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Compiling the following code: #include <vector> std::vector<int> *factory() { std::vector<int> *p = new std::vector<int>; p->reserve(10); return p; } with -Wall -O -fno-exceptions yields the following error in gcc-4.0.0 and gcc-4.0-20050602: .../include/c++/4.0.0/bits/vector.tcc: In member function 'void std::vector<_Tp, _Alloc>::reserve(size_t) [with _Tp = int, _Alloc = std::allocator<int>]': .../include/c++/4.0.0/bits/vector.tcc:78: warning: control may reach end of non-void function 'typename _Alloc::pointer std::vector<_Tp, _Alloc>::_M_allocate_and_copy(size_t, _ForwardIterator, _ForwardIterator) [with _ForwardIterator = int*, _Tp = int, _Alloc = std::allocator<int>]' being inlined The warning comes from a catch/rethrow block deep in the bowels of stl which (because of -fno-exceptions) evaluates to if(false). This is a false positive warning, which would be fine except for shops whose policy is to always compile with -Werror -Wall; there, the warning is fatal. And because this problem is not confined to just one user source file, it's hard to work around. This problem does not occur in gcc-3.4.3 nor in recent gcc-4.1 snapshots. cf. discussion here: http://gcc.gnu.org/ml/libstdc++/2005-06/msg00073.html
And I already filed this way back, see PR 19699 which I am closing this bug as a dup. See comment #2 in that PR from RTH: There is zero chance that this will be fixed for 4.0. That's exactly why Ian implemented the minimalistic check that he did in solving PR19583 and related. You may retartget this pr to fixing the silliness in libstdc++, if you want. *** This bug has been marked as a duplicate of 19699 ***
> You may retartget this pr to fixing the silliness in libstdc++, if you want. And the "silliness" would be? Personally, I'm finding quite a bit of silliness in this remark, to tell you the truth and indeed, mainline is ok, probably the current compiler judges that "silliness" not so silly, after all. When -fno-exceptions, the catch becomes simply an 'if (false)' and I don't see why the implementors of v3 have necessarily to care about the branch not returning.
(In reply to comment #2) > > You may retartget this pr to fixing the silliness in libstdc++, if you want. Paolo I copied the quote fully when I really should not have. RTH did not know we could not fix the if(0) part in libstdc++ at the time he wrote that comment, if you read the next comment in that bug I explain why it cannot be fixed in libstdc++ (comment #3 in PR 19699): (In reply to comment #2) > You may retartget this pr to fixing the silliness in libstdc++, if you want. Actually it is only silliness as try/catch is changed to be "if (true)"/"if (false)" if we don't have exceptions. There is nothing can be done to the libstdc++ headers to fix this.
(In reply to comment #3) > Paolo I copied the quote fully when I really should not have. RTH did not know we could not fix the > if(0) part in libstdc++ at the time he wrote that comment, if you read the next comment in that bug I > explain why it cannot be fixed in libstdc++ (comment #3 in PR 19699): > (In reply to comment #2) > > You may retartget this pr to fixing the silliness in libstdc++, if you want. > Actually it is only silliness as try/catch is changed to be "if (true)"/"if (false)" if we don't have exceptions. > There is nothing can be done to the libstdc++ headers to fix this. I disagree. libstdc++ can do exactly what everybody else does in such cases: add a dummy (unreachable) "return __result" at the end, after both try and catch blocks. This will be a hack, no doubt, a pragmatic one. I don't think that emitting false positive diagnostic that the user does not control is a good thing. I have been working in a verification (research) department for 10 years, and I can assure you that false positives, which can't be turned off, are worse than no diagnostic at all. Such diagnostics *will* turn customers/users away from your tool, or at best "just" ignore diagnostics. I have used a (bought) tool that gave a false positive every 500 lines of code. For 500 KLOC it would give 1K false positive. Now, try to find out a single true bug out of the noise of 1000 false positives. I can tell you that the tool was dropped very fast. We write tools used for verification and testing. Our customers are more likely to be willing to live with uncovered events, than to get false positives. Think of millions of tests/events that cause even 0.1% of false positive. These may overshadow hundreds of real bugs. Same with gcc, if something as central as vector.reserve() give false positive diagnostics then the sheer volume of warnings will render either the warning or reserve() unusable. I suggest to reopen PR19699 against libstdc++ (or maybe open a new one)
I concur with the last post -- a dummy return at the end would solve the problem and seems like an acceptable solution for a release branch. W.
Note Mark has said in the past (and in a private mail which was supposed to be a public one too) that warnings cannot cause this to be a rejects valid.
It sure as hell is for those shops that require -Werror. But ok, I'll be happy if it's fixed for 4.0.2.
(In reply to comment #6) > Note Mark has said in the past (and in a private mail which was supposed to be a public one too) that > warnings cannot cause this to be a rejects valid. For the same reason, PR 21183 should be reopened or merged with this PR: > When compiling a C++ program with -Wall, I get the following compiler warning > >/usr/local/bin/../lib/gcc/i686-pc-linux-gnu/4.0.0/../../../../include/c++/4.0.0/bits/stl_uninitialized.h:113: >warning: control may reach end of non-void function '_ForwardIterator > std::__uninitialized_copy_aux(_InputIterator, _InputIterator, ... > The problem seems to be in __uninitialized_copy_aux() in stl_uninitialized.h > > The catch statement block beginning on line 89 merely exits the routine without > actually returning a value.
(In reply to comment #7) > It sure as hell is for those shops that require -Werror. > > But ok, I'll be happy if it's fixed for 4.0.2. I think that your argument (as phrased) does not hold. Maybe you meant "It sure as hell is for those shops that require -Werror, in this particular instance". (Pardon my language, I just quoted the original ;-) Consider: 1. 1: int main() 2: { 3: int a; 4: never_return(); // the halting problem 5: return a; // Uninitialized variable? 6: } This is perfectly valid code and well defined, yet -Wall -Werror will reject it. No compiler will ever be able to determine if line 5 is ever reached. 2. 1: int main() 2: { 3: int a; 4: if(foo()) 5: a= bar(); 6: if(foo()) 7: return a; // Uninitialized variable? 8: return 0; 9: } Again, no compiler will be able to prove that a nontrivial foo() does not change over time (unless declared const/pure/whatever). And as a result, -Wall -Werror will reject valid code. In both examples, the user has a simple work-around, initialize 'a'. Adding initialization will make the code more stable, as foo() is no longer constrained to be const/pure (forgive me for not remembering if it is called pure or const). In contrast (as mentioned in comment #4), this PR and PR 21183 do not give the user the tools to shut this specific diagnostic instance up.
This is a compiler bug. If they don't fix it in 4.0.x, then we'll have to work around it in libstdc++ 4.0 sources only. People have suggested a patch to add a dummy return. If that's posted, it seems to be perfectly acceptable to me. I suggest that be posted... -benjamin
OK, I'm testing a patch now.
Created an attachment (id=9071) [edit] proposed workaround
I have a patch for the compiler here: http://gcc.gnu.org/ml/gcc-patches/2005-07/msg00049.html which will fix this problem.
That's certainly prettier than my header changes. I'll give it a shot. Thanks!
Fixed by: 2005-07-08 Geoffrey Keating <geoffk@apple.com> * tree-inline.c (expand_call_inline): Prevent 'may reach end' warning in system headers.
*** Bug 24498 has been marked as a duplicate of this bug. ***