Bug 1515209

Created attachment 1355685 [details]
code to reproduce this bug with C++

Description of problem:
gdb reports the wrong contents of a variable, while printing the content of the variable to stdout provides good results

Version-Release number of selected component (if applicable):
8.0.1-30.fc26

How reproducible:
Compile this code
```
$ cat test.cpp
#include <iostream>

int main(int argc, char** argv){

    double rvalue = 0.3;
    std::cout << "- " << rvalue << "\n";
    double a,b;

    rvalue = 0.3;

    std::cout << ": " << rvalue << "\n";
    return rvalue < 1. ? 0 : 1;

}
```
with gcc/g++ and run it inside gdb. the contents of rvalue are printed to the screen in a bogus fashion. Only after the second assignment of "rvalue", gdb prints the correct result. Motivates wrong conclusions in practice.

Steps to Reproduce:
1. g++ -mavx -g test.cpp
2. gdb ./a.out
3. (gdb) b test.cpp:5
4. (gdb) rrun
5. (gdb) next
6. (gdb) p rvalue
$1 = 7.0607433379231282e-184


Actual results:
$1 = 7.0607433379231282e-184

Expected results:
$1 = 0.29999999

Additional info:
- the example above is a contrived example that reproduces the behavior found with a real development project
- Also found this behavior on fc25 with 7.12.1-48.fc25 before upgrading
- Also observed this on Ubuntu 16.04.3 with 7.11.1-0ubuntu1~16.5
- running the code, prints the right results to stdout
- the behavior can be reproduced with C