Re: [RFA] [tree-optimization/80576] Handle non-constant sizes in DSE

[Marc Glisse <marc dot glisse at inria dot fr>]

On Fri, 16 Aug 2019, Jeff Law wrote:

> This patch improves our ability to detect dead stores by handling cases
> where the size memcpy, memset, strncpy, etc call is not constant.  This
> addresses some, but not all, of the issues in 80576.
>
> The key here is when the size is not constant we can make conservative
> decisions that still give us a chance to analyze the code for dead stores.
>
> Remember that for dead store elimination, we're trying to prove that
> given two stores, the second store overwrites (partially or fully) the
> same memory locations as the first store.  That makes the first store
> either partially or fully dead.
>
> When we encounter the first store, we set up a bitmap of bytes written
> by that store (live_bytes).  We then look at subsequent stores and clear
> the appropriate entries in the bitmap.
>
> If the first store has a nonconstant length argument we can use the
> range of the length argument (max) and the size of the destination
> object to make a conservative estimation of how many bytes are written.
>
> For the second store the conservative thing to do for a non-constant
> length is to use the minimum of the range of the length argument.

So I guess it won't handle things like

void f(char*p,int n){
  __builtin_memset(p,3,n);
  __builtin_memset(p,7,n);
}

where we know nothing about the length, except that it is the same? Or do 
you look at symbolic ranges?

> This doesn't come up a lot in practice.  But it also happens to put some
> of the infrastructure in place to handle strcpy and strcpy_chk which are
> needed to fully resolve 80576.
>
> Bootstrapped and regression tested on x86, x86_64, ppc64le, ppc64,
> ppc32, aarch64, sparc, s390x and probably others.  Also verified that
> the tests work on the various *-elf targets in my tester.
>
> OK for the trunk?

ENOPATCH

--
Marc Glisse