Re: [PATCH v4 30/57] tcg/sparc64: Allocate %g2 as a third temporary

[Peter Maydell]

On Mon, 8 May 2023 at 16:17, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 5/5/23 13:19, Peter Maydell wrote:
> > On Wed, 3 May 2023 at 08:17, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> >>
> >> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> >> ---
> >>   tcg/sparc64/tcg-target.c.inc | 15 +++++++--------
> >>   1 file changed, 7 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/tcg/sparc64/tcg-target.c.inc b/tcg/sparc64/tcg-target.c.inc
> >> index e997db2645..64464ab363 100644
> >> --- a/tcg/sparc64/tcg-target.c.inc
> >> +++ b/tcg/sparc64/tcg-target.c.inc
> >> @@ -83,9 +83,10 @@ static const char * const 
> >> tcg_target_reg_names[TCG_TARGET_NB_REGS] = {
> >>   #define ALL_GENERAL_REGS     MAKE_64BIT_MASK(0, 32)
> >>   #define ALL_QLDST_REGS       (ALL_GENERAL_REGS & ~SOFTMMU_RESERVE_REGS)
> >>
> >> -/* Define some temporary registers.  T2 is used for constant generation.  
> >> */
> >> +/* Define some temporary registers.  T3 is used for constant generation.  
> >> */
> >>   #define TCG_REG_T1  TCG_REG_G1
> >> -#define TCG_REG_T2  TCG_REG_O7
> >> +#define TCG_REG_T2  TCG_REG_G2
> >> +#define TCG_REG_T3  TCG_REG_O7
> >>
> >>   #ifndef CONFIG_SOFTMMU
> >>   # define TCG_GUEST_BASE_REG TCG_REG_I5
> >> @@ -110,7 +111,6 @@ static const int tcg_target_reg_alloc_order[] = {
> >>       TCG_REG_I4,
> >>       TCG_REG_I5,
> >>
> >> -    TCG_REG_G2,
> >>       TCG_REG_G3,
> >>       TCG_REG_G4,
> >>       TCG_REG_G5,
> >> @@ -492,8 +492,8 @@ static void tcg_out_movi_int(TCGContext *s, TCGType 
> >> type, TCGReg ret,
> >>   static void tcg_out_movi(TCGContext *s, TCGType type,
> >>                            TCGReg ret, tcg_target_long arg)
> >>   {
> >> -    tcg_debug_assert(ret != TCG_REG_T2);
> >> -    tcg_out_movi_int(s, type, ret, arg, false, TCG_REG_T2);
> >> +    tcg_debug_assert(ret != TCG_REG_T3);
> >> +    tcg_out_movi_int(s, type, ret, arg, false, TCG_REG_T3);
> >>   }
> >
> > Why do we need to change this usage of TCG_REG_T2 but not
> > any of the others ?
>
> To match the comment above.

To expand, what I mean is "when I'm reviewing this patch, what
do I need to know in order to know whether any particular
instance of TCG_REG_T2 should be changed to _T3 or not?".
All the sites where we *don't* change T2 to T3 are now
using a different register, so there is presumably some
logic for how we tell whether that's safe or not. The
"no behaviour change" option would be to change all of them.

thanks
-- PMM