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[compiler-rt][XRay] Support tail call sleds

Description

[compiler-rt][XRay] Support tail call sleds

Summary:
This change depends on D23986 which adds tail call-specific sleds. For
now we treat them first as normal exits, and in the future leave room
for implementing this as a different kind of log entry.

The reason for deferring the change is so that we can keep the naive
logging implementation more accurate without additional complexity for
reading the log. The accuracy is gained in effectively interpreting call
stacks like:

A()
  B()
    C()

Which when tail-call merged will end up not having any exit entries for
A() nor B(), but effectively in turn can be reasoned about as:

A()
B()
C()

Although we lose the fact that A() had called B() then had called C()
with the naive approach, a later iteration that adds the explicit tail
call entries would be a change in the log format and thus necessitate a
version change for the header. We can do this later to have a chance at
releasing some tools (in D21987) that are able to handle the naive log
format, then support higher version numbers of the log format too.

Reviewers: echristo, kcc, rSerge, majnemer

Subscribers: mehdi_amini, llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D23988

Details

Committed
dberrisOct 13 2016, 4:56 PM
Differential Revision
D23988: [compiler-rt][XRay] Support tail call sleds
Parents
rL284177: Improve comments.
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