Problem
When running the release inside Visual Studio and outside Visual Studio, the following code produces various results. I’m working with Visual Studio 2008 and.NET 3.5. I’ve also tried.NET 3.5 Service Pack 1 (SP1).
The JIT should kick in when operating outside of Visual Studio. Either (a) there’s something subtle going on with C# that I’m overlooking, or (b) the JIT is broken. I’m not optimistic that the JIT will fail, but I’m running out of options…
When using Visual Studio, the following is the output:
0 0,
0 1,
1 0,
1 1,
When you run release outside of Visual Studio, you’ll get the following results:
0 2,
0 2,
1 2,
1 2,
What is the explanation for this?
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();
}
}
}
Asked by Philip Welch
Solution #1
It’s a bug in the JIT optimizer. It unrolls the inner loop but fails to update the oVec.y value:
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
When you set oVec.y to 4, the problem goes away because there are too many calls to unroll.
Here’s an example of a workaround:
for (int x = 0; x < 2; x++) {
for (int y = 0; y < 2; y++) {
oDoesSomething.Do(new IntVec(x, y));
}
}
UPDATE: This error was resolved in the version 4.0.30319 jitter, which was re-checked in August 2012. However, the jitter in v2.0.50727 is still visible. After all this time, it appears improbable that they will repair this in the old version.
Answered by Hans Passant
Solution #2
This appears to be a true JIT compilation problem. I’d file a report with Microsoft and see what they have to say. Surprisingly, I discovered that the x64 JIT does not suffer from the same issue.
This is my interpretation of the x86 JIT.
// 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
To me, this appears to be a failed optimization…
Answered by Nick Guerrera
Solution #3
I took your code and pasted it into a new Console App.
So it’s the x86 JIT that’s creating the code wrong. I’ve removed my earlier text concerning loop reordering and the like. A few other answers on here have confirmed that the JIT is unwinding the loop incorrectly when on x86.
To fix the problem, modify IntVec’s declaration to a class, which works in all flavors.
I believe this should be posted on MS Connect….
-1 to Microsoft!
Answered by Andras Zoltan
Post is based on https://stackoverflow.com/questions/2056948/net-3-5-jit-not-working-when-running-the-application