[Mesa-dev] [PATCH 2/3] i965: Use implicit backend_reg copy-constructor.

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> In order to do this, we have to change the signature of the
> backend_reg(brw_reg) constructor to take a reference to a brw_reg in
> order to avoid unresolvable ambiguity about which constructor is
> actually being called in the other modifications in this patch.
>
> As far as I understand it, the rule in C++ is that if multiple
> constructors are available for parent classes, the one closest to you in
> the class heirarchy is closen, but if one of them didn't take a
> reference, that screws things up.

I believe that the reason is that the ordering between more and less
derived classes is only defined for values of the same kind, i.e. a
conversion from 'const src_reg &' to 'const backend_reg &' is considered
a better match than 'const src_reg &' to 'const brw_reg &', and a
conversion from 'src_reg' to backend_reg' would have been considered
better than 'src_reg' to 'brw_reg', but that doesn't necessarily
disambiguate between the conversions to 'const backend_reg &' and
'brw_reg'.  Anyway there was no good reason to pass the brw_reg object
by value.  For the series:

Reviewed-by: Francisco Jerez <currojerez at riseup.net>

> ---
>  src/mesa/drivers/dri/i965/brw_shader.h | 2 +-
>  src/mesa/drivers/dri/i965/brw_vec4.cpp | 6 ++----
>  2 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_shader.h b/src/mesa/drivers/dri/i965/brw_shader.h
> index 718a225..abf04cf 100644
> --- a/src/mesa/drivers/dri/i965/brw_shader.h
> +++ b/src/mesa/drivers/dri/i965/brw_shader.h
> @@ -42,7 +42,7 @@
>  struct backend_reg : public brw_reg
>  {
>     backend_reg() {}
> -   backend_reg(struct brw_reg reg) : brw_reg(reg) {}
> +   backend_reg(const struct brw_reg &reg) : brw_reg(reg) {}
>  
>     bool equals(const backend_reg &r) const;
>  
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> index a23fd82..60b2fed 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> @@ -79,9 +79,8 @@ src_reg::src_reg(struct brw_reg reg) :
>  }
>  
>  src_reg::src_reg(const dst_reg &reg) :
> -   backend_reg(static_cast<struct brw_reg>(reg))
> +   backend_reg(reg)
>  {
> -   this->reg_offset = reg.reg_offset;
>     this->reladdr = reg.reladdr;
>     this->swizzle = brw_swizzle_for_mask(reg.writemask);
>  }
> @@ -137,9 +136,8 @@ dst_reg::dst_reg(struct brw_reg reg) :
>  }
>  
>  dst_reg::dst_reg(const src_reg &reg) :
> -   backend_reg(static_cast<struct brw_reg>(reg))
> +   backend_reg(reg)
>  {
> -   this->reg_offset = reg.reg_offset;
>     this->writemask = brw_mask_for_swizzle(reg.swizzle);
>     this->reladdr = reg.reladdr;
>  }
> -- 
> 2.4.9
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