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Comment by AntonErtl | Hacker News Reader

I would be pretty miffed if I wrote a, say, Pascal compilers with array bounds checks, and the compiler "optimized" the checks away just because accessing a[i] would be undefined behaviour. OTOH, in a loop like

for (i=0; i<16; i++) { if (i>=0 && i<16) do something with a[i] else report an error;

you can certainly optimize the if to if(1). That's an optimization*.

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No, it would not. Compilers can remove boundary checks that provably happen after accessing an item of an array, not those before the array is accessed.

For your example, the Pascal code (at least, I hope this is Pascal; it has been a while since I wrote any):

  if (i>=0 and i<16) then
  begin
    x := a[i]
  end
  else
  begin
    ReportError;
  end

is equivalent to the C code:

  if (i>=0 && i<16)
  {
    if(i>=0 && i<16)
    {
       x = a[i]
    } else {
      RuntimeAbort(“Array access outside bounds");
    }
  } else {
    ReportError();
  }

A good Pascal compiler, like a good C compiler, would optimise away that second boundary check and the call to RuntimeAbort. Neither compiler is allowed to optimise away the first check.

The point of my posting was that you don't need "optimizations" to optimize away redundant bounds checks, optimization* is able to do it just fine. Sorry if I did not get that across clearly. What does your "No, it would not" refer to?

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