Re: [Tinycc-devel] undefined sanitizer

[Michael Matz]

Hi,

On Sat, 22 Jun 2019, Vincent Lefevre wrote:

> > > I keep having fun.
> > > In attach compile report under -fsanitize=undefined in gcc or clang.
> > > Take care.
> >
> > I don't think we should care about alignment of 4 (when 8 would be needed).
> > The 64bit platforms we support all handle misaligned memory accesses quite
> > fine, and doing it "correctly" just wastes space.
>
> Even if there are no issues with the processor, undefined behavior
> may yield the generation of bad code due to optimizations.

Indeed.  The thing is that such "mis"-alignment isn't generically 
undefined behaviour (and hence shouldn't even be part of 
-fsanitize=undefined).  It's implementation defined what it means for a 
pointer to an object type to be correctly aligned (e.g. one where the 
natural alignment of all types is 1 is fully conforming).  Accessing 
something via an incorrectly aligned pointer is undefined, but what 
incorrectly aligned means is implementation defined.

Usually that's ultimately dictated by the capabilities of the underlying 
hardware (and not by -fsanitize when it ignores those capabilities!). 
Our hosts support alignment of 4 even for 8byte quantities.

Now, you do have a point in that some compilers assume larger natural 
alignment (e.g. that a void* has alignment 8), and that there are 
transformations like bit propagation of low bits of addresses, or 
vectorization making use of instructions actually requiring large 
alignment, that might be affected by assumptions about alignment, leading 
to wrong code when those assumptions are violated.  When that actually 
happens we can revisit, until then I personally prefer the space savings.


Ciao,
Michael.