4 ms·

What's wrong with this code? I don't see it. (disclaimer: I am a programmer, not a compiler)

• umanwizard 11y ago
  The (expected) last iteration will try to access d[16] (which is UB) in the statement dd=d[++k].
• Someone 11y ago
  This: k<16; dd=d[++k] , whenever the code reached the end of the loop, effectively becomes dd=d[++k] if(k<16) goto startOfLoop; That, in turn is equivalent to: k+=1 dd=d[k] if(k<16) goto startOfLoop In the second line, k is used to index into array d. Conforming programs do not access memory outside of arrays, so the compiler can infer that 0 <= k < 16 Given that, it is clear that the goto startOfLoop branch will always be taken. Inside the loop, the program accesses some memory, but that data never is visible to the outside world, so the compiler can optimize away the memory reads and writes. The compiler is that aggressive because it has to be in real world programs. For example, compilers using C as their target instruction set may generate bounds checks for every array access, trusting the C compiler to remove as many as possible of them. Macros are another common cause of superfluous source statements that programmers will expect the compiler to optimize away. I think it will be very tricky to detect cases where compilers should be wary of applying 'obvious' optimizations.
  • AntonErtl 11y ago
    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*.
    • Someone 11y ago
      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.
      • AntonErtl 11y ago
        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?
• bmm6o 11y ago
  Raymond Chen has a series of blog posts (e.g. https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=633/ https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=...) that cover some surprising consequences that undefined behavior can have on program optimization. I'm kind of sympathetic to the point of view that these are abuses of the freedoms granted to the compiler, and that it would be better for it to just generate instructions that match the illegal code. But as others have pointed out, you can generally get the compiler to warn you when it detects something like this, and anyone who uses C and doesn't turn on "treat warnings as errors" gets what they deserve. If you want a language that does runtime bounds checking there are plenty to choose from. See also this related SO question, and ones it links to: http://stackoverflow.com/questions/24296571/why-does-this-loop-produce-warning-iteration-3u-invokes-undefined-behavior-an http://stackoverflow.com/questions/24296571/why-does-this-lo...