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

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman 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();
+
----------------
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.


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  https://reviews.llvm.org/D67545/new/

https://reviews.llvm.org/D67545