don't actually build .a files on Linux, instead pass the original .o files to the final link

[bsilver8192]

In #492, @ulfjack mentioned that "We don't actually build .a files on Linux, where we instead pass the original .o files to the final link (or at least, we have code to do that, and I think it's enabled by default). This is faster than building intermediate .a files and uses less disk space." That sounds nice, but doesn't seem to actually work:

$ bazel build --subcommands //:main
INFO: Found 1 target...
>>>>> # //:main [action 'Creating runfiles tree bazel-out/local-fastbuild/bin/main.runfiles']
...
>>>>> # //:gen_files [action 'Executing genrule //:gen_files']
...
>>>>> # //:a [action 'Compiling a.c']
...
>>>>> # //:main [action 'Compiling main.c']
...
>>>>> # //:a [action 'Linking liba.a']
...
>>>>> # //:main [action 'Linking main']
...
Target //:main up-to-date:
  bazel-bin/main
INFO: Elapsed time: 2.655s, Critical Path: 0.61s

BUILD:

genrule(
  name = 'gen_files',
  outs = ['a.c', 'main.c'],
  cmd = 'echo "int main() {}" > $(location main.c) && touch $(location a.c)',
)

cc_library(
  name = 'a',
  srcs = ['a.c'],
)

cc_binary(
  name = 'main',
  srcs = ['main.c'],
  deps = [':a'],
)

[ulfjack]

The CROSSTOOL file needs to declare that it supports that mode by setting
supports_start_end_lib = true. Maybe the cc_configure rule doesn't do that
by default? Maybe it depends on the compiler version?

On Wed, Jun 15, 2016 at 11:53 PM, bsilver8192 [email protected]
wrote:

In #492 #492, @ulfjack
https://github.com/ulfjack mentioned that "We don't actually build .a
files on Linux, where we instead pass the original .o files to the final
link (or at least, we have code to do that, and I think it's enabled by
default). This is faster than building intermediate .a files and uses less
disk space." That sounds nice, but doesn't seem to actually work:

$ bazel build --subcommands //:mainINFO: Found 1 target...>>>>> # //:main [action 'Creating runfiles tree bazel-out/local-fastbuild/bin/main.runfiles']...>>>>> # //:gen_files [action 'Executing genrule //:gen_files']...>>>>> # //:a [action 'Compiling a.c']...>>>>> # //:main [action 'Compiling main.c']...>>>>> # //:a [action 'Linking liba.a']...>>>>> # //:main [action 'Linking main']...Target //:main up-to-date: bazel-bin/mainINFO: Elapsed time: 2.655s, Critical Path: 0.61s

BUILD:

genrule(
name = 'gen_files',
outs = ['a.c', 'main.c'],
cmd = 'echo "int main() {}" > $(location main.c) && touch $(location a.c)',
)

cc_library(
name = 'a',
srcs = ['a.c'],
)

cc_binary(
name = 'main',
srcs = ['main.c'],
deps = [':a'],
)

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[bsilver8192]

That would be the problem. Those appear to be a gold-only flags, and cc_configure.bzl doesn't attempt to auto-detect using gold. If I add linker_flag: "-fuse-ld=gold" (necessary on my Debian Jessie system to use gold) and change to supports_start_end_lib: true, then it works.

Would it make sense to have cc_configure.bzl try -fuse-ld=gold and set stuff appropriately if it works? Or maybe somehow detect if the default linker is gold?

[philwo]

Topic area is C++ auto configuration, so assignee is Damien. :)

[dslomov]

@damienmg Any more information that is needed here?

[damienmg]

Sorry lost track with my personnal issues, it is indeed something that we should do, triaged.

[hlopko]

Should be fixed by cc3d79c, closing. Thank you bsilver8192!