[llvm-commits] [llvm] r171600 - /llvm/trunk/include/llvm/ADT/DenseMap.h

[Chandler Carruth]

On Fri, Jan 4, 2013 at 9:14 PM, NAKAMURA Takumi <geek4civic at gmail.com>wrote:

> Author: chapuni
> Date: Fri Jan  4 23:14:23 2013
> New Revision: 171600
>
> URL: http://llvm.org/viewvc/llvm-project?rev=171600&view=rev
> Log:
> DenseMap: Appease -fstrict-aliasing on g++-4.4.
>
> With DenseMapInfo<Enum>, it is miscompiled on g++-4.4.
>
>     static inline Enum getEmptyKey() { return Enum(<arbitrary int/unsigned
> value>); }
>
>     isEauql(getEmptyKey(), ...)
>
> The compiler mis-assumes the return value is not aliased to Enum.
>

This doesn't make a lot of sense to me.

Aliasing has to do with pointers and references. The return value is a
value, and can't alias anything....

Not sure what is actually happening though.


>
> Modified:
>     llvm/trunk/include/llvm/ADT/DenseMap.h
>
> Modified: llvm/trunk/include/llvm/ADT/DenseMap.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/DenseMap.h?rev=171600&r1=171599&r2=171600&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/ADT/DenseMap.h (original)
> +++ llvm/trunk/include/llvm/ADT/DenseMap.h Fri Jan  4 23:14:23 2013
> @@ -430,7 +430,8 @@
>      incrementNumEntries();
>
>      // If we are writing over a tombstone, remember this.
> -    if (!KeyInfoT::isEqual(TheBucket->first, getEmptyKey()))
> +    const KeyT EmptyKey = getEmptyKey();
> +    if (!KeyInfoT::isEqual(TheBucket->first, EmptyKey))
>        decrementNumTombstones();
>
>      return TheBucket;
>
>
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