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<div class="moz-cite-prefix">memoryIsNotModifiedBetween is precisely
the sort of expensive walk we shouldn't be doing... I'm surprised
it hasn't caused any serious issues yet. Ideally, what we should
be doing is using MemorySSA to find a dependency from the memset:
if the closest dependency is the malloc, there aren't any stores
between the memset and the malloc. (But we aren't using MemorySSA
in DSE yet; see <a class="moz-txt-link-freetext" href="https://reviews.llvm.org/D40480">https://reviews.llvm.org/D40480</a>.)<br>
<br>
But yes, memoryIsNotModifiedBetween has the right meaning.<br>
<br>
-Eli<br>
<br>
On 5/21/2018 7:48 AM, Dávid Bolvanský wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAOrgDVPHB9J56QedfUDD9+onN_FJtv0dcj4mit6_cJAAhJdF1w@mail.gmail.com">
<div dir="ltr"><span style="font-size:12.8px">"memory accesses
between the malloc and the memset</span><span
style="font-size:12.8px"> without an expensive linear scan of
the block/function" </span>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">(1) do you mean just use
"memoryIsNotModifiedBetween" function in DSE to check it?</span>
<div><br>
</div>
<div>x = maloc(..);</div>
<div>memset(x, ...) </div>
<div><br>
</div>
<div>(2) GetUnderlyingObject would give me Value * (from
malloc) ?</div>
<div>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">Also another case:</span></div>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">memset(s, 0, len); //
len > 1</span></div>
<div><span style="font-size:12.8px">return strlen(s); //
optimize to 0</span></div>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">(3) How to check
memset and strlen pairs? I have a strlen call, I have a
"Value *" for "s". What is the best way to construct
memset + strlen pairs?</span></div>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">(4) Can this be somehow
generalized (and not only for strlen)? So
GetStringLength in ValueTracking would be taught about
this info (string is empty after memset)</span></div>
<div><span style="font-size:12.8px"><br>
</span></div>
<div><span style="font-size:12.8px">(5) new malloc
memset folding / memset + strlen case should be
implemented in DSE, right?</span></div>
</div>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2018-05-17 21:36 GMT+02:00 Friedman,
Eli <span dir="ltr"><<a
href="mailto:efriedma@codeaurora.org" target="_blank"
moz-do-not-send="true">efriedma@codeaurora.org</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div class="m_6141790905467921389moz-cite-prefix">The
fundamental problem with trying to do that sort of
transform in instcombine is that you don't have access
to MemoryDependenceAnalysis or MemorySSA; you need a
data structure like that to figure out whether there are
any memory accesses between the malloc and the memset
without an expensive linear scan of the block/function.
(You can sort of get around the problem in simple cases
by adding arbitrary limits to the number of you scan,
but it doesn't generalize well.)<span class="HOEnZb"><font
color="#888888"><br>
<br>
-Eli</font></span>
<div>
<div class="h5"><br>
<br>
On 5/17/2018 12:17 PM, Dávid Bolvanský wrote:<br>
</div>
</div>
</div>
<div>
<div class="h5">
<blockquote type="cite">
<div dir="ltr">As we talked in <a
href="https://reviews.llvm.org/D45344"
target="_blank" moz-do-not-send="true">https://reviews.llvm.org/<wbr>D45344</a>,
the problem was dead stores. And I know why :D
There was just -instcombine pass. I forgot to do
-dse before -instcombine so this is why I did
custom "store removal" code there.
<div><br>
</div>
<div>I would like to finish malloc + llvm.memset
folding. Yes, you told you would like to see the
whole foldMallocMemset in DSE but extend it for
llvm.memset in InstCombine... is it really so
bad to do? </div>
<div>We have standard malloc + memset folding
there, so a few new lines should not do bad
things.</div>
<div><br>
</div>
<div>If I reopen D45344, reupload patch with
removed my custom "store removal" code, It could
be ok, no? The patch as is worked/works for me
for malloc + llvm.memset folding, I would just
add -dse to tests to handle dead stores.<br>
<div><br>
</div>
<div><br>
</div>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2018-05-17 21:00
GMT+02:00 Friedman, Eli <span dir="ltr"><<a
href="mailto:efriedma@codeaurora.org"
target="_blank" moz-do-not-send="true">efriedma@codeaurora.org</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0
0 0 .8ex;border-left:1px #ccc
solid;padding-left:1ex"><span>On 5/17/2018
8:58 AM, Dávid Bolvanský via llvm-dev wrote:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex"> Hello,<br>
<br>
I would like to find a way to do this
removal properly. I found DSE and
"eliminateNoopStore" can be useful for
this thing.<br>
<br>
What I mean?<br>
int *test = malloc(15 * sizeof(int));<br>
test[10] = 12; < ----- remove this
store<br>
memset(test,0,sizeof(int) * 15);<br>
</blockquote>
<br>
</span> This is classic dead store
elimination, and it's already handled by DSE.
At least, we optimize the following testcase:<br>
<br>
#include <stdlib.h><br>
#include <string.h><br>
void bar(int*);<br>
void f() {<span><br>
int *test = malloc(15 * sizeof(int));<br>
test[10] = 12;<br>
</span> memset(test,0,sizeof(int) * 15);<br>
bar(test);<br>
}<br>
<br>
You should be able to look at the existing
code to understand how it's handled (using
MemoryDependenceAnalysis).<span
class="m_6141790905467921389HOEnZb"><font
color="#888888"><br>
<br>
-Eli<br>
<br>
-- <br>
Employee of Qualcomm Innovation Center,
Inc.<br>
Qualcomm Innovation Center, Inc. is a
member of Code Aurora Forum, a Linux
Foundation Collaborative Project<br>
<br>
</font></span></blockquote>
</div>
<br>
</div>
</blockquote>
<p><br>
</p>
<pre class="m_6141790905467921389moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
</div>
</div>
</div>
</blockquote>
</div>
<br>
</div>
</blockquote>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
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