Debugging at the edge of reason
May 1, 2019
Wes Hinsley
5 minute read

What do you do if this code…

printf("Before loop\n");
while ((a>0) && (b>0)) {
  printf("Inside loop\n");
  // Other loop code
}
if (a>0) printf("a>0\n");
if (b>0) printf("b>0\n");
if ((a>0) && (b>0)) printf("Both a>0 and b>0!");

…produces this output?

Before loop
a>0
b>0
Both a>0 and b>0!

If a and b are both above zero, then why didn’t the while-loop do any iteration; why didn’t we see Inside loop in the output?

This was a situation we were in with DDE 0.9.0, but only on Windows, and only when compiled with 32-bit gcc. With 64-bits, or linux, the loop behaved itself perfectly.

But this is almost the end of the story. The issue was first caught by a test that failed in Appveyor, which we use for continuous integration tests in Windows. DDE is a package that solves ordinary differential equations (ODEs), delay differential equations (DDEs) and discrete-time difference (or recursion) equations, perhaps involving delays - and all its tests were passing except one: about the simplest case possible, a function that always returns 1.

  deriv <- function(t, y, p) { 1 }
  dde::dopri(0, c(0, 1), deriv, 0)

On 64-bit, it correctly returned (1,1); on 32-bit, it returned (1,0).

Primary Approaches

At first, we wondered if it was a memory issue specific to 32-bit. We confirmed that sizeof(size_t) is 4 on 32-bit, but 8 on 64-bit, as are all pointers. Other types were the same across platforms. But there were no obvious violations; Valgrind on linux picked up nothing.

Resorting to random black-box-bashing: dde:dopri(0, c(0, 1.121), deriv, 0) and smaller numbers failed the test. dde:dopri(0, c(0, 1.122), deriv, 0) and larger numbers all passed. Eureka? Nope - a mysterious red herring, alas.

In the real-life code, the (a>0) and (b>0) were a bit more complicated - but not much; no functions involved, just comparisons between members of a struct, so no sensible way they could exit the loop, and then appear to have met the criteria for looping in retrospect, as we were seeing.

Secondary Flailings

The next level of desperation involved dumping every member of the object at intervals with Rprintf. This revealed some apparent corruption - surely evidence of some bad memory writes! Wrong again. This time it was a bug in my debugging, as the object had been freed by then so could rightly return undefined values.

However, printing everything at regular points eventually did expose a genuine difference between platforms in one index variable; its value seemed to differ across platforms immediately after the while loop above, which must have terminated early (or didn’t get entered at all), under certain 32-bit circumstances. Printing things within the loop - or before it - somtimes changed the behaviour and made things work, but it would be poor form to submit a PR that claims to fix a functional problem by printing a ton of waffle before-hand.

My github commit titles around this time are telling. Total Confusion Reigns towards Wits’ End and later a Deeper Confusion that’s somehow worse than the previous total one. Eventually, as hope ebbed away, the crucial summit arrived at a commit aptly named Another bizarre effort.

The question now moves from “why doesn’t it work” to, “why does it”:-

bool cond = (a>0) && (b>0);
while (((a>0) && (b>0)) || cond) {
  // a and b may get changed
  cond = (a>0) && (b>0);
}

The syntax is surely equivalent to the code at the top, if more wordy with some redundancy. A good PR might recommend cleaning it up back into the code at the top. Nevertheless, looking at a and b twice for each decision caused correct entry and exit of the loop, and the appveyor tests passed fully.

The Final Insult

Good programmers should be cautious to blame their compiler, as in almost every case where suspicion falls on gcc, or Intel’s or Microsoft’s compiler, eventually analysis hunts down a memory leak or bad array write, or uninitialised variable; the compiler is shown to be correct and sane; the programmer a bit less so (probably on both points by that time).

In this case though, gcc was to blame, and the optimised code even with -O1 changed the behaviour of the loop, breaking the simple test. It didn’t optimise the loop away entirely, since all the many other tests worked consistently throughout the saga; it caused a subtle and rare change in behaviour - and only for 32-bits. If I had edited %USERPROFILE%\Documents\.R\Makevars and set the Rtools-specific flags for gcc like this:

CFLAGS=-O0 -g -std=gnu99 -Wall

a bit earlier (which produces the right answer), I might have doubted gcc sooner. Or would I? Faulty compiler optimisations and memory usage bugs can have similar side effects; both can do strange things when the code is tickled by an innocuous print command. Both can disappear when you turn optimisation off. (User memory bugs can also seemingly disappear when you turn optimisation on…)

We briefly looked at the assembler code produced by the compiler, (-S on the gcc command line. It helps if the fragments of code you’re comparing have exactly matching line numbers, since assembler code is made of gotos.) But while we could tell the code was different with -O1, it would take a special kind of assembler programmer to show us why exactly the behaviour changed as it did.

In the end, we cleaned up my bizarre effort slightly, using a variable some distance away in terms of scope to decide loop iterations, which produced both better-looking code, and coaxed the compiler away from its more outlandish optimisations.

Conclusion

So what to say about all this? Firstly, where there is platform-dependent code, running CI tests on multiple platforms and architectures is essential. Without that, we might have been chasing this issue blindly from a user report after release.

Secondly, debugging this sort of code is hard anyway, and the tools for debugging the R/C combination on Windows are limited. And thirdly, Rtools could consider using a more recent gcc compiler (4.9.3 comes from June 2015).

But until then; on the bright side it’s solid character-building stuff, and common sense and simple debugging tests do eventually prevail, along with patience and occasional prayer. On the other hand, since the gcc used in Rtools 3.5 is necessarily the same as the toolchain used for building R itself on windows, perhaps we’re all doomed. But only in 32-bits. Probably.




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