I've now reproduced and analyzed the problem on i386 with GCC 4.5, i.e.
building python3.2-3.2~rc1 using the 4.5.1-1ubuntu3 compiler.
It turns out that there is no particular bug here (the CFG looks
identical in the -fprofile-generate and -fprofile-use cases), but
instead we're running in a known limitation of the profiling code: it is
not thread-safe.
This means that when using profiling in a multi-threaded application,
profile counters may not be fully accurate since counter updates may
have been skipped due to race conditions in accessing the counters.
This is consistent with the symptoms we're seeing: the problem
disappears on my (single-core) ARM board, but is present on
i386/amd64/ppc all of which presumably are built on multi-core machines,
which increases the chance of actually running into one of those race
conditions. The functions where the problem shows up likewise tend to
be places where high thread contention is expected (e.g. synchronization
primitives).
The recommended way to deal with this problem is to use the -fprofile-
correction flag, which will employ heuristics to attempt to adjust
incorrect counters, instead of simply aborting compilation. See the
manual page:
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates. When
this option is specified, GCC will use heuristics to correct or
smooth out such inconsistencies. By default, GCC will emit an error
message when an inconsistent profile is detected.
I've added the -fprofile-correction flag to the build_all_use_profile
rule in Makefile.pre.in, and with this change I was able to successfully
build the package (only i386 tested).
The issues with 4.6 must be some other problem (note that the profiling
code was significantly rewritten between 4.5 and 4.6, so we may well run
into new problems here). I'll look into this next.
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https://bugs.launchpad.net/bugs/598462
Title:
[PR38292] corrupted profile info with -O[23] -fprofile-use
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