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[Attributor] Deduce "no-recurse" function attribute
Needs ReviewPublic

Authored by jdoerfert on Apr 1 2019, 10:39 AM.

Details

Summary

Deduce the "no-recurse" function attribute through "backward-reasoning".
Thus, if no call site in the function may cause recursion, the function
does not recurse. Note that "backward-reasoning" is the only reasoning
the Attributor performs right now.

Impact on the statistics (-stats) for LLVM-TS + Spec2006, totaling
almost 80% more localized globals (probably due to the intrinsic
white list used).

CHANGED: attributor                   NumAttributesManifested                  612 ->      14331 ( +2241.667%)
CHANGED: attributor                   NumAttributesValidFixpoint             26069 ->      39788 (   +52.626%)
  ADDED: attributor                   NumFnNoRecurse                           n/a ->      13719
CHANGED: functionattrs                NumNoRecurse                           61932 ->      49305 (   -20.388%)
CHANGED: globalopt                    NumLocalized                              37 ->         66 (   +78.378%)

Note: Missing "no-recurse" deduction compared to functionattrs is

completely, or at least in large parts, due to a bug in the
existing code, see: https://llvm.org/PR41336

Possible improvements:

  • Perform "forward-reasoning": If all call sites are in no-recurse functions then the function musst be no-recurse.
  • Eliminate dead call sites: Similar to the preliminary reasoning in the no-return abstract attribute we can ignore dead call sites.