Commit Graph

5 Commits

Author SHA1 Message Date
Rafael Espindola 2b27b2f5a7 Don't create empty sections just to look like gas.
We are long past the time when this much bug for bug compatibility was
useful.

llvm-svn: 251970
2015-11-03 20:02:22 +00:00
Rafael Espindola bda1980917 Write sections mostly in one pass.
During ELF writing, there is no need to further relax the sections, so we
should not be creating fragments. This patch avoids doing so in all cases
but debug section compression (that is next).

Also, the ELF format is fairly simple to write. We can do a single pass over
the sections to write them out and compute the section header table.

llvm-svn: 236235
2015-04-30 14:21:49 +00:00
Rafael Espindola 88d1f632cf Write the section header string table directly to the output stream.
Instead of accumulating the content in a fragment first, just write it
to the output stream.

Also put it first in the section table, so that we never have to worry
about its index being >= SHN_LORESERVE.

llvm-svn: 236145
2015-04-29 20:25:24 +00:00
Rafael Espindola 607da974b2 Write relocation sections contiguously.
Linkers normally read all the relocations upfront to compute the references
between sections. Putting them together is a bit more cache friendly.

I benchmarked linking a Release+Asserts clang with gold on a vm. I tried all
4 combinations of --gc-sections/no --gc-section hot and cold cache.

I cleared the cache with

echo 3 > /proc/sys/vm/drop_caches

and warmed it up by running the link once before timing the subsequent ones.

With cold cache and --gc-sections the time goes from

1.86130781665 +- 0.01713126697463843 seconds
to
1.82370735105 +- 0.014127522318814516 seconds

With cold cache and no --gc-sections the time goes from

1.6087245435500002 +- 0.012999066825178644 seconds
to
1.5687122041500001 +- 0.013145850126026619 seconds

With hot cache and no --gc-sections the time goes from

0.926200939 ( +-  0.33% ) seconds
to
0.907200079 ( +-  0.31% ) seconds

With hot cache and gc sections the time goes from

1.183038049 ( +-  0.34% ) seconds
to
1.147355862 ( +-  0.39% ) seconds

llvm-svn: 235165
2015-04-17 08:11:38 +00:00
Tim Northover a630fb0b67 Put ELF COMDAT relocations into the relevant COMDAT group.
Patch from Игорь Пашев  (I do hope we support utf-8 commit messages; I
also hope he'll forgive me for transliterating it as Igor Pashev in
case things go horribly wrong).

llvm-svn: 186034
2013-07-10 20:58:17 +00:00