----- On Jul 9, 2020, at 1:42 PM, Mathieu Desnoyers mathieu.desnoy...@efficios.com wrote:
> ----- On Jul 9, 2020, at 1:37 PM, Segher Boessenkool > seg...@kernel.crashing.org > wrote: > >> On Thu, Jul 09, 2020 at 09:43:47AM -0400, Mathieu Desnoyers wrote: >>> > What protects r17 *after* this asm statement? >>> >>> As discussed in the other leg of the thread (with the code example), >>> r17 is in the clobber list of all asm statements using this macro, and >>> is used as a temporary register within each inline asm. >> >> That works fine then, for a testcase. Using r17 is not a great idea for >> performance (it increases the active register footprint, and causes more >> registers to be saved in the prologue of the functions, esp. on older >> compilers), and it is easier to just let the compiler choose a good >> register to use. But maybe you want to see r17 in the generated >> testcases, as eyecatcher or something, dunno :-) > > Just to make sure I understand your recommendation. So rather than > hard coding r17 as the temporary registers, we could explicitly > declare the temporary register as a C variable, pass it as an > input operand to the inline asm, and then refer to it by operand > name in the macros using it. This way the compiler would be free > to perform its own register allocation. > > If that is what you have in mind, then yes, I think it makes a > lot of sense. Except that asm goto have this limitation with gcc: those cannot have any output operand, only inputs, clobbers and target labels. We cannot modify a temporary register received as input operand. So I don't see how to get a temporary register allocated by the compiler considering this limitation. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
