[llvm-dev] DenseMap/ValueMap: is M[New]=M[Old] valid ?
Martin Storsjö via llvm-dev
llvm-dev at lists.llvm.org
Thu Sep 26 22:58:44 PDT 2019
I actually ran into an elusive bug with this exact same issue some time
ago as well, see https://bugs.llvm.org/show_bug.cgi?id=42065 and
https://reviews.llvm.org/D62624.
The strange thing about that bug was that it only showed up if built with
GCC; Clang dereferenced the right hand side reference before evaluating
the left hand side, as long as the value type was as small as the
reference itself.
// Martin
On Thu, 26 Sep 2019, David Blaikie via llvm-dev wrote:
> Yep - You'd have to separate them for correctness if "NewKey" might not
> already be in "M".
>
> On Thu, Sep 26, 2019 at 2:48 PM Jeroen Dobbelaere via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I have a question about llvm/ADT/DenseMap.h and
> llvm/IR/ValueMap.h:
>
> When you have a:
> MapType M;
>
> is it safe to do:
> M[NewKey] = M[OldKey];
>
> or do you need to do it in two steps:
> auto tmp = M[OldKey]; // ensure the reference to M[OldKey] is
> copied, before reassigning.
> M[NewKey] = tmp; // might reallocate
>
> aka, will a possible allocation for M[NewKey] invalidate the
> reference that M[OldKey] returns ?
>
> Greetings,
>
> Jeroen Dobbelaere
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