[Bug translator/34414] New: @choose_defined / trinary-expression over-evaluation

[fche at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=34414

            Bug ID: 34414
           Summary: @choose_defined / trinary-expression over-evaluation
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
          Assignee: systemtap at sourceware dot org
          Reporter: fche at redhat dot com
  Target Milestone: ---

When elaborating @defined($x) ? $x : $y,
  var_expanding_visitor::visit_defined_op resolves the condition and
  then calls abort_provide(), which sets aborted_p so the default
  update_visitor::visit_ternary_expression skips both arms. The tree
  is left as a ternary with a constant condition and still-unexpanded
  target symbols in the arms. A later expansion pass walks that ternary
  again without short-circuiting and elaborates the unused arm as
  well. That makes scripts like @defined($mode) ? $mode : $nosuchvar
  fail Pass 2 on $nosuchvar even when @defined($mode) is true (seen on
  semok/thirtysix.stp / recent Fedora kernels). The intended semantics
  are C-like: only the taken arm should be expanded. A fix is to teach
  var_expanding_visitor to short-circuit ternaries whose condition
  is already a literal—expand only the taken arm and provide that
  expression directly. Note also that on current kernels $mode is not
  @defined at every do_sys_open PC (often optimized out while $flags
  remains), so tests that assume uniform $mode availability need a
  stabler local even after the translator fix.

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