How do I do inline assembly on the IPhone?

I've gotten this to work, thanks to some inside help over at the Apple Devforums, you should sign up if you're a dedicated IPhone developer.

First thing's first, it's __asm__(), not plain asm().

Secondly, by default, XCode generates a compilation target that compiles inline assembly against the ARM Thumb instruction set, so usat wasn't recognized as a proper instruction. To fix this, do "Get Info" on the Target. Scroll down to the section "GCC 4.0 - Code Generation" and uncheck "Compile for Thumb". Then this following snippet will compile just fine if you set the Active SDK to "Device"

inline int asm_saturate_to_255 (int a) {
  int y;
  __asm__("usat %0, #8, %1\n\t" : "=r"(y) : "r"(a));
  return y;
}

Naturally, now it won't work with the IPhone Simulator. But TargetConditionals.h has defines you can #ifdef against. Namely TARGET_OS_IPHONE and TARGET_IPHONE_SIMULATOR.