https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035
--- Comment #13 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 6 Jun 2023, ptk.prasertsuk at gmail dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035 > > --- Comment #11 from Pontakorn Prasertsuk <ptk.prasertsuk at gmail dot com> > --- > (In reply to rguent...@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? Well, we do. For the C++ new case we did and it did break real world programs.
