Implement defined but not used diagnostic for statics

[llvmbot]

Bugzilla Link	2236
Resolution	FIXED
Resolved on	Mar 12, 2010 00:57
Version	unspecified
OS	NetBSD
Blocks	llvm/llvm-bugzilla-archive#2417
Reporter	LLVM Bugzilla Contributor

Extended Description

Nate's recent changes cause a bogus diagnostic:

static void a (void);
void b (void) { a (); }
static void a(void) {}

/tmp/bug.c:3:13: warning: static 'a' defined but not used

[llvmbot]

Patch off for review.

[llvmbot]

Fixed here, and added the example as a test. Let me know if you find anything else that doesn't work :)

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20080414/005374.html

[llvmbot]

So this ... system ... doesn't let me reply to its mails....

No, this is still broken Nate. Apparently --emit-llvm is required.

That sends warning bells ringing in my mind ... this diagnostic is easy to do with simple semantic analysis, no codegen or optimization should be necessary.

[llvmbot]

Neil, can you be clearer about what specifically is broken? I added your test above to the test suite and it is passing.

If your complaint is more generally "we shouldn't need to do code generation to get defined but not used" warnings, that's fine, but that's not in the scope of this bug, which was incorrect warnings emitted by CodeGen. Can you provide more examples of -emit-llvm doing the wrong thing? If so, we probably want to switch over to the function redeclaration stuff.

[llvmbot]

Testcase
clang -pedantic-errors -emit-llvm on this file shows the problem.

Sorry I tried to reduce it as much as possible, but for some reason it disappears once the file gets too small.

-emit-llvm is required for some reason; that explains my prior comment. It seems strange that to get the warning we must be generating code.

[llvmbot]

OK, simple test case:

static void a (void);
static void b (void) { a(); }
void c (void) { b(); }
static void a (void) {}

Output as shown below. Why does it differ with -emit-llvm?

$ clang -emit-llvm /tmp/bug.c
/tmp/bug.c:4:13: warning: static 'a' defined but not used
static void a (void) {}
^
1 diagnostic generated.
$ clang /tmp/bug.c
$

[llvmbot]

It differs with -emit-llvm because that's the phase of compilation where the warning is emitted. I agree with you that ideally, we would flag this warning regardless of whether -emit-llvm is passed or not, but the warning has to be emitted from something like an ASTConsumer which is invoked after the entire AST is built. If you'd like to go ahead and disable the warning until it is in the "right place", by all means do so. The main value of PR 1998 was not emitting unused statics into the output IR. Getting a handy warning out of it was "free".

[llvmbot]

Nate - the point of this bug report is that the diagnostic is wrong; I think you've missed that.

The -emit-llvm requirement was just a side comment about its undesirability. Please checkout the 4-line testcase I posted.

[llvmbot]

Nate - I opened up a separate issue about the -emit-llvm thing so it's not confused with the point of this report. Sorry for the confusion.

[lattner]

I'm going to disable the current warning. We should move it to sema someday, the decl merging discussion will hopefully make it more clear how to handle this case.

[llvmbot]

Just as a placeholder, we need used but not defined as well. I thought there was an open issue for this but can't find it now but the same machinery should do both.

[lattner]

The original issue is fixed

[nunoplopes]

mentioned in issue llvm/llvm-bugzilla-archive#2417