Friday, September 8, 2017

Follow-up on “Why undefined behavior may call a never-called function”

I have recieved several questions on the previous blog post about what happens for more complex cases, such as
#include <cstdlib>

typedef int (*Function)();

static Function Do;

static int EraseAll() {
  return system("rm -rf /");
}

static int LsAll() {
  return system("ls /");
}

void NeverCalled() {
  Do = EraseAll;
}

void NeverCalled2() {
  Do = LsAll;
}

int main() {
  return Do();
}
where the compiler will find three possible values for Do: EraseAll, LsAll, and 0.

The value 0 is eliminated from the set of possible values for the call in main, in the same way as for the simpler case, but the compiler cannot change the indirect call to a direct call as there are still two possible values for the function pointer, and clang generates the expected
main:
        jmpq    *Do(%rip)
But a compiler could transform the line
return Do();
to
if (Do == LsAll)
  return LsAll();
else
  return EraseAll();
that has the same surprising effect of calling a never-called function. This transformation would be silly in this case as the cost of the extra comparison is similar to the cost of the eliminated indirect call, but it may be a good optimization when the compiler can determine that the result will be faster (for example, if the functions can be simplified after inlining). I don’t know if this is implemented in clang/LLVM — I could not get this to happen when writing some small test-programs. But, for example, GCC’s implementation of devirtualization can do it if -fdevirtualize-speculatively is enabled, so this is not a hypothetical optimization (GCC does, however, not take advantage of undefined behavior in this case, so it will not insert calls to never-called functions).

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