CppLink actions having access to fdo_profile_path variable

[vinhdaitran]

Description of the feature request:

C++ rules explicitly pipes //command_line_option:fdo_profile to fdo_profile_path build variable for CppCompileAction. fdo_profile_path build variable is then used in several features in cc toolchain. autofdo is one of the using features.

autofdo is implicitly enabled and expands some fdo-specific flags on c/c++ compile actions when fdo_profile_path is available. This works well in making sure we're able to expand the same set of flags in Bazel as in Soong for CFlags.

In AOSP, Soong also sets some fdo-specific flags to LdFlags. AOSP has its own Bazel cc toolchain and we can modify our custom autofdo feature to expand the flags on the link actions as well. However, Bazel Cc++ rules doesn't not pipe the fdo_profile_path variable for CppLinkAction yet. Hence, the dynamic fdo-specific flags don't have access to fdo_profile_path build variable to expand the flags correctly.

It would be better if there's some mechanism to pipe build variables for C++ toolchain to use for this specific use case.

What underlying problem are you trying to solve with this feature?

There is a workaround to this problem. To set the flags on link actions, we can use user_link_flags and additional_linker_inputs attributes such as

filegroup(
  name = "libart_fdo",
  srcs = select({
    "...": ["libart_arm64.afdo"],
    "...": ["libart_arm.afdo"],
  })
)
cc_shared_library(
  name = "foo",
  user_link_flags = ["-fprofile-sample-use=$(location :libart_fdo)"],
  additional_linker_inputs = [":libart_fdo"],
)

But this is counter-intuitive to the existence of fdo_profile rule where we use fdo_profile rule to set the compile flags correctly and use ^ to set the link flags correctly. Another problem is that this approach results to duplicate profiles. fdo_profile symlinks the profile to a separate directory which is referenced in the compile flags. However, the link flags reference a different location though they should be the same.