[Boost-build] From where do compiler options come ?

Subject: [Boost-build] From where do compiler options come ?
From: Edward Diener (eldiener_at_[hidden])
Date: 2015-09-27 11:38:54

I am trying to understand from where compilers options come for a 
particular toolset when running b2.

I can see the compiler options generated if I add a -d2 switch to a b2 
command. That is not a problem. But looking at the particular .jam file 
for a particular toolset I can not understand what causes the options I 
see on the command line.

As an example, using the clang-linux.jam file I can see the way in which 
various features generate compiler options, such as:

toolset.flags clang-linux.compile OPTIONS <warnings>off : -w ;
toolset.flags clang-linux.compile OPTIONS <warnings>on  : -Wall ;
toolset.flags clang-linux.compile OPTIONS <warnings>all : -Wall -pedantic ;
toolset.flags clang-linux.compile OPTIONS <warnings-as-errors>on : -Werror ;

I can also see the command line generated in bjam form, such as:

actions compile.c++.without-pth {
   "$(CONFIG_COMMAND)" -c -x c++ $(OPTIONS) $(USER_OPTIONS) -D$(DEFINES) 
-I"$(INCLUDES)" -o "$(<)" "$(>)"
}

and when I compile a particular source. For instance in the output with 
-d2 I might see:

clang-linux.compile.c++.without-pth 
C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\lambda\test\algorithm_test.test\clang-linux-3.8\debug\algorithm_test.obj

   "C:/Programming/VersionControl/bninja_installed_clang/bin/clang++" -c 
-x c++ -D__MINGW_FORCE_SYS_INTRINS -Wno-unused-local-typedef 
-Wno-dll-attribute-on-redeclaration -O0 -g -fno-inline -Wall -g 
-march=i686 -m32  -DBOOST_ALL_NO_LIB=1 -I"..\..\.." -o 
"C:\Programming\VersionControl\modular-boost\build\boost\bin.v2\libs\lambda\test\algorithm_test.test\clang-linux-3.8\debug\algorithm_test.obj" 
"algorithm_test.cpp"

So I know that the action compile.c++.without-pth is being used. But I 
don't understand how the $(OPTIONS) $(USER_OPTIONS) are generated. Is 
there a -dn b2 switch which will show me how this happens ?

What I am particularly trying to track down is what generates the -Wall 
on the command line for clang ? Because -Wall gets generated, presumably 
from a <warnings>on somewhere, all other -W options are ignored ( 
overridden ). Is <warnings>on automatically generated by Boost Build for 
every toolset somewhere ? I don't think this is the case. But I don't 
understand how this is set nor can I find in the clang.jam, or the 
gcc.jam from which clang.jam inherits much of its functionality, any 
particular line which is somehow setting <warnings>on automatically. Nor 
is it being set in the library which I am compiling or in my 
user-config.jam.