With this patch, compiler generated profile variables will have its own COMDAT
name for ELF format, which syncs the behavior with COFF. Tested with clang
PGO bootstrap. This shows a modest reduction in object sizes in ELF format.
Differential Revision: https://reviews.llvm.org/D68041
llvm-svn: 373241
For COFF, a comdat group is really a symbol marked
IMAGE_COMDAT_SELECT_ANY and zero or more other symbols marked
IMAGE_COMDAT_SELECT_ASSOCIATIVE. Typically the associative symbols in
the group are not external and are not referenced by other TUs, they are
things like debug info, C++ dynamic initializers, or other section
registration schemes. The Visual C++ linker reports a duplicate symbol
error for symbols marked IMAGE_COMDAT_SELECT_ASSOCIATIVE even if they
would be discarded after handling the leader symbol.
Fixes coverage-inline.cpp in check-profile after r372020.
llvm-svn: 372182
This fixes relocations against __profd_ symbols in discarded sections,
which is PR41380.
In general, instrumentation happens very early, and optimization and
inlining happens afterwards. The counters for a function are calculated
early, and after inlining, counters for an inlined function may be
widely referenced by other functions.
For C++ inline functions of all kinds (linkonce_odr &
available_externally mainly), instr profiling wants to deduplicate these
__profc_ and __profd_ globals. Otherwise the binary would be quite
large.
I made __profd_ and __profc_ comdat in r355044, but I chose to make
__profd_ internal. At the time, I was only dealing with coverage, and in
that case, none of the instrumentation needs to reference __profd_.
However, if you use PGO, then instrumentation passes add calls to
__llvm_profile_instrument_range which reference __profd_ globals. The
solution is to make these globals externally visible by using
linkonce_odr linkage for data as was done for counters.
This is safe because PGO adds a CFG hash to the names of the data and
counter globals, so if different TUs have different globals, they will
get different data and counter arrays.
Reviewers: xur, hans
Differential Revision: https://reviews.llvm.org/D67579
llvm-svn: 372020
Summary:
I hadn't realized that instrumentation runs before inlining, so we can't
use the function as the comdat group. Doing so can create relocations
against discarded sections when references to discarded __profc_
variables are inlined into functions outside the function's comdat
group.
In the future, perhaps we should consider standardizing the comdat group
names that ELF and COFF use. It will save object file size, since
__profv_$sym won't appear in the symbol table again.
Reviewers: xur, vsk
Subscribers: eraman, hiraditya, cfe-commits, #sanitizers, llvm-commits
Tags: #clang, #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D58737
llvm-svn: 355044
Summary:
The motivating use case is eliminating duplicate profile data registered
for the same inline function in two object files. Before this change,
users would observe multiple symbol definition errors with VC link, but
links with LLD would succeed.
Users (Mozilla) have reported that PGO works well with clang-cl and LLD,
but when using LLD without this static registration, we would get into a
"relocation against a discarded section" situation. I'm not sure what
happens in that situation, but I suspect that duplicate, unused profile
information was retained. If so, this change will reduce the size of
such binaries with LLD.
Now, Windows uses static registration and is in line with all the other
platforms.
Reviewers: davidxl, wmi, inglorion, void, calixte
Subscribers: mgorny, krytarowski, eraman, fedor.sergeev, hiraditya, #sanitizers, dmajor, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D57929
llvm-svn: 353547
Rong Xu