Created attachment 1479738 [details]
Test code

Description of problem:
I have a certain loop in my code where I get unexpected values in variables updated in the loop body. The problem is related to conversion or comparison between double and integer types.

Version-Release number of selected component (if applicable):
4.9.3-1.el7

How reproducible:
Every time

Steps to Reproduce:
1. On Linux,
i686-w64-mingw32-g++ -mms-bitfields -O2 -I/usr/i686-w64-mingw32/sys-root/mingw/include -L/usr/i686-w64-mingw32/sys-root/mingw/lib testLoop.cpp -o testLoop.exe
2. Copy testLoop.exe and the necessary MinGW libs to a Windows system (I assume you know how.)
3. On Windows,
cd where-you-put-the-files
testLoop.exe

Actual results:
 0 0 0 0
 0.344262 0 0 0
 0.688525 0 0 0
 1.03279 1 1 1
 1.37705 1 1 1
 1.72131 1 1 1
 2.06557 2 2 2
 2.40984 2 2 2
 2.7541 2 2 2
 3.09836 3 3 3
 3.44262 3 3 3
 3.78689 3 3 3
 4.13115 4 4 4
 4.47541 4 4 4
 4.81967 4 4 4
 5.16393 5 5 5
 5.5082 5 5 5
 5.85246 5 5 5
 6.19672 6 6 6
 6.54098 6 6 6
 6.88525 6 6 6
 7.22951 7 7 7
 7.57377 7 7 7
 7.91803 7 7 7
 8.2623 8 8 8
 8.60656 8 8 8
 8.95082 8 8 8
 9.29508 9 9 9
 9.63934 9 9 9
 9.98361 9 9 9
 10.3279 10 10 10
 10.6721 10 10 10
 11.0164 11 11 11
 11.3607 11 11 11
 11.7049 11 11 11
 12.0492 12 12 12
 12.3934 12 12 12
 12.7377 12 12 12
 13.082 13 13 13
 13.4262 13 13 13
 13.7705 13 13 13
 14.1148 14 14 14
 14.459 14 14 14
 14.8033 14 14 14
 15.1475 15 15 15
 15.4918 15 15 15
 15.8361 15 15 15
 16.1803 16 16 16
 16.5246 16 16 16
 16.8689 16 16 16
 17.2131 17 17 17
 17.5574 17 17 17
 17.9016 17 17 17
 18.2459 18 18 18
 18.5902 18 18 18
 18.9344 18 18 18
 19.2787 19 19 19
 19.623 19 19 19
 19.9672 19 19 19
 20.3115 20 20 20
 20.6557 20 20 20
SHOULD NOT BE HERE
 21 20 21 21

Expected results:
 0 0 0 0
 0.344262 0 0 0
 0.688525 0 0 0
 1.03279 1 1 1
 1.37705 1 1 1
 1.72131 1 1 1
 2.06557 2 2 2
 2.40984 2 2 2
 2.7541 2 2 2
 3.09836 3 3 3
 3.44262 3 3 3
 3.78689 3 3 3
 4.13115 4 4 4
 4.47541 4 4 4
 4.81967 4 4 4
 5.16393 5 5 5
 5.5082 5 5 5
 5.85246 5 5 5
 6.19672 6 6 6
 6.54098 6 6 6
 6.88525 6 6 6
 7.22951 7 7 7
 7.57377 7 7 7
 7.91803 7 7 7
 8.2623 8 8 8
 8.60656 8 8 8
 8.95082 8 8 8
 9.29508 9 9 9
 9.63934 9 9 9
 9.98361 9 9 9
 10.3279 10 10 10
 10.6721 10 10 10
 11.0164 11 11 11
 11.3607 11 11 11
 11.7049 11 11 11
 12.0492 12 12 12
 12.3934 12 12 12
 12.7377 12 12 12
 13.082 13 13 13
 13.4262 13 13 13
 13.7705 13 13 13
 14.1148 14 14 14
 14.459 14 14 14
 14.8033 14 14 14
 15.1475 15 15 15
 15.4918 15 15 15
 15.8361 15 15 15
 16.1803 16 16 16
 16.5246 16 16 16
 16.8689 16 16 16
 17.2131 17 17 17
 17.5574 17 17 17
 17.9016 17 17 17
 18.2459 18 18 18
 18.5902 18 18 18
 18.9344 18 18 18
 19.2787 19 19 19
 19.623 19 19 19
 19.9672 19 19 19
 20.3115 20 20 20
 20.6557 20 20 20

Additional info:
I get the "expected results" if
1. Remove "-O2" from the compile command.
2. I compile and run natively on my 64-bit Linux system.
3. Build a 64-bit Windows version, with our without "-O2" - e.g.
x86_64-w64-mingw32-g++ -O2 -mms-bitfields -I/usr/x86_64-w64-mingw32/sys-root/mingw/include -L/usr/x86_64-w64-mingw32/sys-root/mingw/lib testLoop.cpp -o testLoop.exe

1.) leads me to conclude that there must be an issue with how loops are optimised when -O2 is enabled.

I get similar issues with
for(double pos=0; pos<classValues.size(); pos+=step) {
  size_t index=static_cast<size_t>(pos);
  ...

I'm testing on an old workstation running 64-bit Windows 7 enterprise.