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[clang] Speedup line offset mapping computation

Authored by serge-sans-paille on Apr 1 2021, 1:18 PM.

Description

[clang] Speedup line offset mapping computation

Clang spends a decent amount of time in the LineOffsetMapping::get(...)
function. This function used to be vectorized (through SSE2) then the
optimization got dropped because the sequential version was on-par performance
wise.

This provides an optimization of the sequential version that works on a word at
a time, using (documented) bithacks to provide a portable vectorization.

When preprocessing the sqlite amalgamation, this yields a sweet 3% speedup.

Differential Revision: https://reviews.llvm.org/D99409