[llvm-bugs] [Bug 39741] New: [X86] DomainReassignment pass replaces GR32/GR64 with VK32/VK64 on KNL where VK32/VK64 aren't legal register classes

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39741

            Bug ID: 39741
           Summary: [X86] DomainReassignment pass replaces GR32/GR64 with
                    VK32/VK64 on KNL where VK32/VK64 aren't legal register
                    classes
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: craig.topper at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

The domain reassigment pass tries to replace GPR registers with their
equivalent width mask registers, but VK32/VK64 aren't legal without AVX512BW. I
guess for the test cases we have the register coalescer manages to get rid of
the VK32 reference or the register allocator avoids spilling it.

But I really don't think there's any guarantee that we can't end up generating
a VK32/VK64 spill because of this.

I tried disabling the pass when AVX512BW is not enabled, but got some extra
moves in our lit tests. Probably need some extra DAG combiner or lowering work
to recover.

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