From: Marc Glisse <marc.glisse@inria.fr>
To: Jeff Law <law@redhat.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [RFA] [tree-optimization/80576] Handle non-constant sizes in DSE
Date: Fri, 16 Aug 2019 19:55:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.21.1908162006590.11513@stedding.saclay.inria.fr> (raw)
In-Reply-To: <8b836cef-7aa1-fd3d-5585-c2a6fe74bed6@redhat.com>
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
next prev parent reply other threads:[~2019-08-16 18:10 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-16 18:10 Jeff Law
2019-08-16 19:55 ` Marc Glisse [this message]
2019-08-16 20:41 ` Jeff Law
2019-08-16 22:49 ` Martin Sebor
2019-08-22 0:30 ` Jeff Law
2019-08-22 18:50 ` Martin Sebor
2019-08-23 16:50 ` Jeff Law
2019-08-19 14:23 ` Richard Biener
2019-08-22 2:12 ` Jeff Law
2019-08-22 11:14 ` Richard Biener
2019-08-23 20:27 ` Jeff Law
2019-08-26 10:07 ` Richard Biener
2019-09-03 21:24 ` Jeff Law
2019-09-09 20:10 ` Jeff Law
2019-09-16 9:12 ` Richard Biener
2019-09-16 9:18 ` Richard Biener
2019-08-22 15:53 ` Martin Sebor
2019-08-23 16:50 ` Jeff Law
2019-08-16 21:50 ` Jeff Law
2019-08-16 22:19 ` Marc Glisse
2019-08-16 22:43 ` Jeff Law
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