[cfe-commits] [Patch 2 of 5] Proposed fix for PR12850

[Andy Gibbs]

Hi,

Attached here is the second of five patches for PR12850.

This patch is where all the main functionality resides.  A new class,
FunctionCache, has been created inside the lib/AST directory.  This class
implements two caches of previously evaluated constexpr functions
according to their FunctionDecl* and evaluated arguments (as APValues).
The two caches are divided into a global cache where the evaluation can
be determined as valid across all evaluation contexts, and a local cache
that is specific to the current evaluation context (see IsSupported).

When a potentially cachable constexpr function is being evaluated, a
marker will be inserted into the cache.  Once the evaluation has been
completed successfully, this marker is replaced with the resultant
value of the evaluation.  If this value itself is determined to be
uncachable, or if the evaluation cannot be completed, the marker is
removed completely from the cache.  Additionally to this, now if a
circular dependency is discovered, an error message can now be shown
to the user.

If, later, the same function is evaluated with the same evaluated
parameters (and it was previously cached), the previously evaluated
result can be pulled from the cache.  In order to be sure that the
parameters really are the same, firstly it must match the same hash
value (see CalculateHash) and then the cached and current values are
directly compared (see IsAPValueEqual).  The comparison is intended
to be on the conservative side so that cache misses should occur
rather than invalid matches.  For the majority of APValue kinds (i.e.
Int, Float, ComplexInt, ComplexFloat, Vector, Array, Struct) the
comparison is straight forward.  For LValue it is more involved, since
it must walk through the LValue path and additionally StringLiterals
are handled as a special case so that two identical strings will still
match.  At the present time Union, AddrLabelDiff and MemberPointer
kinds are *not* supported.

Hopefully this gives enough detail to understand the code in the patch.
I will be very willing to give more explanations to specific questions
that come up... :o)

Thanks
Andy


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