[Bug translator/17246] New: Elide write-only tapset globals

["jistone at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17246

            Bug ID: 17246
           Summary: Elide write-only tapset globals
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: translator
          Assignee: systemtap at sourceware dot org
          Reporter: jistone at redhat dot com

Following on PR17242, it would be nice if we could elide tapset globals which
are initialized, but never actually used.  Write-only user globals are chosen
for automatic probe end display in add_global_var_display(), but tapset globals
should ideally be fine to remove.

However, there's this caution in dead_assignment_remover::visit_assignment():

  // NB: Not so fast!  The left side could be an array whose
  // index expressions may have side-effects.  This would be
  // OK if we could replace the array assignment with a
  // statement-expression containing all the index expressions
  // and the rvalue... but we can't.
  // Another possibility is that we have an unread global variable
  // which are kept for probe end value display.

So it refuses to remove assignments to any global variables at all.

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