[llvm-bugs] [Bug 32255] New: [X86] Should we use ExeDepsFix to optimize unused input to gather instructions?

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https://bugs.llvm.org/show_bug.cgi?id=32255

            Bug ID: 32255
           Summary: [X86] Should we use ExeDepsFix to optimize unused
                    input to gather instructions?
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: craig.topper at gmail.com
                CC: llvm-bugs at lists.llvm.org

If the gather mask is all ones or the input is undef, we force the input to a
zero vector during lowering. It may be better to instead force the input to
undef and let ExeDepsFix handle clearance checking.

We need to be careful though because I think ExeDepsFix will first try to tie
the input to the VSIB index register since that's already an input. We would
need to teach ExeDepsFix about the early clobber restriction on the source
register that makes that illegal.

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