Building precompiled header with GN - c++

Question

I'm trying to write a GN file that generates .gch files. I've gone through the GN documentation here
I just don't understand how to implement this, i am novice when it comes to GN. So let me explain what i'm trying to achieve.
I want GN to create the following pair of compiler commands:
my-clang -x c++-header pch/hello_world.h -o pch/hello_world.h.gch
my-clang -include pch/hello_world.h -o hello.out -c hello_world.cpp
I expected to be able to create a source set in my GN file like this:
source_set("source_set0") {
precompiled_source = "//pch/hello_world.h"
precompiled_header = "../pch/hello_world.h"
cflags = [
"-include$precompiled_header"
]
sources = [
"//hello_world.cpp",
]
}
This however does not actually generate the pre-compiled header.
I expect I need to have another source_set or equivalent to specifically compile the header file, however my understanding is that the .gch suffix is required for the compiler to recognise the file as a pre-compiled header.
So i figured that i'd need to convince GN to create the specific output file based on the target
hello_world.h file output hello_world.h.gch
hello_world.cpp output hello_world.o
Ideally i'd like to be able to modify my tool specification:
tool("cxx") {{
command = "\"{cxx_exe}\" ... -c {{{{source}}}} -o {{{{output}}}}"
outputs = [
"{{{{source_out_dir}}}}/{{{{target_output_name}}}}.{{{{source_name_part}}}}.o",
**SOME CONDITION**
"{{{{source_out_dir}}}}/{{{{target_output_name}}}}.{{{{source_name_part}}}}.h.gch",
]
}}
However my reading of Tool Variables indicates that the above functionality is only available for linker_tools.
It seems as well that GN needs some convincing to associate cxx tool with .h files.
Has anyone got any experience with this or able to point me in the right direction.
TL;DR
I want to compiler header files with GN to generated .gch file
I want to use the gch in later step of compile
Following the documentation didn't help, am i misunderstanding
How to convince GN to compiler .h files
Is there i need to make sure that the compilation of the header file happens first
Thanks for your time
p.s.
cpp file contents:
int main (void){
const char* greeting = "Hello world";
print_greeting(greeting);
}
.h file contents
#pragma once
#include <iostream>
void print_greeting(const char* greeting){
std::printf("%s", greeting);
}

Answer 1

I got to the bottom of this, I modified the tool:
tool("cxx") {{
command = "\"{cxx_exe}\" ... -c {{{{source}}}} -o {{{{output}}}}"
precompiler_header_type = "gcc"
outputs = [
"{{{{source_out_dir}}}}/{{{{target_output_name}}}}.{{{{source_name_part}}}}.o",
**SOME CONDITION**
"{{{{source_out_dir}}}}/{{{{target_output_name}}}}.{{{{source_name_part}}}}.h.gch",
]
}}
Then update my gn file:
config("precompiled_header"){
precompiled_header="../pch/hello_world.h"
precompiled_source="//pch/hello_world.h"
}
source_set("source_set1") {
public_configs = [
":precompiled_header"
]
sources = [
"//hello_world.cpp",
]
}
In hindsight this is actually what the documentation said to do. The command lines that are created by GN now look like this
ninja.exe -t commands
"my-clang++.exe" ... -O2 -g -x c++-header -c ../pch/hello_world.h -o obj/pch/source_set1.hello_world.h-cc.gch
"my-clang++.exe" ... -O2 -g -include obj/pch/source_set1.hello_world.h-cc -c ../hello_world.cpp -o obj/source_set1.hello_world.o
Hope this helps someone in the future