[PATCH] D52268: [AST] Squeeze some bits in LinkageComputer

[Bruno Ricci via Phabricator via cfe-commits]

riccibruno added a comment.

In https://reviews.llvm.org/D52268#1241033, @rjmccall wrote:

> `LinkageComputer` isn't actually persisted anywhere, right?  And there's maybe one computer active at once?  So this compression is theoretically saving one pointer of stack space but forcing a bunch of bit-manipulation every time these fields are accessed.


It is not persisted but this saves one pointer per entry in the map. Another factor is that hashing a pair involves hashing
each component and then combining the result, which is comparatively much more expansive than just hashing a `PointerIntPair`,
which involves only a shift and a xor. The field storing the `LVComputationKind` is never directly read but only used to differentiate
various kinds of computations in the map. I went back and instrumented the lookup function `LinkageComputer::lookup` with `rdtsc`,
and (with all the usual caveats about microbenchmarks and `rdtsc`) I get that this cuts the number of ticks spent inside `lookup`
from about 8e6 to 3.5e6. Now of course taking a step back this represent only milliseconds and is firmly in the category of
"way to small to bother",  but now we might as well do it.


Repository:
  rC Clang

https://reviews.llvm.org/D52268