Basic IPA optimizations based on profile.
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ipa-profile pass implements the following analysis propagating profille
inter-procedurally.
- Count histogram construction. This is a histogram analyzing how much
time is spent executing statements with a given execution count read
from profile feedback. This histogram is complete only with LTO,
otherwise it contains information only about the current unit.
The information is used to set hot/cold thresholds.
- Next speculative indirect call resolution is performed: the local
profile pass assigns profile-id to each function and provide us with a
histogram specifying the most common target. We look up the callgraph
node corresponding to the target and produce a speculative call.
This call may or may not survive through IPA optimization based on decision
of inliner.
- Finally we propagate the following flags: unlikely executed, executed
once, executed at startup and executed at exit. These flags are used to
control code size/performance threshold and code placement (by producing
.text.unlikely/.text.hot/.text.startup/.text.exit subsections).
Entry in the histogram.
1.13.2