0000000a xor esi,esi ; oVec.x = 0
for (oVec.y = 0; oVec.y < 2; oVec.y++) {
0000000c mov edi,2 ; oVec.y = 2, WRONG!
oDoesSomething.Do(oVec);
00000011 push edi
00000012 push esi
00000013 mov ecx,ebx
00000015 call dword ptr ds:[00170210h] ; first unrolled call
0000001b push edi ; WRONG! does not increment oVec.y
0000001c push esi
0000001d mov ecx,ebx
0000001f call dword ptr ds:[00170210h] ; second unrolled call
for (oVec.x = 0; oVec.x < 2; oVec.x++) {
00000025 inc esi
00000026 cmp esi,2
00000029 jl 0000000C
当你让oVec。Y增量为4，展开的调用太多了。
一个解决办法是:
for (int x = 0; x < 2; x++) {
for (int y = 0; y < 2; y++) {
oDoesSomething.Do(new IntVec(x, y));
}
}
更新:在2012年8月重新检查，这个错误在4.0.30319版本被修复。但仍然存在于v2.0.50727抖动中。在这么长时间之后，他们似乎不太可能在旧版本中修复这个问题。
