Hello Benoît,
On 08/12/2013 12:08 AM, Benoît Thébaudeau wrote:
On Sunday, August 11, 2013 10:58:36 PM, Jeroen Hofstee wrote:
To be EABI compliant (r9 is a platform specific register) and as a prepration for building u-boot with clang/llvm (with does / will support r9 as reserved register), store the pointer to gd in r9.
If r9 is reserved, I understand that its current usage may conflict with clang's but why would gd have to be stored in r9 for clang? Moreover, if r9 is reserved for clang (reserved for what?), why can it be used for gd?
I know Albert already responded to this, but for completeness: The reserved is from a compiler point of view, which is perhaps a bit llvm nomenclature, Reserved means the compiler should _not_ use it as a general purpose register; it is reserved for the platform, U-boot in this case.
I'm also wondering if r9 as initialized by relocate.S is not sometimes used by GCC to handle position-independent code within generated code, i.e. like the static base feature described for r9 as a possible usage in the ARM EABI AAPCS document (5.1.1). If this is the case, changing r9 to gd would break GCC code at runtime.
This is not needed / supported on U-boot arm. U-boot will actually error at compile time if there are any symbols which are not pc relative.
Regards, Jeroen