Visual Studio 2015 Update 3 - C++ Compiler bug?

Question

We observe a strange case where in VS2015 Update3 compiler will omit part of the code for no obvious reason.

We found out that

• This happens in VS2015 Update3 (Help|About says 14.0.25431.01 Update 3, cl.exe version 19.00.24215.1)
• This doesn't happen in VS2015 Update2 (Help|About says 14.0.25123.00 Update 2, cl.exe version 19.00.23918)
• This happens only when optimization is turned on (for example, in default Release configuration)
• Happens in both x86 and x64
• Happens when code snippet is inserted into brand new "Win32 Console Application" (I mean, no fancy commandline options required)

We managed to minimize the culprit code to this snippet:

#include <stdio.h>
#include <tchar.h>
#include <stdlib.h>

int _tmain(int, _TCHAR*[])
{
    volatile int someVar = 1;

    const int indexOffset = someVar ? 0 : 1;    // Loop omitted
    // const int indexOffset = !someVar;        // Loop omitted
    // const int indexOffset = 0;               // Good
    // const int indexOffset = 1;               // Good
    // const int indexOffset = someVar;         // Good
    // const int indexOffset = someVar + 1;     // Good

    for (int i = 1 - indexOffset; i < 2 - indexOffset; ++i)
    {
        printf("Test passed\n");
    }

    return 0;
}

For the lines that say "Loop omitted", the entire loop body is omitted by compiler. Why? To my knowledge, there is no undefined behavior involved.

---

Disassembly for the first "Loop omitted":

int _tmain(int, _TCHAR*[])
{
01151010  push        ebp  
01151011  mov         ebp,esp  
01151013  push        ecx  
    volatile int someVar = 1;
01151014  mov         dword ptr [ebp-4],1  

    const int indexOffset = someVar ? 0 : 1;    // Loop omitted
0115101B  mov         eax,dword ptr [someVar]  
    // const int indexOffset = !someVar;        // Loop omitted
    // const int indexOffset = 0;               // Good
    // const int indexOffset = 1;               // Good
    // const int indexOffset = someVar;         // Good
    // const int indexOffset = someVar + 1;     // Good

    for (int i = 1 - indexOffset; i < 2 - indexOffset; ++i)
    {
        printf("Test passed\n");
    }

    system("pause");
0115101E  push        offset string "pause" (011520F8h)  
01151023  call        dword ptr [__imp__system (0115205Ch)]  
01151029  add         esp,4  
    return 0;
0115102C  xor         eax,eax  
}
0115102E  mov         esp,ebp  
01151030  pop         ebp  
01151031  ret

---

Test project: http://dropmefiles.com/S7mwT

---

Try it online!

• Go to http://webcompiler.cloudapp.net/
• Put sample code to the editor
• Put /O2 to Additional compiler flags
• Check Run executable after compilation

---

Bug report: https://developercommunity.visualstudio.com/content/problem/71906/compiler-optimization-code-generation-bug.html

Answer 1

Yes, it's a bug. Specifically, it's a bug in the new SSA optimizer introduced in VS2015 Update 3. The undocumented command line option -d2SSAOptimizer- tells the compiler backend to use the old optimizer instead, which causes the bug to not manifest.

FYI, you can minimize your repro to:

int main()
{
    volatile int someVar = 1;

    const int indexOffset = someVar ? 0 : 1;

    for (int i = 1 - indexOffset; i < 2 - indexOffset; ++i)
    {
        return 0;
    }
    return 1;
}

which will help the compiler developers localize the problem more quickly.

---

Addition from Codeguard (I decided that Casey's answer should be THE answer): I have received reply from Microsoft (Gratian Lup, author of blog post Introducing a new, advanced Visual C++ code optimizer):

Yes, this is indeed a bug in the SSA Optimizer itself - usually most bugs reported as being in the new optimizer are in other parts, sometimes exposed now after 20 years.

It's in a small opt. that tries to remove a comparison looking like (a - Const1) CMP (a - Const2), if there is no overflow. The issue is that your code has (1 - indexOffset) CMP (2 - indexOffset) and subtraction is not commutative, of course - but the optimizer code disregards that and handles (1 - indexOffset) as if it's (indexOffset - 1).

A fix for this issue will be released in the next larger update for VS2017. Until then, disabling the SSA Optimizer would be a decent workaround. Disabling optimizations for only this function may be a better approach if it doesn't slow down things too much. This can be done with #pragma optimize("", off): https://msdn.microsoft.com/en-us/library/chh3fb0k.aspx