D:\depot_tools\chromium\src\build\config\coverage\BUILD.gn

Comment out these lines as follows:

# "-fcoverage-mapping",

# "-mllvm",
# "-limited-coverage-experimental=true",

Modify the ldflag for the 64-bit clang_rt library to match your desired
LLVM build location. For example:

ldflags += [
"D:/src_builds/llvm-monorepo/llvm-build-polly/lib/clang/9.0.0/lib/windows/clang_rt.profile-x86_64.lib"
]

----------------

Edit Chromium's compiler config.

D:\depot_tools\chromium\src\build\config\compiler\BUILD.gn

Modify these lines around #1860 to instead set NOICF and NOREF:

common_optimize_on_ldflags += [ "/OPT:NOICF" ] # Redundant COMDAT folding.
common_optimize_on_ldflags += [ "/OPT:NOREF" ] # Remove unreferenced data.

----------------

Edit the Windows build configuration.

D:\depot_tools\chromium\src\build\config\win\BUILD.gn

Modify these lines around #154 to instead set NOICF and NOREF:

"/OPT:NOREF",
"/OPT:NOICF",

----------------

For your first stage PGO build, add the following argument to your build:

use_clang_coverage = true

Note, Thin/Full LTO is not needed for the first stage, so do not enable it.

----------------

Set the following environment variable from the command line:

set LLVM_PROFILE_FILE=code-%p.profraw

----------------

Compile the build. Reordering likely will fail for chrome.exe, so just copy
chrome.exe from the initialexe directory up to the build directory.

copy D:\depot_tools\chromium\src\out\pgo\initialexe\chrome.exe
D:\depot_tools\chromium\src\out\pgo\chrome.exe

----------------

Go to the build directory, and profile the build as desired.

chrome.exe ----no-sandbox

----------------

Copy llvm-profdata.exe from your LLVM bin directory into the build
directory, and merge the raw profile data files.

llvm-profdata.exe merge -output=code.profdata code-*.profraw

----------------

Edit Chromium's compiler config.

D:\depot_tools\chromium\src\build\config\compiler\BUILD.gn

Around line #291, locate cflags += [ "/X" ]. Add the following to the next
line:

cflags += [ "-fprofile-instr-use=D:/builds/pgo/code.profdata",
"-Wno-profile-instr-unprofiled", "-Wno-error=profile-instr-out-of-date", ]

Modify these lines around #1860 to restore ICF and REF:

common_optimize_on_ldflags += [ "/OPT:ICF" ] # Redundant COMDAT folding.
common_optimize_on_ldflags += [ "/OPT:REF" ] # Remove unreferenced data.

----------------

Edit the Windows build configuration.

D:\depot_tools\chromium\src\build\config\win\BUILD.gn

Modify these lines around #154 to restore ICF and REF:

"/OPT:REF",
"/OPT:ICF",

----------------

Modify the build arguments again. Disable the coverage argument.

use_clang_coverage = false

Enable the ThinLTO arguments if using LTO.

use_thin_lto = true
thin_lto_enable_optimizations = true

----------------

Run the build process again. Enjoy the PGO build.... hopefully. :)

----------------

Save the code.profdata file. It should be usable for upto a week or two (or
more; YMMV) of build revisions without too much concern.

Just remember to insert the profile use directive into the compiler config
again when building.

cflags += [
"-fprofile-instr-use=D:/depot_tools/chromium/src/out/pgo/code.profdata",
"-Wno-profile-instr-unprofiled", "-Wno-error=profile-instr-out-of-date", ]