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Hi, Scott,</div>
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Thanks for reporting this problem. We should get a bug filed on this issue at bugs.llvm.org. If you're not able to do this, please let us know, and someone else can take care of it.</div>
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-Hal</div>
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<div class="PlainText">Hal Finkel<br>
Lead, Compiler Technology and Programming Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Scott Manley via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> Thursday, May 2, 2019 4:14 PM<br>
<b>To:</b> llvm-dev<br>
<b>Subject:</b> [llvm-dev] llvm is illegally vectorizing with a recurrence on skylake</font>
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<div>Hi -- I have found a bug in an HPC code where llvm is vectorizing a loop on Skylake that has an obvious recurrence. I derived a small test case based on the original benchmark below:</div>
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<div>/*****************************************************************/</div>
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<div>static void __attribute__ ((always_inline)) one(</div>
<div> const int *restrict in, const int *const end,</div>
<div> const unsigned shift, int *const restrict index,</div>
<div> int *const restrict out)</div>
<div>{</div>
<div> do {</div>
<div> int a_idx = *in>>shift;</div>
<div> int b_idx = index[a_idx]; </div>
<div> out[b_idx] = *in; // <-- reccurence as index[a_idx] can be the</div>
<div> index[a_idx]++; // same and incremented within the vector</div>
<div> } while(++in!=end); // which leads to incorrect results</div>
<div>}</div>
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<div>#ifndef NO_TWO</div>
<div>static void __attribute__ ((noinline)) two(</div>
<div> const int *restrict in, const int *const end,</div>
<div> const unsigned shift, int *const restrict index,</div>
<div> int *const restrict out)</div>
<div>{</div>
<div> do out[index[(*in>>shift)]++]=*in; while(++in!=end);</div>
<div>}</div>
<div>#endif</div>
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<div>void parent(</div>
<div> int digits, int n, int *restrict work, int * restrict idx,</div>
<div> int *restrict shift, int **restrict indicies)</div>
<div>{</div>
<div> int *in = work;</div>
<div> int *dst = work+n;</div>
<div>// int *indicies[1024];</div>
<div>// int shift[1024];</div>
<div> int d;</div>
<div> for(d=1;d!=digits-1;++d) {</div>
<div> int *t;</div>
<div> one(in,in+n,shift[d],indicies[d],dst);</div>
<div> t=in,in=dst,dst=t;</div>
<div> }</div>
<div>#ifndef NO_TWO</div>
<div> two(in,in+n,shift[d],indicies[d],idx);</div>
<div>#endif</div>
<div>}</div>
</div>
<div>/*****************************************************************/<br>
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clang -S -O2 -Rpass=loop-vectorize small.c -march=skylake-avx512</div>
<div>small.c:6:3: remark: vectorized loop (vectorization width: 16, interleaved count: 1) [-Rpass=loop-vectorize]</div>
<div> do {</div>
<div> ^</div>
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<div>I believe the problem to be a issue with dependency information getting destroyed because if you remove the two() function (or compile one() on its own, or prevent inlining of one()), it correctly prevents vectorization. <br>
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<div>clang -S -O2 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize small.c -march=skylake-avx512 -DNO_TWO</div>
<div>small.c:6:3: remark: loop not vectorized [-Rpass-missed=loop-vectorize]</div>
<div> do {</div>
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<div>I did trace it down to possibly being something within DepChecker->areDepsSafe() as it returns true for the incorrect case.</div>
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<div>Thanks,</div>
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<div>Scott</div>
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