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Right. If you do:
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<div class=""> int * x;</div>
<div class=""> int ** y=&x // or address of any pointer;</div>
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<div class="">, you should find the value y either multiple of 4 or 8 (for offending architecture).</div>
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Hui Zhou</div>
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T: 630-252-3430</div>
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<div class="">On Jun 20, 2019, at 1:09 PM, Nick Radcliffe via discuss <<a href="mailto:discuss@mpich.org" class="">discuss@mpich.org</a>> wrote:</div>
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<div style="margin-top: 0px; margin-bottom: 0px;" class="">Actually, scratch that. My example isn't valid for this case. I'll have to take another look at the user's code to see why it's failing.<br class="">
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<font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>Nick Radcliffe<br class="">
<b class="">Sent:</b><span class="Apple-converted-space"> </span>Thursday, June 20, 2019 1:04:34 PM<br class="">
<b class="">To:</b><span class="Apple-converted-space"> </span><a href="mailto:discuss@mpich.org" class="">discuss@mpich.org</a><br class="">
<b class="">Cc:</b><span class="Apple-converted-space"> </span>Raffenetti, Kenneth J.<br class="">
<b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [mpich-discuss] Incorrect error checking?</font>
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<div style="margin-top: 0px; margin-bottom: 0px;" class="">Ken,<br class="">
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<div style="margin-top: 0px; margin-bottom: 0px;" class="">The reason I brought this up is that a user's (valid) code was failing due to this parameter check. It's possible to use an address that's not 8 byte aligned to store a pointer to an int, i.e.,</div>
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<div style="margin-top: 0px; margin-bottom: 0px;" class=""> int *x;</div>
<div style="margin-top: 0px; margin-bottom: 0px;" class=""> int *y;<br class="">
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<div class=""> x = malloc(16);<br class="">
y = &x[1];</div>
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<div style="margin-top: 0px; margin-bottom: 0px;" class="">I would generally expect y to be 4 byte aligned in the above case. So sometimes this parameter check will cause a correct program to fail, right?<br class="">
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<div id="x_divRplyFwdMsg" dir="ltr" class=""><font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>Raffenetti, Kenneth J. via discuss <<a href="mailto:discuss@mpich.org" class="">discuss@mpich.org</a>><br class="">
<b class="">Sent:</b><span class="Apple-converted-space"> </span>Wednesday, June 19, 2019 2:44:40 PM<br class="">
<b class="">To:</b><span class="Apple-converted-space"> </span><a href="mailto:discuss@mpich.org" class="">discuss@mpich.org</a><br class="">
<b class="">Cc:</b><span class="Apple-converted-space"> </span>Raffenetti, Kenneth J.<br class="">
<b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [mpich-discuss] Incorrect error checking?</font>
<div class=""> </div>
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<div class="x_BodyFragment"><font size="2" class=""><span style="font-size: 11pt;" class="">
<div class="x_PlainText">Upon further thought, it's just trying to catch a common error, and uses<span class="Apple-converted-space"> </span><br class="">
alignment to detect it since the type information is lost thru the<span class="Apple-converted-space"> </span><br class="">
interface. In testing on x86_64, it doesn't appear that it will catch<span class="Apple-converted-space"> </span><br class="">
the intended error.<br class="">
<br class="">
As Thomas pointed out in another reply, there are also some issues with<span class="Apple-converted-space"> </span><br class="">
the implementation, though it is correct that we require intptr_t to<span class="Apple-converted-space"> </span><br class="">
build MPICH. At the very least, we could patch to use sizeof(void *) vs.<span class="Apple-converted-space"> </span><br class="">
sizeof(intptr_t). Or we can still remove the check since the use-case is<span class="Apple-converted-space"> </span><br class="">
so narrow. The preprocessor guard also depends on a AC_TRY_RUN that I'd<span class="Apple-converted-space"> </span><br class="">
prefer to delete, to be honest.<br class="">
<br class="">
Ken<br class="">
<br class="">
On 6/19/19 9:27 AM, Raffenetti, Kenneth J. via discuss wrote:<br class="">
> Nick,<br class="">
><span class="Apple-converted-space"> </span><br class="">
> I agree this check looks like nonsense. IMO, we can just delete it and<br class="">
> the others like it. I will create a PR.<br class="">
><span class="Apple-converted-space"> </span><br class="">
> Ken<br class="">
><span class="Apple-converted-space"> </span><br class="">
> On 6/18/19 5:25 PM, Nick Radcliffe via discuss wrote:<br class="">
>> I recently noticed that a parameter check in MPII_Win_get_attr looks a<br class="">
>> bit off:<br class="">
>><br class="">
>><br class="">
>> /* A common user error is to pass the address of a 4-byte<br class="">
>> * int when the address of a pointer (or an address-sized int)<br class="">
>> * should have been used. We can test for this specific<br class="">
>> * case. Note that this code assumes sizeof(intptr_t) is<br class="">
>> * a power of 2. */<br class="">
>> if ((intptr_t) attribute_val & (sizeof(intptr_t) - 1)) {<br class="">
>> MPIR_ERR_SETANDSTMT(mpi_errno, MPI_ERR_ARG, goto<br class="">
>> fn_fail, "**attrnotptr");<br class="">
>> }<br class="">
>><br class="">
>><br class="">
>> The comment indicates that the check is testing "attribute_val" to see<br class="">
>> if a pointer to int was passed in, rather than a double pointer or<br class="">
>> pointer to an integer that can store an address. But the check seems to<br class="">
>> only be testing for 8 byte alignment (and failing if not aligned). Am I<br class="">
>> missing something here?<br class="">
>><br class="">
>><br class="">
>><br class="">
>> Nick Radcliffe Software Engineer| Cray Inc.<br class="">
>><br class="">
>> 2131 Lindau Ln #1000 | Bloomington, MN55425<br class="">
>><br class="">
>> +1-651-605-8864 nradclif@<a href="http://cray.com" class="">cray.com</a> <<a href="mailto:email@cray.com" class="">mailto:email@cray.com</a>> www.cray.com<br class="">
>> <<a href="http://www.cray.com/" class="">http://www.cray.com</a>><br class="">
>><br class="">
>><br class="">
>> _______________________________________________<br class="">
>> discuss mailing list <a href="mailto:discuss@mpich.org" class="">discuss@mpich.org</a><br class="">
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>><br class="">
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