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<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">The value exiting the loop of the linear clause list-item seems not correct in clang openmp codegen. According to OpenMP 4.5 standard Section 2.15.3.7, the value corresponding to the sequentially
last iteration of the associated loop(s) is assigned to the original list item.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">Let’s look at the following test case:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">#include<iostream><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">using namespace std;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">int main() {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> int b = 10, M = 10;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> int a[10], i;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> #pragma omp parallel num_threads(4)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> #pragma omp for linear(b: 2)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> for (i = 0; i < M; i++)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> a[i] = b;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> cout << b << endl;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> for (i = 0; i < M; i++)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> cout << a[i] << " ";<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> cout << endl;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> return 0;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">The result of `clang++ temp.cpp -fopenmp && ./a.out` is<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">30<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">10 12 14 16 18 20 22 24 26 28<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">If we change the above test case as follows<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> #pragma omp parallel num_threads(4)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> #pragma omp for linear(b: 2)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> for (i = 0; i < M; i++) {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:24.0pt"><span lang="EN-US" style="font-size:12.0pt">a[i] = b;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:24.0pt"><span lang="EN-US" style="font-size:12.0pt">b = 10;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">The result is the same. The exiting value in clang openmp codegen is implemented as (logical last iteration + 1) * linear_step + linear_base, which is (M – 1 + 1) * 2 + 10 for this case. This
is apparently incorrect.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">Here, there are two problems regarding to the Openmp standard.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:18.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span lang="EN-US" style="font-size:12.0pt"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US" style="font-size:12.0pt">The example linear_in_loop.1.c in openmp-examples-5.1.pdf is as follows:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">j = 0;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">#pragma omp parallel<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">#pragma omp for linear(j:1)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">for ( i = 0; i < N; i += 2 ) {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:12.0pt"><span lang="EN-US" style="font-size:12.0pt">b[j] = a[i] * 2.0f;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:12.0pt"><span lang="EN-US" style="font-size:12.0pt">j++;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">`j++` is really not necessary here. The compiler will calculate the `j` for each iteration according to the openmp standard (the value of the new list item on each iteration of the associated
loop(s) corresponds to the value of the original list item before entering the construct plus the logical number of the iteration times linear-step) and `j++` do the redundant calculation, which is not efficient.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="margin-left:18.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span lang="EN-US" style="font-size:12.0pt"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US" style="font-size:12.0pt">Which value should be assigned to the original list item?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">If it is the value of the new list item after entering the construct, the results for the above two cases should be 28, 28. In this case, the value of “(M – 1) * linear_step + original_b” is assigned
to the original b, where “M – 1” is the logical last iteration.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">If it is the value of the new list item corresponding to the sequentially last iteration of the loop before exiting the construct, the results for the above two cases should be 28, 10. In this
case, the value of the privatized variable b is assigned the the original b.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">All the best,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt">Peixin<o:p></o:p></span></p>
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