[Scons-users] smartlink and reiterating compile flags on the link step

[Bill Deegan]

Andrew,




On Sat, Oct 31, 2015 at 10:52 AM, Andrew C. Morrow <
andrew.c.morrow at gmail.com> wrote:

>
> Hi All -
>
> It seems to me that when using smartlink and therefore using the C or C++
> compiler to drive the link step for programs and shared libraries, that the
> various C and C++ language compile flags should be reiterated on the link
> step. In particular, without regard to ordering:
>
> C link for static links should include: $CCFLAGS and $CFLAGS
> C link for shared links should include $CCFLAGS, $CFLAGS, $SHCCFLAGS, and
> $SHCFLAGS
> C++ link for static links should include $CCFLAGS and $CXXFLAGS
> C++ link for shared links should include $CCFLAGS, $SHCCFLAGS, $CXXFLAGS
> and $SHCXXFLAGS
>
> The rationale is that the presence of certain compile flags may mean that
> the effects of other compile flags are deferred until link time. In
> particular, if -flto is passed to the compile step, the desired
> optimization level should be passed to both the compiler invocation for
> objects, and to the linker invocation for programs or shared libraries.
> Currently, that latter part doesn't happen with smartlink.
>

I'm not sure I agree.
If we were to modify the code as you speak and there were flags for the
c/c++ compiler which we did not want to get passed to the linker as well,
how would we prevent that?



>
> See, for instance the section titled "Be sure you get link-time
> optimization flags right" here:
> http://hubicka.blogspot.com/2014/04/linktime-optimization-in-gcc-2-firefox.html
>
> I've also verified that building a C++ library (with g++ as the smartlink
> selected linker) with
>
> CCFLAGS="-flto -g -O3" LINKFLAGS='-flto -fuse-ld=gold'
>
> vs
>
> CCFLAGS="-flto -g -O3" LINKFLAGS='$CCFLAGS -fuse-ld=gold'
>
>

> generate different binaries (interestingly, when using clang++ to drive
> the link step, there is no difference).
>

What about also passing "-g -O3" in your linkflags?
It seems that in your case, those flags must be altering the outputted
binary?


>
> Is there a way to configure smartlink  (which isn't really documented) so
> that the appropriate set of compiler flags are reiterated on the link step?
> Changing $LINKFLAGS and $SHLINKFLAGS doesn't seem quite right as they don't
> differentiate between C and C++ linking. Or is this an improvement that
> should be made to smartlink?
>

Seems like LINKCFLAGS LINKCXXFLAGS and SH* would be a reasonable way to
address the need for c or c++ specific link flags?

-Bill
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