Attached is a simple middle end implementation of detection of
mismatched pairs of calls to C++ new and delete, along with
a substantially enhanced implementation of -Wfree-nonheap-object.
The latter option has been in place since 2011 but detected only
the most trivial bugs.

Unlike the Clang -Wmismatched-new-delete which diagnoses
declarations of "overloaded operator new() and operator delete()
functions that do not have a corresponding free store function
defined within the same scope", this patch detects mismatches
between calls to allocation and deallocation functions, such as
calling free() on the result of new, of delete on the result of
array new.  The functionality provided by Clang can be added on
top of what this feature does and since they are so close I think
it's fine to have both under the same option (a new level could
be introduced to distinguish the two).

The -Wfree-nonheap-object enhancement lets the warning detect all
calls to free, realloc, or C++ delete, with pointers that can be
proven not to point to the first byte of an allocated object.

The patch relies on the well-tested compute_objsize() function
for the determination of pointer provenance and makes use of
the changes in the following patch submitted for review just
yesterday:
https://gcc.gnu.org/pipermail/gcc-patches/2020-November/557807.html

As usual, I tested on x86_64-linux with Glibc & Binutils/GDB with
no new false positives.

Martin

PS A few words on the implementation choices:

The new code is in builtins.c only because -Wfree-nonheap-object
is there.  I still plan to move all of the invalid access checking
code into its own module or pass at some point but I didn't want
to make this improvement contingent on that restructuring.
Even though it's all in builtins.c, the code is called from calls.c.
This is so that simple mismatches can be diagnosed even when free
isn't handed in builtins.c (i.e., without optimization).
The warning makes no attempt to analyze the CFG or handle
conditional mismatches.  That will have to wait until the code
is moved to a GIMPLE pass.