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resend: Re: [PATCH] strace: Fix crash caused over-optimization
- From: Daniel Santos <daniel dot santos at pobox dot com>
- To: cygwin-patches at cygwin dot com, Corinna Vinschen <corinna-cygwin at cygwin dot com>
- Date: Wed, 19 Apr 2017 13:50:36 -0500
- Subject: resend: Re: [PATCH] strace: Fix crash caused over-optimization
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-------- Forwarded Message --------
Subject: Re: [PATCH] strace: Fix crash caused over-optimization
Date: Wed, 19 Apr 2017 10:57:02 -0500
From: Daniel Santos <email@example.com>
To: firstname.lastname@example.org, Corinna Vinschen
On 04/18/2017 05:04 AM, Corinna Vinschen wrote:
On Apr 17 03:39, Daniel Santos wrote:
I actually did try that, although I had guessed it wouldn't (and shouldn't)
work. I believe that the reason is that rather the accesses are volatile or
not, gcc can see nothing else using it and memset can be a treated as a
compiler built-in (per the C spec, maybe C89?), so it can presume the
outcome. If there's a cleaner way to do this, I would really love to learn
that. __attribute__ ((used)) only works on variables with static storage.
Now I suspect that I may have found a little bug in gcc, because if I call
memset by casting it directly as a volatile function pointer, it is still
optimized away, and it should not:
((void *(*volatile)(void *, int, size_t))memset) (buf, 0, sizeof (buf));
And most interestingly, if I first assign a local volatile function pointer
to the address, then gcc properly does *not* optimize it away:
void *(*volatile vol_memset)(void *, int, size_t) = memset;
vol_memset (buf, 0, sizeof (buf));
I'm actually really glad for your response and that I played with this
because I need to make sure that this problem doesn't exist in gcc7. I have
changes going into gcc8 shortly and this could mask problems from my test
program where I cast functions as volatile w/o assigning using a local.
What about using RtlSecureZeroMemory instead?
Well that's surprising. Yes, it does solve the problem and I presume
would be more portable. :) It even inlines the "memset", but uses a
single byte rep stos. Technically, I think that a double-word stos
could be used in this case, but I doubt that matters much.