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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236235 91177308-0d34-0410-b5e6-96231b3b80d8
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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236145 91177308-0d34-0410-b5e6-96231b3b80d8
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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235165 91177308-0d34-0410-b5e6-96231b3b80d8
