Re: Implement removal of malloc/free pairs with NULL check

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Nov 01, 2024 at 01:34:45PM +0100, Jan Hubicka wrote:
> > 
> > malloc(SIZE_MAX / 2 + 1) does not succeed.
> 
> Is malloc required to return NULL for block of half of address size or
> it is glibc behavior? Some time ago we was discussing possibility to

I don't see C23 saying that explicitly (though it e.g. says that
calloc is guaranteed to return NULL if nmemb * size computation wraps around),
but I think it is derived from the fact that on objects larger than half
of the address space one can't (at least on arches where
ptrdiff_t/size_t/void * have the same precision) perform correct pointer
arithmetics.

So, I think for calloc-like functions we need to be careful.

Or we could just use the ranger and do this only if the size is known
from value ranges not to be larger than half of the address space and
for calloc-like that there is no overflow?

> > Please reconsider? Why to we need to match LLVM here?
> 
> Matching llvm is not really the goal. I implemented it since it is
> useful optimization for code that builds small objects on heap and
> compiler can optimize away their use.  This is relatively common for
> code with higer abstraction penalty. My original motivation was
> std::vector and std::string consumers, but I think there is C code doing
> similar things, too, so it would be better to support both C and C++
> allocations.

Are most of such cases with variable sized allocations or fixed ones?

	Jakub