rsmith added inline comments.
================
Comment at: clang/lib/Sema/SemaDecl.cpp:14361
+/// attributes are applied to declarations.
+void Sema::AddKnownFunctionAttributesForReplaceableGlobalAllocationFunction(
+ FunctionDecl *FD) {
----------------
lebedev.ri wrote:
> rsmith wrote:
> > This should all be done by CodeGen, not by injecting source-level
> > attributes.
> I don't agree, can you explain why this should be done in codegen?
Generally, it's a design goal for the Clang AST to represent the original
program source (http://clang.llvm.org/docs/InternalsManual.html#faithfulness).
But I suppose these attributes are all marked "implicit", and it's a good idea
to express these hints uniformly to codegen and the static analyzer and so
on... OK, I can make peace with doing this here.
================
Comment at: clang/lib/Sema/SemaDecl.cpp:14414-14432
+ // C++2a [basic.stc.dynamic.allocation]p3:
+ // For an allocation function [...], the pointer returned on a successful
+ // call shall represent the address of storage that is aligned as follows:
+ // (3.2),(3.3) Otherwise, [...] the storage is aligned for any object
+ // that does not have new-extended alignment [...].
+ //
+ // NOTE: we intentionally always manifest this basic alignment, because it is
----------------
lebedev.ri wrote:
> rsmith wrote:
> > This is incorrect. The pointer returned by `operator new` is only suitably
> > aligned for any object that does not have new-extended alignment **and is
> > of the requested size**. And the pointer returned by `operator new[]` is
> > suitably aligned for any object **that is no larger than the requested
> > size**. (These are both different from the rule for `malloc`, which does
> > behave as you're suggesting here.) For example:
> >
> > Suppose the default new alignment and the largest fundamental alignment are
> > both 16, and we try to allocate 12 bytes. Then:
> >
> > * `operator new` need only return storage that is 4-byte aligned (because
> > that is the largest alignment that can be required by a type `T` with
> > `sizeof(T) == 12`)
> > * `operator new` need only return storage that is 8-byte aligned (because
> > that is the largest alignment that can be required by a type `T` with
> > `sizeof(T) <= 12`)
> > * `malloc` must return storage that is 16-byte aligned (because that is
> > the largest fundamental alignment)
> So essentially, if we can't evaluate the requested byte count as a
> constant/constant range,
> we must simply give up here, on the most interesting case of variable
> allocation size?
> That is surprisingly extremely pessimizing from C++ :)
>
>
The benefit from not requiring all allocations to be padded to a multiple of
the largest fundamental alignment is probably worth a lot more than allowing
the optimizer to assume a higher alignment for pointers from direct calls to
`operator new` with a non-constant argument.
We can at least look for a constant argument in CodeGen and emit the alignment
hint there. (Though it's probably better to do that in the middle-end, since
the argument might be reduced to a constant via inlining, especially for uses
of `std::allocator` and similar.)
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D73380/new/
https://reviews.llvm.org/D73380
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