On Tue, Apr 9, 2019 at 10:58 AM Jeff Law <law@xxxxxxxxxx> wrote: > > On 4/7/19 10:44 AM, William Tambe wrote: > > When STATIC_CHAIN_REGNUM is defined and the static chain register is > > not a fixed register, I am assuming that the prologue need to save it > > and that df_regs_ever_live_p must be checked to determine whether to > > save it. > Again, it depends on the ABI and what it says about the register. Most > (if not all) the ABIs I've worked with through the years use a static > chain that is call clobbered and thus you don't have to do anything special. > > > > > > The uncertainty arise when one cannot tell which GCC internal variable > > must be checked beside df_regs_ever_live_p ; ie: > > https://github.com/gcc-mirror/gcc/blob/gcc-8_3_0-release/gcc/config/fr30/fr30.c#L145 > I'm not familiar with the fr30. But AFAICT its static chain (%r12) is > clobbered. > > In general it's better to include the actual code you're referencing. I > don't have gcc-8.3.0 handy and the line number references are going to > be "off". > > I'm guessing you're referring to this: > > > /* Tell prologue and epilogue if register REGNO should be saved / restored. > > The return address and frame pointer are treated separately. > > Don't consider them here. */ > > #define MUST_SAVE_REGISTER(regno) \ > > ( (regno) != RETURN_POINTER_REGNUM \ > > && (regno) != FRAME_POINTER_REGNUM \ > > && df_regs_ever_live_p (regno) \ > > && ! call_used_regs [regno] ) > > > > #define MUST_SAVE_FRAME_POINTER (df_regs_ever_live_p (FRAME_POINTER_REGNUM) || frame_pointer_needed) > > #define MUST_SAVE_RETURN_POINTER (df_regs_ever_live_p (RETURN_POINTER_REGNUM) || crtl->profile) > > Which looks pretty normal. For a typical register you just need to know > if it's call saved (! call_used_regs) and if it's ever used > (df_regs_ever_live). > > Frame pointers are sometimes handled specially (and the frame pointer is > different than the static chain). Frame pointers are often eliminated > and references turned into a stack pointer address. That in turn allows > the frame pointer to be used by the register allocator. > > On the fr30 the return address appears to be handled specially as well. > Not really sure why -- perhaps the call to mcount isn't fully exposed > or something similar. It's not terribly uncommon to handle a few things > specially when profiling. > > > > > > Hence the reason for asking for examples showing how to properly have > > the DF entry block define registers so that it is enough to check only > > df_regs_ever_live_p in the prologue on whether to save them. > > > > Are they currently such examples ? > So the first question I would ask is whether or not your static chain is > a call clobbered register is not. If not, why -- most ports have a call > clobbered static chain. My static chain register is not marked in CALL_USED_REGISTERS , which I assume mean, it is not call clobbered. My preference would be to have it call clobbered so I wouldn't need to do anything special. > > Jeff
