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Silence GCC warning about broken strict aliasing rules

Authored by kimgr on Aug 12 2018, 5:39 AM.



The as<T>() method would trigger the following warning on GCC <7:

warning: dereferencing type-punned pointer will break
    strict-aliasing rules [-Wstrict-aliasing]

  return *reinterpret_cast<T *>(Union.buffer);

Union.buffer is guaranteed to be aligned to whatever types it contains,
and json::Value maintains the invariant that it only calls as<T>() for a
T it has previously placement-newed into Union.buffer. This should
follow the rules for strict aliasing.

Older GCCs might be incorrectly identifying this as a strict-aliasing
violation and undefined behavior, which could lead to misoptimizations
down the line. We've seen no evidence of such behavior.

No functional change intended.

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Event Timeline

kimgr created this revision.Aug 12 2018, 5:39 AM
kimgr edited the summary of this revision. (Show Details)Aug 12 2018, 6:31 AM
kimgr abandoned this revision.Aug 12 2018, 9:24 AM

Better solution in D50608.