On 11/19/2012 11:28 AM, Florian Weimer wrote: > Note how %0 and %3 are assigned the same register. Changing "=r" to > "+r" fixes this. If an output-only operand must be modified last, the > original register assignment would be correct, but the Extended Asm > documentation does not require such a thing. It does. "Unless an output operand has the `&' constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced." Have a look at the documentation for "earlyclobber". It's perfectly reasonable to reuse a register used for an input as an output, and gcc would generate far worse code if it couldn't do this. Andrew.
