#include <iostream>

void foo(size_t size, const double * J, double * omega)
{
    for( size_t i=0; i<size; ++i )
        omega[i] = - J[ i*(size+1) ]; // если без минуса, то все работает нормально.
}

int main(int argc, char *argv[])
{
    const size_t size=6;
    double J[size*size] = {
        11.,12.,13.,14.,15.,16.,
        21.,22.,23.,24.,25.,26.,
        31.,32.,33.,34.,35.,36.,
        41.,42.,43.,44.,45.,46.,
        51.,52.,53.,54.,55.,56.,
        61.,62.,63.,64.,65.,66.,
        };

    double omega[size] ={0};

    foo(size, J, omega);

    for(size_t i=0; i< size; ++i)
        if( omega[i] != -J[i*(size+1)] )
            std::cout<< "Error!!! : i="<<i<<" omega=" << omega[i]  << " -J=" << -J[i*(size+1)] << std::endl;

    return 0;
}

#include <iostream>
#include <iterator>
#include <numeric>
using namespace std;

__forceinline
size_t index(size_t i, size_t size)
{
    return i*(size+1);
}

__declspec(noinline) // affects bug
void foo(size_t size, const double * const X, double * const Y)
{
    for(size_t i=0; i!=size; ++i) // replacing size_t with unsigned affects bug
        Y[i] = -X[index(i, size)]; // negation affects bug
}

int main()
{
    const size_t size = 100;
    cout << "size=" << size << endl;
    
    double X[size*size], Y[size];
    iota(begin(X), end(X), 1000.);

    foo(size, X, Y);

    size_t errors = 0;
    for(size_t i=0; i!=size; ++i)
    {
        const double x = X[index(i, size)], y = -Y[i];
        if(x != y)
        {
            ++errors;
            if(errors < 5)
                cout << "i=" << i
                     << " x=" << x << " y=" << y << endl;
        }
    }
    cout << "total errors=" << errors << endl;
}

cl
cl -Od main.cpp >> out.txt 2> err.txt
main.exe
del main.exe
cl -O2 main.cpp >> out.txt 2> err.txt
main.exe
del main.exe

C:\vs2012_fp_bug>test.cmd

C:\vs2012_fp_bug>cl
Оптимизирующий 32-разрядный компилятор Microsoft (R) C/C++ версии 16.00.40219.01 для 80x86
(C) Корпорация Майкрософт (Microsoft Corporation). Все права защищены.

использование: cl [ параметр... ] имя_файла... [ /link параметр_компоновки... ]

C:\vs2012_fp_bug>cl -Od main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>cl -O2 main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>test.cmd

C:\vs2012_fp_bug>cl
Оптимизирующий компилятор Microsoft (R) C/C++ версии 16.00.40219.01 для x64
(C) Корпорация Майкрософт (Microsoft Corporation). Все права защищены.

использование: cl [ параметр... ] имя_файла... [ /link параметр_компоновки... ]

C:\vs2012_fp_bug>cl -Od main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>cl -O2 main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>test.cmd

C:\vs2012_fp_bug>cl
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.50727.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

C:\vs2012_fp_bug>cl -Od main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>cl -O2 main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
i=1 x=1101 y=1001
i=3 x=1303 y=1203
i=5 x=1505 y=1405
i=7 x=1707 y=1607
total errors=50

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>test.cmd

C:\vs2012_fp_bug>cl
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.50727.1 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

C:\vs2012_fp_bug>cl -Od main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
total errors=0

C:\vs2012_fp_bug>del main.exe

C:\vs2012_fp_bug>cl -O2 main.cpp  1>>out.txt 2>err.txt

C:\vs2012_fp_bug>main.exe
size=100
i=1 x=1101 y=1001
i=3 x=1303 y=1203
i=5 x=1505 y=1405
i=7 x=1707 y=1607
total errors=50

C:\vs2012_fp_bug>del main.exe

D:\WB\tests\test_msvc_o2>cl -O2 main.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.61030 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

main.cpp
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\INCLUDE\xlocale(336) : warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
Microsoft (R) Incremental Linker Version 11.00.61030.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

D:\WB\tests\test_msvc_o2>main.exe
Error!!! : i=1 omega=-12 -J=-22
Error!!! : i=3 omega=-34 -J=-44

D:\WB\tests\test_msvc_o2>cl -O2 main.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.61030 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

main.cpp
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\INCLUDE\xlocale(336) : warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
Microsoft (R) Incremental Linker Version 11.00.61030.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

D:\WB\tests\test_msvc_o2>main.exe
Error!!! : i=1 omega=-12 -J=-22
Error!!! : i=3 omega=-34 -J=-44
Error!!! : i=5 omega=-56 -J=-66

#include <iostream>

void foo(size_t size, const double * J, double * omega)
{
    for( size_t i=0; i<size; ++i )
        omega[i] = - J[ i*(size+1) ]; // если без минуса, то все работает нормально.
}

int main(int argc, char *argv[])
{
    const size_t size=6;
    double J[size*size] = {
        11.,12.,13.,14.,15.,16.,
        21.,22.,23.,24.,25.,26.,
        31.,32.,33.,34.,35.,36.,
        41.,42.,43.,44.,45.,46.,
        51.,52.,53.,54.,55.,56.,
        61.,62.,63.,64.,65.,66.,
        };

    double omega[size] ={0};

    foo(size, J, omega);

    for(size_t i=0; i< size; ++i)
        if( omega[i] != -J[i*(size+1)] )
            std::cout<< "Error!!! : i="<<i<<" omega=" << omega[i]  << " -J=" << -J[i*(size+1)] << std::endl;

    return 0;
}

Error!!! : i=1 omega=-12 -J=-22
Error!!! : i=3 omega=-34 -J=-44
Error!!! : i=5 omega=-56 -J=-66

// Bug in VS2012 11.0.50727.1 x64 Release /O2
// Output is: i=1 Y=-1001 X=1005

#include <iostream>
#include <iterator>
#include <numeric>
using namespace std;

size_t index(size_t i, size_t size)
{
    return i*(size+1);
}

__declspec(noinline) // affects bug
void foo(size_t size, const double * const X, double * const Y)
{
    for(size_t i=0; i!=size; ++i) // replacing size_t with unsigned affects bug
        Y[i] = -X[index(i, size)]; // negation affects bug
}

int main()
{
    const size_t size=4;
    double X[size*size], Y[size];
    iota(begin(X), end(X), 1000.);
    foo(size, X, Y);
    const size_t i = 1;
    cout << "i=" << i << " Y=" << Y[i] << " X=" << X[index(i, size)] << endl;
}

; Function compile flags: /Ogtpy
;    COMDAT ?foo@@YAX_KQEBNQEAN@Z
_TEXT    SEGMENT
size$ = 16
X$ = 24
Y$ = 32
?foo@@YAX_KQEBNQEAN@Z PROC                ; foo, COMDAT
; File c:\vs2012_fp_bug\main.cpp
; Line 14
$LN18:
    sub    rsp, 8
; Line 15
    xor    r9d, r9d
    mov    r11, rdx
    mov    r10, rcx
    test    rcx, rcx
    je    $LN1@foo
    movapd    xmm1, XMMWORD PTR __xmm@80000000000000008000000000000000
    cmp    rcx, 4
    jb    $LN11@foo
; Line 16
    mov    QWORD PTR [rsp+16], rbx
    lea    rbx, QWORD PTR [rcx+1]
    lea    rdx, QWORD PTR [r8-8]
    lea    rdx, QWORD PTR [rdx+rcx*8]
    lea    rcx, QWORD PTR [rcx*2-1]
    lea    rbx, QWORD PTR [r11+rbx*8]
    lea    rcx, QWORD PTR [rcx+1]
    lea    rcx, QWORD PTR [r11+rcx*8]
    cmp    r8, rcx
    ja    SHORT $LN12@foo
    cmp    rdx, rbx
    jae    SHORT $LN16@foo
$LN12@foo:
    mov    rax, r10
    mov    QWORD PTR [rsp], rdi
    lea    rbx, QWORD PTR [r10+1]
    and    eax, 3
; Line 15
    lea    rdi, QWORD PTR [r10+1]
    mov    rdx, r10
    sub    rdx, rax
    shl    rbx, 5
    mov    rcx, r8
    mov    rax, r11
    add    rdi, rdi
    npad    4
$LL3@foo:
; Line 16
    movupd    xmm0, XMMWORD PTR [rax]
    add    r9, 4
    lea    rcx, QWORD PTR [rcx+32]
    xorpd    xmm0, xmm1
    movupd    XMMWORD PTR [rcx-32], xmm0
    movupd    xmm0, XMMWORD PTR [rax+rdi*8]
    add    rax, rbx
    xorpd    xmm0, xmm1
    movupd    XMMWORD PTR [rcx-16], xmm0
    cmp    r9, rdx
    jne    SHORT $LL3@foo
    mov    rdi, QWORD PTR [rsp]
$LN16@foo:
    mov    rbx, QWORD PTR [rsp+16]
$LN11@foo:
; Line 15
    cmp    r9, r10
    je    SHORT $LN1@foo
    lea    rax, QWORD PTR [r10+1]
    lea    rdx, QWORD PTR [r10*8+8]
    imul    rax, r9
    lea    rcx, QWORD PTR [r11+rax*8]
    npad    3
$LL10@foo:
; Line 16
    movsdx    xmm0, QWORD PTR [rcx]
    inc    r9
    add    rcx, rdx
    xorpd    xmm0, xmm1
    movsdx    QWORD PTR [r8+r9*8-8], xmm0
    cmp    r9, r10
    jne    SHORT $LL10@foo
$LN1@foo:
; Line 17
    add    rsp, 8
    ret    0
?foo@@YAX_KQEBNQEAN@Z ENDP                ; foo

const double x = X[index(i, size)], y = -Y[i];
        if(x != y)