tree 1c12e7a82a56aad78fb4a298dad40e5b6403d178
parent 364d3205045b694a6c89a8d9ad932e7c105d0f6c
author Cherry Zhang <cherryyz@google.com> 1593203749 -0400
committer Cherry Zhang <cherryyz@google.com> 1593993379 +0000

[dev.link] cmd/link: emit ELF relocations in mmap

Currently, ELF relocations are generated sequentially in the heap
and flushed to output file periodically. In fact, in some cases,
the output size of the relocation records can be easily computed,
as a relocation entry has fixed size. We only need to count the
number of relocation records to compute the size.

Once the size is computed, we can mmap the output with the proper
size, and directly write relocation records in the mapped memory.
It also opens the possibility of writing relocations in parallel
(not done in this CL).

Note: on some architectures, a Go relocation may turn into
multiple ELF relocations, which makes size calculation harder.
This CL does not handle those cases, and it still writes
sequentially in the heap there.

Linking cmd/compile with external linking,

name          old time/op    new time/op    delta
Asmb2            190ms ± 2%     141ms ± 4%  -25.74%  (p=0.000 n=10+10)

name          old alloc/op   new alloc/op   delta
Asmb2_GC        66.8MB ± 0%     8.2MB ± 0%  -87.79%  (p=0.008 n=5+5)

name          old live-B     new live-B     delta
Asmb2_GC         66.9M ± 0%     55.2M ± 0%  -17.58%  (p=0.008 n=5+5)

Change-Id: If7056bbe909dc90033eef6b9c4891fcca310602c
Reviewed-on: https://go-review.googlesource.com/c/go/+/240399
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>