On Mon, 18 Apr 2016, Ramana Radhakrishnan wrote: > On Mon, Apr 18, 2016 at 2:26 PM, Alexander Monakov <amona...@ispras.ru> wrote: > > On Thu, 14 Apr 2016, Szabolcs Nagy wrote: > >> looking at [2] i don't see why > >> > >> func: > >> mov x9, x30 > >> bl _tracefunc > >> <function body> > >> > >> is not good for the kernel. > >> > >> mov x9, x30 is a nop at function entry, so in > >> theory 4 byte atomic write should be enough > >> to enable/disable tracing. > > > > Overwriting x9 can be problematic because GCC has gained the ability to > > track > > register usage interprocedurally: if foo() calls bar(), and GCC has already > > emitted code for bar() and knows that it cannot change x9, it can use that > > knowledge to avoid saving/restoring x9 in foo() around calls to bar(). See > > option '-fipa-ra'. > > > > If there's no register that can be safely used in place of x9 here, then > > the backend should emit the entry/pad appropriately (e.g. with an unspec > > that > > clobbers the possibly-overwritten register). > > Can you not use one of x16 / x17 the intra-procedure-call scratch > registers as per the PCS ?
As long as: - there's a guarantee that the call to _tracefunc wouldn't need some kind of veneer that would clobber those; I don't know enough about AArch64 to say; - and GCC is not smart enough to be aware that intra-TU calls to 'func' (the function we're instrumenting) don't touch x16/x17. And GCC should be that smart, if it's not, it's a bug, right? :) Thanks. Alexander
