Incorrect code generation when referencing linker symbols?

Adding version info:

$ arm-none-eabi-gcc -v
Using built-in specs.
COLLECT_GCC=C:\Program Files (x86)\GNU Tools ARM Embedded\4.9 2015q3\bin\arm-none-eabi-gcc.exe
COLLECT_LTO_WRAPPER=c:/program\ files\ (x86)/gnu\ tools\ arm\ embedded/4.9\ 2015q3/bin/../lib/gcc/arm-none-eabi/4.9.3/lto-wrapper.exe
Target: arm-none-eabi
Configured with: /home/build/work/GCC-4-9-build/src/gcc/configure --build=i686-linux-gnu --host=i686-w64-mingw32 --target=arm-none-eabi --prefix=/home/build/work/GCC-4-9-build/install-mingw --libexecdir=/home/build/work/GCC-4-9-build/install-mingw/lib --infodir=/home/build/work/GCC-4-9-build/install-mingw/share/doc/gcc-arm-none-eabi/info --mandir=/home/build/work/GCC-4-9-build/install-mingw/share/doc/gcc-arm-none-eabi/man --htmldir=/home/build/work/GCC-4-9-build/install-mingw/share/doc/gcc-arm-none-eabi/html --pdfdir=/home/build/work/GCC-4-9-build/install-mingw/share/doc/gcc-arm-none-eabi/pdf --enable-languages=c,c++ --disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath --disable-libssp --disable-libstdcxx-pch --disable-nls --disable-shared --disable-threads --disable-tls --with-gnu-as --with-gnu-ld --with-headers=yes --with-newlib --with-python-dir=share/gcc-arm-none-eabi --with-sysroot=/home/build/work/GCC-4-9-build/install-mingw/arm-none-eabi --with-libiconv-prefix=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-gmp=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-mpfr=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-mpc=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-isl=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-cloog=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-libelf=/home/build/work/GCC-4-9-build/build-mingw/host-libs/usr --with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm' --with-pkgversion='GNU Tools for ARM Embedded Processors' --with-multilib-list=armv6-m,armv7-m,armv7e-m,cortex-m7,armv7-r
Thread model: single
gcc version 4.9.3 20150529 (release) [ARM/embedded-4_9-branch revision 227977] (GNU Tools for ARM Embedded Processors)

Hi Stuart,

GCC is following the C standard here, more precisely its section 6.5.6 paragraph 9 (for C99):

When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array elements. (...) In other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of an array object, the expression (P)-(Q) has the value i−j provided the value fits in an object of type ptrdiff_t.

You are computing the difference of the *address* of two pointers to char. A pointer has 4 bytes in ARM so it needs to divide the result of the difference by 4, hence a right shift by 2.

Best regards.

Hi Stuart,

Just a little tip. I am assuming you are trying to compute the size of bss. You will want to declare __bss_start__ and __bss_end__ as:
extern char __bss_start__;
extern char __bss_end__;

And then you leave your subtraction expression as is, taking their addresses. That way, because the sizeof(char) = 1, you will not need to multiply the result and you will get the size of bss.

Cheers,
Andre