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<div class="moz-cite-prefix">On 9/30/13 11:22 AM, Alexey Samsonov
wrote:<br>
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<div class="gmail_quote">On Mon, Sep 30, 2013 at 7:48 PM, John
Criswell <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:criswell@illinois.edu" target="_blank">criswell@illinois.edu</a>></span>
wrote:<br>
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<div>On 9/30/13 9:40 AM, Alexey Samsonov wrote:<br>
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<div dir="ltr">Hi llvmdev!
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<div>There are cases when we want our
instrumentation passes for Sanitizer tools to
insert llvm.memset.* calls (basically, we want
to mark certain region of user memory as
(un)addressable by writing magic values for
"shadow" of that memory region). llvm.memset
are convenient:</div>
<div>(1) we don't have to manually emit all
these n-byte stores in a cycle.</div>
<div>(2) llvm.memset can be inlined as a
platform-specific fast instructions (e.g.
SSE).</div>
<div>But there will be a problem if llvm.memset
is lowered into a regular memset() call:
sanitizer runtime libraries intercept all
memset() calls and treat them as function
calls made by user, in particular checking
that its arguments point to an addressable
"user" memory, not some sanitizer-specific
memory regions.</div>
<div><br>
</div>
<div>Can you suggest a way to ensure
llvm.memset() is not transformed into memset
function()? This intrinsic has
<isvolatile> argument, which limits
possible optimization of this call, does it
make sense to add yet another argument, that
would forbid transforming it into function
calls?</div>
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Dumb question: why not run the ASan instrumentation
passes first and then run the pass that inserts the
calls to llvm.memset()?<br>
<br>
Alternatively, why not put the llvm.memset and
load/store instrumentation into a single pass? That
way, the pass can determine which memsets it added
itself and which are ones from the original program that
need instrumentation.<br>
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<div>Sorry, I didn't understand your suggestions. Maybe I
poorly described the problem. We need a way to teach
CodeGen that some llvm.memset intrinsics can't be lowered
into memset function call (those, that were added by ASan
instrumentation pass), and some can (all the others).
Otherwise the program would crash on ASan-added memset()
at runtime.</div>
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<br>
Ah. I think I see: you're not instrumenting memset(); you have a
replacement memset() implementation in your run-time library. As
such, you don't want your calls to llvm.memset() to be changed into
memset() because then they'll call your new implementation of
memset(). Is that correct?<br>
<br>
I figured my question was dumb; I just didn't know why.<br>
:)<br>
<br>
Assuming my understanding of the situation is correct, I don't
really have a good answer for you. You could try using vector
stores instead of llvm.memset() and see if the optimizers/code
generators don't change that into memset(). If you can be more
intrusive, you could add an attribute to llvm.memset() that tells
the code generator not to change it to memset(). However, I don't
have an idea of how to do it without changing LLVM and without doing
something that might break in the future.<br>
<br>
-- John T.<br>
<br>
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<div bgcolor="#FFFFFF" text="#000000"> <br>
-- John T.<span class="HOEnZb"><font color="#888888"><br>
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-- <br>
<div>Alexey Samsonov, MSK</div>
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-- <br>
<div>Alexey Samsonov, MSK</div>
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