[PATCH] D15829: [PGO] Clang Option that enables IR level PGO instrumentation

[David Li via cfe-commits]

davidxl added a comment.

For the longer term, one possible solution is to make FE based
instrumentation only used for coverage testing which can be turned on
with -fcoverage-mapping option (currently, -fcoverage-mapping can not
be used alone and must be used together with
-fprofile-instr-generate). To summarize:

A. Current behavior:
--------------------

1. -fprofile-instr-generate turns on FE based instrumentation
2. -fprofile-instr-generate -fcoverage-mapping turns on FE based

instrumentation and coverage mapping data generation.

3. -fprofile-instr-use=<..> assumes profile data from FE instrumentation.



B. Proposed new behavior:
-------------------------

1. -fprofile-instr-generate turns on IR late instrumentation
2. -fcoverage-mapping turns on FE instrumentation and coverage-mapping
3. -fprofile-instr-generate -fcoverage-mapping result in compiler warning
4. -fprofile-instr-use=<> will automatically determine how to use the

profile data.

B.2) above can be done today for improved usability. B.1) needs a
transition period before  the IR based instrumentation becomes
stablized (and can be flipped to the default).  During the transition
period, the behavior of 1) does not change, but a cc1 option can be
used to turn on IR instrumentation (as proposed by Sean).

In the real longer term, I think IR based instrumentation can also be
used for coverage testing too (by disabling some pre-optimizations and
pre-inlining needed for PGO purpose) -- but this is a different topic
to be discussed.

thanks,

David


http://reviews.llvm.org/D15829