Re: [Qemu-devel] [PATCH v3 6/6] tcg/mips: Support r6 SEL{NE, EQ}Z instead of MOVN/MOVZ

[James Hogan]

On Thu, Oct 08, 2015 at 06:31:32PM +0200, Aurelien Jarno wrote:
> On 2015-10-02 13:24, James Hogan wrote:
> > Extend MIPS movcond implementation to support the SELNEZ/SELEQZ
> > instructions introduced in MIPS r6 (where MOVN/MOVZ have been removed).
> > 
> > Whereas the "MOVN/MOVZ rd, rs, rt" instructions have the following
> > semantics:
> >  rd = [!]rt ? rs : rd
> > 
> > The "SELNEZ/SELEQZ rd, rs, rt" instructions are slightly different:
> >  rd = [!]rt ? rs : 0
> > 
> > First we ensure that if one of the movcond input values is zero that it
> > comes last (we can swap the input arguments if we invert the condition).
> > This is so that it can exactly match one of the SELNEZ/SELEQZ
> > instructions and avoid the need to emit the other one.
> > 
> > Otherwise we emit the opposite instruction first into a temporary
> > register, and OR that into the result:
> >  SELNEZ/SELEQZ  TMP1, v2, c1
> >  SELEQZ/SELNEZ  ret, v1, c1
> >  OR             ret, ret, TMP1
> > 
> > Which does the following:
> >  ret = cond ? v1 : v2
> > 
> > Signed-off-by: James Hogan <address@hidden>
> > Cc: Richard Henderson <address@hidden>
> > Cc: Aurelien Jarno <address@hidden>
> > ---
> > Changes in v3:
> > - Switch to using bool eqz to indicate whether to use SELEQZ / MOVZ
> >   instead of SELNEZ / MOVN (Richard).
> > - Add tcg_debug_assert(v2 == ret) for pre-r6 case with comment to remind
> >   reader that it should be guaranteed via constraints (Richard).
> > 
> > Changes in v2:
> > - Combine with patch 6 from v1, and drop functional changes to movcond
> >   implementation pre-r6. We now provide different constraints for
> >   movcond depending on presence of r6. (thanks Richard for feedback).
> > ---
> >  tcg/mips/tcg-target.c | 43 +++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 37 insertions(+), 6 deletions(-)
> 
> Reviewed-by: Aurelien Jarno <address@hidden>

Thanks for the reviews Aurelien!

Cheers
James

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