[Mesa-dev] [PATCH] i965/vec4: Don't coalesce registers in gen6 math ops if reswizzling needed

[Matt Turner]

On Sun, Sep 20, 2015 at 8:48 AM, Antia Puentes <apuentes at igalia.com> wrote:
> Math operations in SandyBridge do not support source swizzling

I can't find any documentation to support this claim, but I remember
that SNB math must be in align1 mode so it can't do swizzles or
writemasking (see commit e14cc504).

But, the documentation actually says "The supported regioning modes
for math instruction are align16, align1 with the following
restrictions: ..."

Would be nice if we could actually find a PRM citation.

> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92033
> ---
>  src/mesa/drivers/dri/i965/brw_ir_vec4.h |  3 ++-
>  src/mesa/drivers/dri/i965/brw_vec4.cpp  | 11 +++++++++--
>  2 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_ir_vec4.h b/src/mesa/drivers/dri/i965/brw_ir_vec4.h
> index 966a410..a48bb68 100644
> --- a/src/mesa/drivers/dri/i965/brw_ir_vec4.h
> +++ b/src/mesa/drivers/dri/i965/brw_ir_vec4.h
> @@ -175,7 +175,8 @@ public:
>
>     bool is_send_from_grf();
>     unsigned regs_read(unsigned arg) const;
> -   bool can_reswizzle(int dst_writemask, int swizzle, int swizzle_mask);
> +   bool can_reswizzle(const struct brw_device_info *devinfo, int dst_writemask,
> +                      int swizzle, int swizzle_mask);
>     void reswizzle(int dst_writemask, int swizzle);
>     bool can_do_source_mods(const struct brw_device_info *devinfo);
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> index ed49cd3..d7192e4 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
> @@ -941,10 +941,17 @@ vec4_visitor::opt_set_dependency_control()
>  }
>
>  bool
> -vec4_instruction::can_reswizzle(int dst_writemask,
> +vec4_instruction::can_reswizzle(const struct brw_device_info *devinfo,
> +                                int dst_writemask,
>                                  int swizzle,
>                                  int swizzle_mask)
>  {
> +

Extra new line.

> +   /* gen6 math instructions can not manage source swizzles */

If we can find documentation, we should update this comment. If not,
let's change it to read

/* Gen6 MATH instructions can not execute in align16 mode, so swizzles
are not allowed. */

(linewrapped as appropriate)

With those changes,

Reviewed-by: Matt Turner <mattst88 at gmail.com>