https://sourceware.org/bugzilla/show_bug.cgi?id=23126
--- Comment #4 from Nick Clifton <nickc at redhat dot com> --- Hi Freddie, > Hmmm... It seems that the problem is not entirely in GAS, but in GCC 8 (RC) > as well... Actually, I think that you have found a separate gas bug^H^H feature... > So it seems that if you have both `-mcpu=cortex-m0 -march=armv6-m` then GAS > has no problem with that. > However if both CPU and ARCH are specified with > directives in the file, then GAS doesn't like this: Right - it turns out that command line options are additive, so if you specify -march=armv6-m -mcpu=cortx-m0 you get the features of both the architecture and cpu enabled, even if they are not the same. (This also applies if the options are specified in the opposite order). But - directives inside the assembler source file override any previous directives or command line options. So if you have: .cpu cortex-m0 .arch armv6-m then you *only* get the features of the V6M architecture and not any other features that might be present in the cortex-m0. The other important point to note here is that the assembler's "armv6-m" architecture does *not* support the SVC instruction. In order to support that you must use the "armv6s-m" architecture. The "s" in that string refers to the OS extension to the base V6M architecture, which in this case means the SVC instruction. Note - the assembler does know that the cortex-m0 supports the SVC instruction, but that is because internally it has a table that says that the cortex-m0 uses the armv6s-m architecture. This explains why your svc-directive.s test fails to assemble. It has the .arch directive after the .cpu directive and it uses the wrong architecture name. With regard to the "ARMv6-M Architecture Reference Manual" entry: In an ARMv6-M implementation that does not include the Unprivileged/ Privileged Extension, execution is always privileged. However in such an implementation, application code might use supervisor calls to maintain a software hierarchy with a system kernel. This does not, to me, mean that the SVC instruction *must* be supported. Rather it says that if the SVC instruction is present in machine code that is running on a v6-M implementation that does not separate privileged from unprivileged execution then this should be permitted. However, I did find this document online, the "Cortex-M0 Devices Generic User Guide", which clearly shows that the SVC instruction is supported: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0432c/CHDCICDF.html So all in all, I think that the assembler is not, technically, wrong. But it is rather confusing in its behaviour. > Do you think I should file a bug report for GCC too? Yes - for a -mcpu=cortex-m0 gcc command line option gcc should be generating a ".arch armv6s-m" directive and not a ".arch armv6-m" directive. (This presumably applies to other cortex-m?? variants as well, although I have not checked). Alternatively it should just generate a .cpu directive and no ".arch" directive, and leave it up to the assembler to divine the architecture involved. Cheers Nick -- You are receiving this mail because: You are on the CC list for the bug. _______________________________________________ bug-binutils mailing list bug-binutils@gnu.org https://lists.gnu.org/mailman/listinfo/bug-binutils
