Strict aliasing and malloc

Hello,

I have a question regarding newlib and the -fstrict-aliasing implied
by turning on O2.

The strict aliasing implied by the ISO standard and enabled in gcc
with O2 (This might be specific to gcc, but could be the case with any
compiler with aliasing optimizations) makes it so you can only cast a
pointer to a compatible type, and a special case is malloc, which
should return an "undeclared type" *.

I however did not find the -fno-strict-aliasing flag in any
configuration or makefile (If I just overlooked it, and the flag is
mandatory that would answer my question)

My question:
In newlib/libc/stdlib/mallocr.c on line 2212 you have the statement:
"top = (mchunkptr)brk;"
Here top is of type "mchunkptr" and brk is a "char *". The standard
says that you can not just alias a incompatible type and dereference
it (unless it's a malloc'ed variable, as it would change it's type
when written to, but how do you inform the compiler?)

As an example, see 4.2.1 (p. 63) in
https://www.cl.cam.ac.uk/~pes20/cerberus/notes30.pdf

So is this allowed? Or am I missing something.

* After asking in the gcc IRC, they mentioned that the way they go
about having the special case for malloc is making sure the libc
library is linked from a library and no LTO is performed.


My main reason for asking is just wanting to know how a malloc
implementation should deal with these restrictions stated by the ISO C
standard, and improve my understanding of the (sometimes confusing)
aliasing rules.

Thanks