[llvm-bugs] [Bug 40292] New: [SCEV] Enormous SCEV growth on linear calculations

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Jan 11 03:23:05 PST 2019


            Bug ID: 40292
           Summary: [SCEV] Enormous SCEV growth on linear calculations
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: max.kazantsev at azul.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 21316
  --> https://bugs.llvm.org/attachment.cgi?id=21316&action=edit

The attached file demonstrates a corner case that makes SCEV hang. opt
-loop-vectorize -S works "infinitely" on this test due to huge SCEV creation.
Sometimes I observe crashes by stack overflow that seem to also be caused by
attempts to create huge SCEVs. Also reproducible with opt -analyze
-scalar-evolution (so it is not something specific for vectorizer).

This is a typical scenario of linear calculations with powers that come from
unrolled loops. We used to face problems of this variety in the past. Maybe
it's time to add some mechanism to restrict SCEV from doing any attempts of
simplifications here. Limiting by depth doesn't seem sufficient.

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