64-bit multiplication pitfalls

I’ve seen several instances of code recently that look something like this:

void madd(int64_t *sum, int32_t x, int32_t y)
  *sum += x * y;

Or this:

void func(int64_t);
  int32_t x, y = ...;
  func(x * y);

Unfortunately, neither of these cases do what the programmer intended.  The intended result was to compute the full 64-bit product from multiplying two 32-bit numbers.  What gets computed instead is the lower 32-bits of the desired product, sign-extended to 64-bits, which is quite different! The assembly produced by x86-64 GCC at -O2 for the first example looks like:

    imull   %edx, %esi    # int32_t multmp = x * y
    movslq  %esi, %rsi    # int64_t exttmp = static_cast<int64_t>(multmp)
    addq    %rsi, (%rdi)  # *sum += exttmp

If the full 64-bit product is desired, one of the arguments needs to be cast to a 64-bit value first.

void madd(int64_t *sum, int32_t x, int32_t y)
  *sum += static_cast<int64_t>(x) * y;

(The standard-ese for this is that operands are automatically promoted based on the types of the operands, not on the type of the result.  Integers smaller than int are promoted to int, which is what you want most of the time. Of course, here we’re dealing with things that are already int-sized[*], so we have to explicitly ask for promotion.)

which produces the desired:

    movslq  %esi, %rsi    # int64_t xtmp = static_cast<int64_t>(x)
    movslq  %edx, %rdx    # int64_t ytmp = static_cast<int64_t>(y)
    imulq   %rdx, %rsi    # int64_t multmp = xtmp * ytmp
    addq    %rsi, (%rdi)  # *sum += multmp

The above examples are semi-obvious instances, but when dealing with types whose sizes are not specified, similar problems occur. Consider replacing int64_t with off_t and int32_t with size_t in the example above. While such code will mostly work (most files are well under 2GB or 4GB in size), off_t and size_t do not need to be the same size: try compiling sizeof off_t with -D_FILE_OFFSET_BITS=64 on your favorite 32-bit Linux sometime.

[*] Assuming we’re on a fairly standard 32-bit or 64-bit machine, of course.

1 response

  1. Zack wrote on :

    You should write forced conversions among primitive numeric types using the “old-fashioned” functional cast notation, int64_t(x). This is because although static_cast is equivalent to an old-style cast for primitive numeric types, even experienced C++ programmers will think it does something subtly different for at least a moment and then have to remind themselves it doesn’t. Smoothing away all such tiny speedbumps is the difference between code which can merely be understood and code which is pleasant to read.