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<p><font size="2" face="Helvetica">Hi all,</font><br>
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<font size="2" face="Helvetica">I would like to propose an improvement of the “almost dead” block elimination in Transforms/Local.cpp so that it will preserve the canonical loop form for loops with a volatile iteration variable.</font><br>
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<font size="2" face="Helvetica">*** Problem statement</font><br>
<font size="2" face="Helvetica">Nested loops in LCALS Subset B (</font><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__codesign.llnl.gov_LCALS.php&d=AwMGaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=aWKfvN4c8lvUSvVn8J0Z2ajTctlBJf0198Au28epBr0&s=4d9dt5ODcDWHHatSrwu5ZYT9ebgVzNEtpOlIR87izCM&e="><font size="2" face="Helvetica"><u>https://codesign.llnl.gov/LCALS.php</u></font></a><font size="2" face="Helvetica">) are not vectorized with LLVM -O3 because the LLVM loop vectorizer fails the test whether the loop latch and exiting block of a loop is the same. The loops are vectorizable, and get vectorized with LLVM -O2 and also with other commercial compilers (icc, xlc). </font><br>
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<font size="2" face="Helvetica">*** Details</font><br>
<font size="2" face="Helvetica">These loops ended up with different loop latch and exiting block after a series of optimizations including loop unswitching, jump threading, simplify-the-CFG, and loop simplify. The fundamental problem here is that the above optimizations cannot recognize a loop with a volatile iteration variable and do not preserve its canonical loop structure.</font><br>
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<font size="2" face="Helvetica">(1) Loop unswitching generates several empty placeholder BBs only with PHI nodes after separating out a shorter path with no inner loop execution from a standard path. </font><br>
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<font size="2" face="Helvetica">(2) Jump threading and simplify-the-CFG passes independently calls TryToSimplifyUnconditionalBranchFromEmptyBlock() in Transforms/Utils/Local.cpp to get rid of almost empty BBs. </font><br>
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<font size="2" face="Helvetica">(3) TryToSimplifyUnconditionalBranchFromEmtpyBlock() eliminates the placeholder BBs after loop unswitching and merges them into subsequent blocks including the header of the inner loop. Before eliminating the blocks, the function checks if the block is a loop header by looking at its PHI nodes so that it can be saved, but the test fails with the loops with a volatile iteration variable. The outer loop is now collapsed into the inner loop with multiple backedges.</font><br>
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<font size="2" face="Helvetica">(4) LoopSimplify checks if a loop with multiple backedges can be separated to nested loops by looking at PHI nodes in the loop header. The test fails with the loops with a volatile iteration variable. </font><br>
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<font size="2" face="Helvetica">(5) LoopSimplify then creates a unique backedge block for the loop, and the loop now has a different loop latch (the unique backedge block created in (3)) and exiting block (a block where the volatile outer loop variable is incremented and tested). </font><br>
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<font size="2" face="Helvetica">*** Proposed solutions</font><br>
<font size="2" face="Helvetica">(1) Make LoopInfo available in Jump Threading and Simplify-the-CFG passes and use LoopInfo to test whether an almost empty BB is a loop header. If yes, do not eliminate the BB.</font><br>
<font size="2" face="Helvetica"> Pros: Leverages existing analysis results, small code changes / Cons: Add pass dependencies </font><br>
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<font size="2" face="Helvetica">(2) Instead of using LoopInfo, iterate through BB’s to identify backedges and loop headers in TryToSimplifyUnconditionalBranchFromEmptyBlock() and use the results to test if a BB is a loop header. If yes, do not eliminate the BB. Jump Threading has functions that do similar cheap loop analysis.</font><br>
<font size="2" face="Helvetica"> Pros: No need to depend on external analysis results / Cons: More lines to be added</font><br>
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<font size="2" face="Helvetica">*** Summary</font><br>
<font size="2" face="Helvetica">I would like to propose an improvement of the “almost dead” block elimination in Transforms/Local.cpp so that it will preserve the canonical loop form for loops with a volatile iteration variable. On top of the current algorithm relying on PHI nodes to recognize loops, actual loop analysis to test if a BB belongs to a loop and etc. can be used. </font><br>
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