[Bug tree-optimization/110035] Missed optimization for dependent assignment statements

["ptk.prasertsuk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035

--- Comment #11 from Pontakorn Prasertsuk <ptk.prasertsuk at gmail dot com> ---
(In reply to rguenther@suse.de from comment #10)
> On Mon, 5 Jun 2023, ptk.prasertsuk at gmail dot com wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035
> > 
> > --- Comment #9 from Pontakorn Prasertsuk <ptk.prasertsuk at gmail dot com> ---
> > (In reply to Richard Biener from comment #8)
> > > (In reply to Pontakorn Prasertsuk from comment #7)
> > > > For the LLVM IR code of the snippet I provided, Clang's alias analysis can
> > > > prove that `new` call has no side effect to other memory location. This is
> > > > indicated by `noalias` keyword at the return value of the `new` call (_Znwm).
> > > > 
> > > > According to Clang's Language Reference:
> > > > "On function return values, the noalias attribute indicates that the
> > > > function acts like a system memory allocation function, returning a pointer
> > > > to allocated storage disjoint from the storage for any other object
> > > > accessible to the caller."
> > > > 
> > > > Is this possible for GCC alias analysis pass?
> > > 
> > > >   MyClass c = a;
> > > >   MyClass *b = new MyClass;
> > > >   *b = c;
> > > 
> > > the point is that 'new' can alter the value of 'a', GCC already knows that
> > > 'b' is distinct from c and a but that's not the relevant thing.  It looks
> > > like LLVM creates wrong-code here.
> > 
> > In what case can 'new' alter 'a'? I thought memory allocation functions such as
> > 'malloc, 'calloc' and 'new' cannot alias other memory locations than its return
> > value.
> 
> 'new' can be overridden by the user, you can declare your own 
> implementation that does fancy stuff behind the scenes, including
> in the above case altering 'a'.  Welcome to C++ ...

I assume you are referring to this case: https://godbolt.org/z/z4Y7YdxWE

Clang indeed assumes that 'new' is non-alias and this feature can be turned off
by using -fno-assume-sane-operator-new

However, can we safely assume that 'malloc' and 'calloc' are non-alias as well?