3 ms·

It just isn't that simple in practice. Modern compilers automatically apply a plethora of different rules in sequence when transforming code. This results in a

It just isn't that simple in practice. Modern compilers automatically apply a plethora of different rules in sequence when transforming code. This results in a long chain of transformations where each operation is perfectly reasonable when considered on its own. Unfortunately, for certain inputs, the sum total occasionally appears demonic. The current state of the art is such that there is no way to reliably prevent this that doesn't also severely reduce the optimization abilities of modern compilers. Realistically, adding such language to the standard would (at best) result in worse optimizations by default and additional compiler flags to reenable the more aggressive "nonstandard" ones. (Such flags already exist for various arithmetic operations.)

a1369209993 5y ago
> where each operation is perfectly reasonable when considered on its own. No, it is not. For example, the transformation: int read_and_discard = *p; // vvvv int read_and_discard = *p; __unsafe_assume_always(p != 0); is not reasonable, since p is not, in fact, always nonnull.
- d110af5ccf 5y ago
  Your examples are far too simplistic. Real world code is going to be far more complex and will tend to resist trivial analysis. For example, please explain how to prevent this without also (inadvertently) preventing the removal of unnecessary null checks when functions are inlined. What about an unnecessary null check that's hidden inside a macro? What about whole program LTO? A macro could be used in a variety of situations. In some of them, you need the null check for safety. In others, the null check in entirely redundant. Short of AGI, the compiler can't actually comprehend the code it operates on. Yet surely you expect it to eliminate "obviously" redundant work? It accomplishes this by applying a large set of fairly simple rules. (There are probably much better examples but I can't think of them off the top of my head.) In practice, when I play with Godbolt I find that the "obvious" cases tend to result in helpful diagnostic messages.
  - a1369209993 5y ago
    > For example, please explain how to prevent this without also (inadvertently) preventing the removal of unnecessary null checks when functions are inlined. What about an unnecessary null check that's hidden inside a macro? If a null check is unnecessary, that's because it is reachable only from the not-null side of some previous null check. The compiler can track that information just fine, using the same tools it uses to track false information derived pointer dereferences. If: if(!p) abort(); // we know p is non-null here, regardless of the dereference use(*p); MACRO(p); expands to: if(!p) abort(); // first null check (this is relevant to optimizing out unnecessary null checks) use(*p); // pointer dereference (this isn't) if(!p) abort(); // second null check (unnecessary *because of the if*) utilize(*p); // more pointer dereference the compiler can optimize out the second if (the "unnecessary null check" you refer to) based on the first if, regardless of whether the pointer dereference is even there.
    - atq2119 5y ago
      This is still too simplistic. Think along the lines of: void foo(T *p) { if (!p) abort(); bar(p); } void bar(T *p) { use(*p); MACRO(p); } In other words, the first null-check and the latter check are in different functions that may not even be in the same compilation unit, or where the call sequence is hard to reason about due to function pointers etc.
      - a1369209993 5y ago
        Assuming bar isn't inlined (eg separate compilation unit, as you mentioned), the null check in MACRO is not unnecessary, because bar can be called from places other than foo, and those places might pass null pointer. This is one of several situations where it might be useful for the compiler to excercise it's perogative to implement operations without regard for undefined behaviour to rewrite `use(*p);` as `if(!p) abort(); use(*p);`, which would make the null check in MACRO unnecessary, but unless it does so, the check is not unnecessary, just insufficient.
- zzo38computer 5y ago
  I disagree. The transformation should be valid; if p is null then the result is undefined and may crash, including the rest of the program after that occurs. However, I do think that it should not make such a transformation for a volatile read/write; in that case it should not make such an assumption (who knows if it is meaningful on the target computer or some sort of emulator or operating system or debugger or whatever).

a1369209993 5y ago

> where each operation is perfectly reasonable when considered on its own. No, it is not. For example, the transformation: int read_and_discard = *p; // vvvv int read_and_discard = *p; __unsafe_assume_always(p != 0); is not reasonable, since p is not, in fact, always nonnull.

d110af5ccf 5y ago
Your examples are far too simplistic. Real world code is going to be far more complex and will tend to resist trivial analysis. For example, please explain how to prevent this without also (inadvertently) preventing the removal of unnecessary null checks when functions are inlined. What about an unnecessary null check that's hidden inside a macro? What about whole program LTO? A macro could be used in a variety of situations. In some of them, you need the null check for safety. In others, the null check in entirely redundant. Short of AGI, the compiler can't actually comprehend the code it operates on. Yet surely you expect it to eliminate "obviously" redundant work? It accomplishes this by applying a large set of fairly simple rules. (There are probably much better examples but I can't think of them off the top of my head.) In practice, when I play with Godbolt I find that the "obvious" cases tend to result in helpful diagnostic messages.
- a1369209993 5y ago
  > For example, please explain how to prevent this without also (inadvertently) preventing the removal of unnecessary null checks when functions are inlined. What about an unnecessary null check that's hidden inside a macro? If a null check is unnecessary, that's because it is reachable only from the not-null side of some previous null check. The compiler can track that information just fine, using the same tools it uses to track false information derived pointer dereferences. If: if(!p) abort(); // we know p is non-null here, regardless of the dereference use(*p); MACRO(p); expands to: if(!p) abort(); // first null check (this is relevant to optimizing out unnecessary null checks) use(*p); // pointer dereference (this isn't) if(!p) abort(); // second null check (unnecessary *because of the if*) utilize(*p); // more pointer dereference the compiler can optimize out the second if (the "unnecessary null check" you refer to) based on the first if, regardless of whether the pointer dereference is even there.
  - atq2119 5y ago
    This is still too simplistic. Think along the lines of: void foo(T *p) { if (!p) abort(); bar(p); } void bar(T *p) { use(*p); MACRO(p); } In other words, the first null-check and the latter check are in different functions that may not even be in the same compilation unit, or where the call sequence is hard to reason about due to function pointers etc.
    - a1369209993 5y ago
      Assuming bar isn't inlined (eg separate compilation unit, as you mentioned), the null check in MACRO is not unnecessary, because bar can be called from places other than foo, and those places might pass null pointer. This is one of several situations where it might be useful for the compiler to excercise it's perogative to implement operations without regard for undefined behaviour to rewrite `use(*p);` as `if(!p) abort(); use(*p);`, which would make the null check in MACRO unnecessary, but unless it does so, the check is not unnecessary, just insufficient.
zzo38computer 5y ago
I disagree. The transformation should be valid; if p is null then the result is undefined and may crash, including the rest of the program after that occurs. However, I do think that it should not make such a transformation for a volatile read/write; in that case it should not make such an assumption (who knows if it is meaningful on the target computer or some sort of emulator or operating system or debugger or whatever).