Hi there, thanks for your questions. > -----Original Message----- > From: John Tytgat [mailto:john.tyt...@aaug.net]
[...] > I've read with interest the > https://wiki.ubuntu.com/ARM/Thumb2PortingHowto page and I'm > wondering about the following : in the "Quick Reference" > section it is mentioned that "bx lr" is a no go for ARMv4 and earlier. The Thumb instruction set did not exist at all before ARMv4T, and so in ARMv4 and earlier architectures do not understand the BX instruction which is used to switch to and from Thumb. > But further on we have: > > #ifdef (___ARM_ARCH_4T__) || defined (__ARM_ARCH_4__) > "mov pc, lr" > #else > "bx lr" > #endif > > Why do we have the __ARM_ARCH_4T__ test ? ARMv4T happily > groks "bx lr", no ? You are right, but to minimise any impact on Debian (who build for ARMv4T but do not make use of Thumb) I count ARMv4T as a non-Thumb architecture. As things stand Debian should work on ARMv4 (non-T) platforms, and it's possible some are still in use; if so, I don't want to break these with our updates. ARMv4T has some particular limitations which require less efficient interworking return sequences for Thumb functions compared with the newer architectures, so Thumb is less likely to be used for general userspace code in Debian (unless they migrate completely to a higher architecture baseline at some point in the future). Strictly speaking, the test could be #if defined(__ARM_ARCH_2__) || defined(__ARM_ARCH_3__) || \ defined(__ARM_ARCH_3M__) || \ defined(__ARM_ARCH_4__) || defined(__ARM_ARCH_4T__) However, this is verbose and easy to get wrong; and ARMv4 is about the oldest baseline used by anyone at present. Reversing the sense of the #if (to detect any compatible architecture, as opposed to rejecting incompatible ones) is difficult to do in an upwards-compatible way because of the way GCC describes the architecture in its predefines. I'll poke the wiki to clarify the rationale here (though discussion is still welcome). Cheers ---Dave -- Ubuntu-mobile mailing list Ubuntu-mobile@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-mobile
