On 04/05 01:24:44, Dmitry Eremin-Solenikov wrote:
> On 04.04.2017 23:34, Brian Brooks wrote:
> > On 04/04 23:27:51, Dmitry Eremin-Solenikov wrote:
> >> On 04.04.2017 23:26, Brian Brooks wrote:
> >>> On 04/04 23:04:10, Dmitry Eremin-Solenikov wrote:
> >>>> On 04.04.2017 21:47, Brian Brooks wrote:
> >>>>> Signed-off-by: Brian Brooks <[email protected]>
> >>>>
> >>>> Brian,
> >>>>
> >>>> how does this fail with clang on ARMv8? Could you please include error
> >>>> message in the commit message?
> >>>
> >>> Well, you can't pass -mcx16 to clang when compiling natively on ARM.
> >>>
> >>> This is what it will look like:
> >>>
> >>> clang: error: argument unused during compilation: '-mcx16'
> >>>
> >>> and Autotools will halt the rest of the build.
> >>
> >> Does this happen during configure stage or when building?
> >
> > You will see that error during build. That is when clang is invoked.
> > But, the bug is in configure.ac where -mcx16 is added to CFLAGS when
> > it shouldn't be.
>
> Then why isn't it detected by configure script?
The configure script is generated by Autoconf. Autoconf takes the
configure.ac file as input. The autoconf tool is invoked when you
run this project's ./bootstrap script.
configure.ac is a combination of shell script and M4 (a macro language).
If something in configure.ac doesn't look like shell, it is likely
a macro.
Lets step through it:
0 if test "${CC}" != "gcc" -o ${CC_VERSION_MAJOR} -ge 5; then
1 my_save_cflags="$CFLAGS"
2
3 CFLAGS=-mcx16
4 AC_MSG_CHECKING([whether CC supports -mcx16])
5 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([])],
6 [AC_MSG_RESULT([yes])]
7 [ODP_CFLAGS="$ODP_CFLAGS $CFLAGS"],
8 [AC_MSG_RESULT([no])]
9 )
10 CFLAGS="$my_save_cflags"
11 fi
Line 0 is specifically checking for whether we are using GCC or
whether whatever compiler we're using's version is greater than
or equal to 5.
Line 1 saves CFLAGS into a temporary variable, and line 10 restores
it. This technique is useful for when you want to avoid altering
CFLAGS until the very end of the configure.ac has been processed.
The reason is that the compiler may be invoked as configure.ac is
processed and you want a pristine set of CFLAGS when you do this.
Line 3 adds -mcx16 to CFLAGS. The original author probably wants
to compile a small program with -mcx16 at this point in the
processing of the configure.ac file. If we know we're not running
on an x86-based chip, we can skip this entirely. More on that later.
Line 4 is a macro to print something to inform the user that we
are checking "whether CC supports -mcx16".
Lines 5-9 will invoke the compiler with a provided program
(an empty program: AC_LANG_PROGRAM([])) and execute lines 6 and 7
on success or lines 7 and 8 on failure. The interesting part is
line 7 which appends CFLAGS to ODP_CFLAGS on success. We know
that CFLAGS contains -mcx16, but we must also be mindful that
there is nothing else in CFLAGS otherwise multiple consecutive
uses of this technique would end up appending redundant things
to CFLAGS.
So, what happens when the configure script is run on ARM with
CC=clang?
checking whether CC supports -mcx16... yes
Wat... umm... let's try something else...
$ touch poof.c
$ clang -mcx16 poof.c
clang: warning: argument unused during compilation: '-mcx16'
(.text+0x30): undefined reference to `main'
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
OK. So, why does it fail when we do a 'make'? Because of -Werror.
$ clang -Werror -mcx16 poof.c
clang: error: argument unused during compilation: '-mcx16'
-Werror is in ODP_CFLAGS, not CFLAGS used throughout the processing
of the configure.ac file.
An understandable argument is to add that flag to CFLAGS, but we can
do better. Much better. We can skip this onzin entirely because we
already know the architecture that we're targeting (the ${host}
variable). Skipping that saves time.
So, we only check for the x86-based -mcx16 flag if we're targeting
an x86-based processor:
if "${host}" == i?86* -o "${host}" == x86*; then
if test "${CC}" != "gcc" -o ${CC_VERSION_MAJOR} -ge 5; then
[..]
fi
fi
It is that simple! :)
> --
> With best wishes
> Dmitry
