[PATCH] D67545: [clang-tidy] Added DefaultOperatorNewCheck.

[Gabor Marton via Phabricator via cfe-commits]

martong added inline comments.


================
Comment at: clang-tools-extra/clang-tidy/cert/DefaultOperatorNewCheck.cpp:51
+  // The alignment used by default 'operator new' (in bits).
+  const unsigned DefaultAlignment = Context.getTargetInfo().getNewAlign();
+
----------------
aaron.ballman wrote:
> lebedev.ri wrote:
> > martong wrote:
> > > martong wrote:
> > > > What is the difference between "default" and "fundamental" alignment? Are they the same? Can they differ in any architecture?
> > > > 
> > > > https://wiki.sei.cmu.edu/confluence/display/cplusplus/MEM57-CPP.+Avoid+using+default+operator+new+for+over-aligned+types
> > > > Here there is no wording of "default alignment" only "fundamental alignment" is mentioned. Based on this I'd call this as `FundamentalAligment`.
> > > > What is the difference between "default" and "fundamental" alignment? Are they the same?
> > > 
> > > `fundamental alignment` of any type is the alignment of std::max_align_t. I.e. `alignof(std::max_align_t)`. 
> > > See C++17 6.11.2.
> > > 
> > > On the other hand, default alignment is the value in `__STDCPP_DEFAULT_NEW_ALIGNMENT__` which may be predefined with `fnew-alignment`
> > > See https://www.bfilipek.com/2019/08/newnew-align.html
> > > 
> > > These values can differ: https://wandbox.org/permlink/yIwjiNMw9KyXEQan
> > > 
> > > Thus, I think we should use the fundamental alignment here, not the default alignment. 
> > > So, `getNewAlign()` does not seem right to me.
> > > @aaron.ballman What do you think?
> > > Thus, I think we should use the fundamental alignment here, not the default alignment.
> > 
> > I have the exact opposite view.
> > If as per `getNewAlign()` the alignment would be okay, why should we not trust it?
> The comment on `getNewAlign()` is:
> ```
>   /// Return the largest alignment for which a suitably-sized allocation with
>   /// '::operator new(size_t)' is guaranteed to produce a correctly-aligned
>   /// pointer.
> ```
> I read that as saying any alignment larger than what is returned by `getNewAlign()` must call the over-aligned operator new variant in C++17 if available. So if the actual call target doesn't have an alignment specifier, it's probably getting the alignment wrong and would be worth diagnosing on.
> I have the exact opposite view.
> If as per getNewAlign() the alignment would be okay, why should we not trust it?

That could lead to a false positive diagnostic if `-fnew-alignment=8` and `alignas(16)` , because `alignof(max_align_t)` is still 16.

See the definidion of `getNewAlign()` which will return with 8 in this case I suppose:
```
  unsigned getNewAlign() const {
    return NewAlign ? NewAlign : std::max(LongDoubleAlign, LongLongAlign);
  }
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67545/new/

https://reviews.llvm.org/D67545