4 ms·

> So the compiler is proving that the array is being overflowed and rather than emitting a warning to that effect <source>:5:13: warning: unsafe buffer acce

> So the compiler is proving that the array is being overflowed and rather than emitting a warning to that effect <source>:5:13: warning: unsafe buffer access [-Wunsafe-buffer-usage] 5 | if (table[i] == v) return true; | ^~~~~ https://godbolt.org/z/zGxnKxvz6 https://godbolt.org/z/zGxnKxvz6 This one is weirdly hard to get a compiler warning out of which is a fair critque, but so many of the "Look what the compiler did to my UB silently!" issues are not at all silent and would have been stopped dead with "-Wall -Wextra -Werror"

iainmerrick 3y ago
As noted elsewhere in this thread, GCC by default does the "optimization" and doesn't warn. No doubt there are other examples where Clang is the one that misbehaves. How are we supposed to know whether our code is being compiled sensibly or not, without poring over the disassembly? Just set all the warning flags and hope for the best?
- UncleMeat 3y ago
  I think that a big problem is that for every compile that seems "not sensible" and is actually not sensible, there are 100s or 1000s of compiles that would look absolutely insane to a human but are actually exactly what you want when you sit down and think about it for a long time. Almost all of the "don't do the overly clever stuff!" proposals would throw away a huge amount of actually productive clever stuff.
  - fluoridation 3y ago
    I think what the GP means by "not sensible" is that proving that the code is broken in order to silently optimize it more aggressively is not sensible. If your theorem proven can find a class of bugs then have it emit diagnostics. Don't only use those bugs to make the code run faster. Yes, make the code run faster, but let me know I may be doing something nonsensical, since chances are that it is nonsensical and it doesn't cost anything at run time.
    - UncleMeat 3y ago
      Right and the next part is the hard part: defining this clearly. What I'm saying is that there is a surprising amount of "wait, actually I do want that" when you dig into this proposal.
      - fluoridation 3y ago
        A reasonable compiler would let you turn off a specific warning for a section of code.
        torstenvl 3y ago
        They pretty much all do. #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wwhatever" // code #pragma clang diagnostic pop
        fluoridation 3y ago
        I was going to comment that GCC doesn't, but it seems it was added as some point since the last time I checked. I know at one time GCC had as a policy not to allow doing that.
        torstenvl 3y ago
        Here's an example: https://pastebin.com/raw/fH0Lj2Zb https://pastebin.com/raw/fH0Lj2Zb
    - mike_hock 3y ago
      A warning is only useful if it prescribes a code transformation that affirms the programmer's intent and silences the warning (unless the warning was a true positive and caught a bug). You cannot simply emit a warning every time you optimize based on UB. There is no `if(obvious out-of-bound access) silently emit nonsense har har har` in the compiler's source code. The compiler doesn't understand intent or the program as a whole. It applies micro transformations that all make sense in isolation. And while the compiler also tries to detect erroneous programming patterns and warn about those, that's exceedingly more difficult.
- moefh 3y ago
  > whether our code is being compiled sensibly or not I'm failing to see what's not sensible about how that code is compiled. The only possible way that function could return false is if you read past the end of the array and the value there happens to be different from `v`. Is it really the more sensible to rely on that, rather than fixing a known behavior in case of array overflow?
  - robinsonb5 3y ago
    If the compiler's going to interpret undefined behaviour as license to do something that runs counter to the programmer's expectations, the most sensible course of action is for the compiler to yell very loudly about it instead of near-silently producing (differently!) broken code. Currently that piece of code doesn't trigger a warning with -Wall. It's not even flagged with -Wextra - it needs -Weverything.
    - moefh 3y ago
      One man's "broken code produced by the compiler" is another man's "excellently optimized code by the compiler". Where to draw the line is not always clear, but here's a very clear-cut example[1] where emitting a warning would be bad. If you don't want to watch the video, it's basically this: - the code technically contains undefined behavior, but it will never be actually triggered by the program - changing the code to remove undefined behavior forces the compiler to emit terrible code Making the compiler yell at the programmer in this case would be terrible, but it's clearly a consequence of what you're asking. [1] https://youtu.be/yG1OZ69H_-o?t=2358 https://youtu.be/yG1OZ69H_-o?t=2358
      - jeffbee 3y ago
        Exactly. I think a lot of this noise is by non-practitioners of the language. The compiler is steel-manning this loop. It is generously interpreting the 4 as irrelevant, and deducing that the loop must always exit early. The author can’t possibly have meant to access beyond the end, because that’s not defined. QED. It seems altogether sensible to me.
- uecker 3y ago
  I agree there should be a warning. But it is not trivial to teach a compiler when to warn or not to not generate too many false positives. Not as good as warning, but UBSan catches this at run-time: https://godbolt.org/z/Mdjn7h8dj https://godbolt.org/z/Mdjn7h8dj
bondant 3y ago
I am not sure this warning is proving any overflow, you can get the same warning by just accessing table[i]. https://godbolt.org/z/Gxd3rK9Ts https://godbolt.org/z/Gxd3rK9Ts

iainmerrick 3y ago

As noted elsewhere in this thread, GCC by default does the "optimization" and doesn't warn. No doubt there are other examples where Clang is the one that misbehaves. How are we supposed to know whether our code is being compiled sensibly or not, without poring over the disassembly? Just set all the warning flags and hope for the best?

UncleMeat 3y ago
I think that a big problem is that for every compile that seems "not sensible" and is actually not sensible, there are 100s or 1000s of compiles that would look absolutely insane to a human but are actually exactly what you want when you sit down and think about it for a long time. Almost all of the "don't do the overly clever stuff!" proposals would throw away a huge amount of actually productive clever stuff.
- fluoridation 3y ago
  I think what the GP means by "not sensible" is that proving that the code is broken in order to silently optimize it more aggressively is not sensible. If your theorem proven can find a class of bugs then have it emit diagnostics. Don't only use those bugs to make the code run faster. Yes, make the code run faster, but let me know I may be doing something nonsensical, since chances are that it is nonsensical and it doesn't cost anything at run time.
  - UncleMeat 3y ago
    Right and the next part is the hard part: defining this clearly. What I'm saying is that there is a surprising amount of "wait, actually I do want that" when you dig into this proposal.
    - fluoridation 3y ago
      A reasonable compiler would let you turn off a specific warning for a section of code.
      - torstenvl 3y ago
        They pretty much all do. #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wwhatever" // code #pragma clang diagnostic pop
        fluoridation 3y ago
        I was going to comment that GCC doesn't, but it seems it was added as some point since the last time I checked. I know at one time GCC had as a policy not to allow doing that.
        torstenvl 3y ago
        Here's an example: https://pastebin.com/raw/fH0Lj2Zb https://pastebin.com/raw/fH0Lj2Zb
  - mike_hock 3y ago
    A warning is only useful if it prescribes a code transformation that affirms the programmer's intent and silences the warning (unless the warning was a true positive and caught a bug). You cannot simply emit a warning every time you optimize based on UB. There is no `if(obvious out-of-bound access) silently emit nonsense har har har` in the compiler's source code. The compiler doesn't understand intent or the program as a whole. It applies micro transformations that all make sense in isolation. And while the compiler also tries to detect erroneous programming patterns and warn about those, that's exceedingly more difficult.
moefh 3y ago
> whether our code is being compiled sensibly or not I'm failing to see what's not sensible about how that code is compiled. The only possible way that function could return false is if you read past the end of the array and the value there happens to be different from `v`. Is it really the more sensible to rely on that, rather than fixing a known behavior in case of array overflow?
- robinsonb5 3y ago
  If the compiler's going to interpret undefined behaviour as license to do something that runs counter to the programmer's expectations, the most sensible course of action is for the compiler to yell very loudly about it instead of near-silently producing (differently!) broken code. Currently that piece of code doesn't trigger a warning with -Wall. It's not even flagged with -Wextra - it needs -Weverything.
  - moefh 3y ago
    One man's "broken code produced by the compiler" is another man's "excellently optimized code by the compiler". Where to draw the line is not always clear, but here's a very clear-cut example[1] where emitting a warning would be bad. If you don't want to watch the video, it's basically this: - the code technically contains undefined behavior, but it will never be actually triggered by the program - changing the code to remove undefined behavior forces the compiler to emit terrible code Making the compiler yell at the programmer in this case would be terrible, but it's clearly a consequence of what you're asking. [1] https://youtu.be/yG1OZ69H_-o?t=2358 https://youtu.be/yG1OZ69H_-o?t=2358
    - jeffbee 3y ago
      Exactly. I think a lot of this noise is by non-practitioners of the language. The compiler is steel-manning this loop. It is generously interpreting the 4 as irrelevant, and deducing that the loop must always exit early. The author can’t possibly have meant to access beyond the end, because that’s not defined. QED. It seems altogether sensible to me.
uecker 3y ago
I agree there should be a warning. But it is not trivial to teach a compiler when to warn or not to not generate too many false positives. Not as good as warning, but UBSan catches this at run-time: https://godbolt.org/z/Mdjn7h8dj https://godbolt.org/z/Mdjn7h8dj

bondant 3y ago

I am not sure this warning is proving any overflow, you can get the same warning by just accessing table[i]. https://godbolt.org/z/Gxd3rK9Ts https://godbolt.org/z/Gxd3rK9Ts