[LLVMdev] Unreachable code in Mutex.cpp

[David Blaikie]

On Fri, Jan 13, 2012 at 11:58 PM, Chris Lattner <clattner at apple.com> wrote:
> On Jan 13, 2012, at 11:17 PM, David Blaikie wrote:
>
>>> On some (linux?) implementations, various pthread APIs are defined as "weak extern" symbols in libc and strong definitions in libpthreads.  The idea of this check is thus to detect if pthreads is linked into the app and enable threads if so.
>>
>> Sorry, right - I should've been more clear. I understand that that's
>> what the declaration/boolean used to do:
>>
>>
>> static const bool pthread_enabled = static_cast<bool>(pthread_mutex_init);
>>
>> But that's not what the code does now - the change you made 5 years
>> ago changed it to a constant:
>>
>> static const bool pthread_enabled = true;
>>
>> (I found this because the return outside the 'if (pthread_unabled) {
>> ... return }' was marked as unreachable by -Wunreachable-code). Since
>> it hasn't actually been doing anything for 5 years - should we just
>> remove the variable & all the conditions (unconditionally using
>> pthreads), or reinstate the smarts it used to have (testing the weak
>> symbol)?
>
> Aha, I'm completely fine with constant folding this all away.  Simple is good.

Sounds good. Committed as r148206 (including a similar fix in
RWMutex.cpp - which propagated the same quirk when it was
copy/pasted/created from Mutex.cpp)

I've attached a patch containing some other fixes to remove
unreachable code in LLVM - this isn't comprehensive, but a start based
on -Wunreachable-code. Is this sort of thing OK to commit as I work
through it? About the most noteworthy parts are:

1) two instances of code deliberately if(false)ed out with comments
explaining that developers can bring it back in once something works
or to aid in debugging. I switched these to #if 0 instead - I realize
this means those code snippets may bitrot since they won't be compiled
anymore. Is this the right thing? Should we have some construct for
"deliberately unreachable" code like this? (perhaps like the
parentheses warning - some quirky syntax people could use when they
intend to create an always false condition)

2) Mostly it was just replacing trailing returns that used to be there
to satisfy GCC with llvm_unreachable instead. (also there were a lot
of break/return/etc after llvm_Unreachables which were easily removed)

- David
-------------- next part --------------
A non-text attachment was scrubbed...
Name: llvm_unreachable_fixes.diff
Type: application/octet-stream
Size: 6129 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120114/0c262c4e/attachment.obj>