Re: [PATCH v1 03/15] tcg: Fix register allocation constraints

[LIU Zhiwei]

On 2024/8/13 19:52, Richard Henderson wrote:
> On 8/13/24 21:34, LIU Zhiwei wrote:
> > From: TANG Tiancheng<tangtiancheng.ttc@alibaba-inc.com>
> >
> > When allocating registers for input and output, ensure they match
> > the available registers to avoid allocating illeagal registers.
> >
> > We should respect RISC-V vector extension's variable-length registers
> > and LMUL-based register grouping. Coordinate with 
> > tcg_target_available_regs
> > initialization tcg_target_init (behind this commit) to ensure proper
> > handling of vector register constraints.
> >
> > Note: While mov_vec doesn't have constraints, dup_vec and other IRs do.
> > We need to strengthen constraints for all IRs except mov_vec, and this
> > is sufficient.
> >
> > Signed-off-by: TANG Tiancheng<tangtiancheng.ttc@alibaba-inc.com>
> > Fixes: 29f5e92502 (tcg: Introduce paired register allocation)
> > Reviewed-by: Liu Zhiwei<zhiwei_liu@linux.alibaba.com>
> > ---
> >   tcg/tcg.c | 20 +++++++++++++-------
> >   1 file changed, 13 insertions(+), 7 deletions(-)
> >
> > diff --git a/tcg/tcg.c b/tcg/tcg.c
> > index 34e3056380..d26b42534d 100644
> > --- a/tcg/tcg.c
> > +++ b/tcg/tcg.c
> > @@ -4722,8 +4722,10 @@ static void tcg_reg_alloc_dup(TCGContext *s, 
> > const TCGOp *op)
> >           return;
> >       }
> >   -    dup_out_regs = tcg_op_defs[INDEX_op_dup_vec].args_ct[0].regs;
> > -    dup_in_regs = tcg_op_defs[INDEX_op_dup_vec].args_ct[1].regs;
> > +    dup_out_regs = tcg_op_defs[INDEX_op_dup_vec].args_ct[0].regs &
> > + tcg_target_available_regs[ots->type];
> > +    dup_in_regs = tcg_op_defs[INDEX_op_dup_vec].args_ct[1].regs &
> > + tcg_target_available_regs[its->type];
>
> Why would you ever have constraints that resolve to unavailable 
> registers?
>
> If you don't want to fix this in the backend, then the next best place 
> is in process_op_defs(), so that we take care of this once at startup, 
> and never have to think about it again.

Hi Richard,

The constraints provided in process_op_defs() are static and tied to the 
IR operations. For example, if we create constraints for add_vec, the 
same constraints will apply to all types of add_vec operations 
(TCG_TYPE_V64, TCG_TYPE_V128, TCG_TYPE_V256). This means the constraints 
don't change based on the specific type of operation being performed.
In contrast, RISC-V's LMUL (Length Multiplier) can change at runtime 
depending on the type of IR operation. Different LMUL values affect 
which vector registers are available for use in RISC-V. Let's consider 
an example where the host's vector register width is 128 bits:

For an add_vec operation on v256 (256-bit vectors), only even-numbered 
vector registers like 0, 2, 4 can be used.
However, for an add_vec operation on v128 (128-bit vectors), all vector 
registers (0, 1, 2, etc.) are available.

Thus if we want to use all registers of vectors, we have to add a 
dynamic constraint on register allocation based on IR types.

Thanks,
Zhiwei

> r~