[llvm-bugs] [Bug 41066] New: X86 ISel gets stuck in infinite loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41066

            Bug ID: 41066
           Summary: X86 ISel gets stuck in infinite loop
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: andrew at scheidecker.net
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

Created attachment 21595
  --> https://bugs.llvm.org/attachment.cgi?id=21595&action=edit
IR that triggers bug

Using SVN revision 356146 (from 2019/03/14), llc gets stuck in a loop on the
attached input, when run with the command-line:

  llc llvm-lock2.ll -mcpu=skylake-avx512

Using llc from the LLVM 6.0 release does not get stuck in a loop.

Running it with e.g. -mcpu=skylake also does not get stuck in a loop. Based on
this, I've assumed this is an X86/AVX512 target specific bug.

Passing -debug prints the following in a loop:

  Combining: t13: v2i64 = or t1518, t2

  Combining: t1518: v2i64 = vselect t1517, t11, t10
  Creating new node: t1519: v2i1 = xor t1517, t24
  Creating new node: t1520: v2i64 = vselect t1519, t10, t11
   ... into: t1520: v2i64 = vselect t1519, t10, t11

  Combining: t13: v2i64 = or t1520, t2

  Combining: t1520: v2i64 = vselect t1519, t10, t11
  Creating new node: t1521: v2i1 = xor t1519, t24
  Creating new node: t1522: v2i64 = vselect t1521, t11, t10
   ... into: t1522: v2i64 = vselect t1521, t11, t10

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