Summary:
Enable BOLT to reorder data sections in a binary based on memory
profiling data.
This diff adds a new pass to BOLT that can reorder data sections for
better locality based on memory profiling data. For now, the algorithm
to order data is primitive and just relies on the frequency of loads to
order the contents of a section. We could probably do a lot better by
looking at what functions use the hot data and grouping together hot
data that is used by a single function (or cluster of functions).
Block ordering might give some hints on how to order the data better as
well.
The new pass has two basic modes: inplace and split (when inplace is
false). The default is split since inplace hasn't really been tested
much. When splitting is on, the cold data is copied to a "cold" version
of the section while the hot data is kept in the original section, e.g.
for .rodata, .rodata will contain the hot data and .bolt.org.rodata will
contain the cold bits. In inplace mode, the section contents are
reordered inplace. In either mode, all relocations to data within that
section are updated to reflect new data locations.
Things to improve:
- The current algorithm is really dumb and doesn't seem to lead to any
wins. It certainly could use some improvement.
- Private symbols can have data that leaks over to an adjacent symbol,
e.g. a string that has a common suffix can start in one symbol and
leak over (with the common suffix) into the next. For now, we punt on
adjacent private symbols.
- Handle ambiguous relocations better. Section relocations that point
to the boundary of two symbols will prevent the adjacent symbols from
being moved because we can't tell which symbol the relocation is for.
- Handle jump tables. Right now jump table support must be basic if
data reordering is enabled.
- Being able to handle TLS. A good amount of data access in some
binaries are happening in TLS. It would be worthwhile to be able to
reorder any TLS sections too.
- Handle sections with writeable data. This hasn't been tested so
probably won't work. We could try to prevent false sharing in
writeable sections as well.
- A pie in the sky goal would be to use DWARF info to reorder types.
(cherry picked from FBD6792876)
Summary:
BOLT sources are being moved under tools/llvm-bolt/src
and tools/llvm-bolt will contain more files such as LICENSE.txt,
README.txt, etc.
Remove trailing white spaces from our sources.
Create llvm.patch by running
> git diff f137ed238db11440f03083b1c88b7ffc0f4af65e include lib > \
tools/llvm-bolt/llvm.patch
README.txt has instructions on checking out sources and applying the
patch.
(cherry picked from FBD7878380)
