[cfe-commits] False positive for -Wunreachable-code

David Blaikie dblaikie at gmail.com
Tue Oct 30 11:19:47 PDT 2012

On Tue, Oct 30, 2012 at 11:08 AM, Abramo Bagnara
<abramo.bagnara at bugseng.com> wrote:
> Il 30/10/2012 18:49, David Blaikie ha scritto:
>> On Tue, Oct 30, 2012 at 10:39 AM, Abramo Bagnara
>> <abramo.bagnara at bugseng.com> wrote:
>>> Il 30/10/2012 18:25, David Blaikie ha scritto:
>>>> On Tue, Oct 30, 2012 at 10:19 AM, Jordan Rose <jordan_rose at apple.com> wrote:
>>>>> On Oct 30, 2012, at 10:17 , David Blaikie <dblaikie at gmail.com> wrote:
>>>>>> On Tue, Oct 30, 2012 at 9:25 AM, Jordan Rose <jordan_rose at apple.com> wrote:
>>>>>>> On Oct 30, 2012, at 2:34 , Abramo Bagnara <abramo.bagnara at bugseng.com> wrote:
>>>>>>>> Il 28/10/2012 08:12, Abramo Bagnara ha scritto:
>>>>>>>>> $ cat p.c
>>>>>>>>> #include <stdio.h>
>>>>>>>>> enum e { a, b = 4 } x = 3;
>>>>>>>>> void g(int v) {
>>>>>>>>> printf("%d\n", v);
>>>>>>>>> }
>>>>>>>>> int main(int argc, char **argv) {
>>>>>>>>> switch (x) {
>>>>>>>>> case a:
>>>>>>>>>   g(0);
>>>>>>>>>   break;
>>>>>>>>> case b:
>>>>>>>>>   g(1);
>>>>>>>>>   break;
>>>>>>>>> default:
>>>>>>>>>   g(2);
>>>>>>>>>   break;
>>>>>>>>> }
>>>>>>>>> }
>>>>>>>>> $ _clang -Wunreachable-code -Wcovered-switch-default -O2 p.c
>>>>>>>>> p.c:17:3: warning: default label in switch which covers all enumeration
>>>>>>>>> values
>>>>>>>>>     [-Wcovered-switch-default]
>>>>>>>>> default:
>>>>>>>>> ^
>>>>>>>>> p.c:18:7: warning: will never be executed [-Wunreachable-code]
>>>>>>>>>   g(2);
>>>>>>>>>     ^
>>>>>>>>> $ ./a.out
>>>>>>>>> 2
>>>>>>>>> Of course -Wcovered-switch-default warning is a perfectly true positive.
>>>>>>>>> My reading of the standard is that nothing prevent an enum to have a
>>>>>>>>> value different from listed enum constants if this value is compatible
>>>>>>>>> with enum range (and code generation seems to agree on that).
>>>>>>>> I've attached the patch for review.
>>>>>>>> The fixed testcase shows well why to hide warnings about undefined
>>>>>>>> behaviour in code actually generated is a bad thing.
>>>>>>> If we do this, we're going to want this under a CFG option at the very least. The static analyzer should continue assuming that an enum input to a switch is always a valid enum constant, in order to keep our false positive rate down.
>>>>>> Yeah, I doubt this'll be any better in the compiler proper, really.
>>>>>> The heuristic exists to, as you rightly point out, reduce false
>>>>>> positives & that rationale exists in the compiler as well.
>>>>>> While, yes, it means we lose some true positives as well, that
>>>>>> probably isn't worth the increase in false positives.
>>>>> I can see Abramo's point, however, that in a more defensive coding style the current -Wunreachable warning can easily be considered a false positive. We don't optimize out the default case in an enum-covered switch.
>>>> Flagging this as unreachable code is a bug & should be fixed - but
>>>> probably in the way I described. Treating it purely as reachable code
>>>> & emitting our 'runtime' diagnostics for code in such situations will
>>>> (I believe - though I haven't run numbers) increase false positive
>>>> rates substantially.
>>>> A trivial example that GCC often warns about & Clang deliberately does not:
>>>> int func(enum X v) {
>>>>   switch (v) {
>>>>   case A: return 1;
>>>>   case B: return 2;
>>>>   ... // fully covered
>>>>   case Z: return 26;
>>>>   }
>>>>   // GCC warns that the function may exit without a return value here,
>>>> Clang does not
>>>>   // the LLVM/Clang codebase has a lot of llvm_unreachables after
>>>> fully covered/exiting
>>>>   // switches like this to silence GCC's warning. Each of those is,
>>>> essentially, a GCC
>>>>   // false positive (in the sense that the code is not buggy).
>>>> }
>>> :-o
>>> Unless I'm missing something, the code is definitely buggy and leads to
>>> undefined behaviour in C++ entering with v = Z + 1. Note that entering
>>> with v = Z + 1, is not per se an undefined behavior: only the missing
>>> return causes that.
>>> Can we at least agree on that?
>> Yes & no. Yes a program exhibits UB if the function is called with v =
>> Z + 1, no the code is not (necessarily) buggy if the function is never
>> called with such invalid values.
>> If code is written in such a way as to not violate that invariant,
>> then the warning is a false positive (it has not found a bug in the
>> code). If people often write code with this invariant then the false
>> positive rate is "high" and the true positive rate is "not so high",
>> so we try to avoid warning & producing more noise than good advice.
>> (it's obviously not a 1:1 ratio, and it's certainly a judgement call)
>>> If we'd agree on that we will easily discover that my proposed change
>>> does not lead to any false positive diagnostics, that GCC is right and
>> Your definition of "false positive" differs from mine/ours. Hopefully
>> my description above helps describe the motivation here.
> Yes, my definition of false positive is different, see:
> http://en.wikipedia.org/wiki/Type_I_and_type_II_errors#False_positive_error
> "is the default unreachable"? ("is there a wolf near the herd?")
> If the message says "warning: will never be executed
> [-Wunreachable-code]" ("Wolf, wolf!") then we have a false positive.
> The idea that a warning is a false positive if it has not found a bug in
> the code is rather weird... do you know any warning that is able to
> always find a bug without knowing the programmer intentions?

Generally, no. They make local assumptions so to make a best effort at
finding bugs. Part of that is also to avoid finding non-bugs because
doing so creates a burden on developers that may eventually eclipse
the benefit they gain from the warning.

We're not trying to write warnings to teach people about the semantics
of their code - they can read books about that. They're meant to be
actionable for a good reason, not just to indicate that the developer
read & understood the diagnostic message & then went on their merry

> That apart, of course if we want a compilation switch that deviates from
> the standard and says that an enum typed expression cannot have any
> value different from enum constants specified we can do that, but really
> we want that?

Not really, no - the compiler implements the standard. For the purpose
of diagnostics we might be interested in adding an attribute to enum
types or variables that could indicate whether that variable or
variables of that type are intended to contain an/all values in the
representable range (to cause us to do things like diagnose the code
after the switch or in the default of a covered switch as reachable,

> What about enum designed to be used to represent a mask?

They happen, certainly. I'm not at all claiming that people never
deliberately put values outside the enum constants into an enum value.
Simply that the signal/noise in terms of bug finding/actionable/useful
warnings isn't considered (by me right now & by whoever implemented
this particular tweak whenever they implemented it) worthwhile to
assume enums contain all values in their representable range. Other
people might have other opinions (as you do) & I the only real way to
assess the situation is to run the diagnostic on a large codebase &
figure out how many cases would require semantically differing changes
(ie: code fixes) versus simple suppressions (like llvm_unreachable).
>From our experience with GCC so far, it seems a fair bit more of the
latter & not much of the former I think.

- David

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