[Bug c++/47282] New: GCC 4.2 gives different result for the same computation with -O3, depending on irrelevant code

[ichudov at algebra dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47282

           Summary: GCC 4.2 gives different result for the same
                    computation with -O3, depending on irrelevant code
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ichudov@algebra.com


Created attachment 22961
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22961
tstdouble.cpp -- compile with gcc-4.2 -O3 -o tstdouble tstdouble.cpp

The attached C++ code depends only on stdio.h and string.h.

It calls the same function (DoubleToInt) twice: once from main( ... ) and once
from Test( ... ). 

Both main() and Test() call DoubleToInt with exact same values of v and factor. 

DoubleToInt include some junk code surrounded by "if( False() ) { ... }". That
junk code, though it never executes, has the effect of altering the result of
calling DoubleToInt, so the second call's result does not match the first
call's result. 

Even taking out a couple of lines from the big condition of if statement inside
if( False() ) { ... }, causes the values to become correct.

Similarly, taking out printf in DoubleToInt, also causes the printed values to
be correct (identical). 

So some flakiness exists in computations, depending on irrelevant code that
never executes. We tried very hard to distill it into smallest possible code. 

This seems to NOT occur in GCC 4.3 and GCC 4.4. It only occurs with -O3.

To compile: 

   gcc-4.2 -O3 -o tstdouble tstdouble.cpp