Skillshare: Finding uninitialised variables in Fortran

Why care about uninitialised variables?

• Can result in a random value at an address in memory (garbage value) being loaded into an unintialised variable.
• Often leads to non-reproducible behaviour of the program.
• Often visible when changing systems or compilers.
• Has a corrosive effect via the requirement to maintain legacy software/environments to preserve the so-called “correct” functioning of some application

! src/examples/ex1.F90
program main
    use iso_fortran_env, only: real32
    implicit none
    real(kind=real32) :: d
    print *, d
    write(*, '(z8)') d ! as hexadecimal
end program main

Compiling the above program and running it three times:

gfortran src/ex1.F90 && for run in {1..3}; do ./a.out; done

Outputs:

  -1.35553568E+33
F685AA88
   2.16538873E-35
 5E6437D
   8.91414933E+30
72E10657

Common approaches for detecting use of uninitialised data:

ifort

• -check uninit
• -ftrapuv

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-check uninit

• Enables run-time checking for uninitialized variables. Only local scalar variables of intrinsic type INTEGER, REAL, COMPLEX, and LOGICAL without the SAVE attribute are checked.
• Unreliable and gives false positives (see ex2.F90)

-ftrapuv

• Initialises stack local variables to an unusual value to aid error detection.
• Floating point variables are initialised to signaling not a number (SNAN) (i.e. enables -init snan)
• Must be used in combination with -fpe0 to produce exceptions.

---

What about arrays?

See ex3.F90.

---

ifx

• LLVM Memory Sanitiser introduced in -check uninit for ifx version 2023.2.0.
  • Similar to valgrind.
  • Hard to diagnose output in complex programs.

---

gfortran

• -fcheck=all
• -finit-real=snan -ffpe-trap=invalid

---

Valgrind?

Using DDT's memory debugging library

• DDT is a debugger for high perfomance computing programs: https://www.linaroforge.com/linaro-ddt/
• Proven to be the most useful in detecting uninitialised variables.

Conclusion:

• There is yet to be a perfect tool for finding all cases of uninitialised variable usage.
• Compilers are continually improving their capability for debugging uninitialised variables.
  • Best to always use the latest compiler when possible.

Resources:

• https://www.nas.nasa.gov/hecc/assets/pdf/training/UnInit_Fix_your_code_2012_10_31.pdf

  • Recording can be found at: https://www.nas.nasa.gov/hecc/support/past_webinars.html

• LLVM Memory Sanitiser: https://www.intel.com/content/www/us/en/developer/videos/townhall-with-intel-fortran-compiler-developers.html