[Mesa-dev] [PATCH 06/23] i965/fs: fix copy/constant propagation regioning checks

[Francisco Jerez]

Samuel Iglesias Gonsálvez <siglesias at igalia.com> writes:

> From: Iago Toral Quiroga <itoral at igalia.com>
>
> We were not accounting for reg_suboffset in the check for the start
> of the region. This meant that would allow copy-propagation even if
> the dst wrote to sub_regoffset 4 and our source read from
> sub_regoffset 0, which is not correct. This was observed in fp64 code,
> since there we use reg_suboffset to select the high 32-bit of a double.
>
I don't think this paragraph is accurate, copy instructions with
non-zero destination subreg offset are currently considered partial
writes and should never have been added to the ACP hash table in the
first place.

> Also, fs_reg::regs_read() already takes the stride into account, so we
> should not multiply its result by the stride again. This was making
> copy-propagation fail to copy-propagate cases that would otherwise be
> safe to copy-propagate. Again, this was observed in fp64 code, since
> there we use stride > 1 often.
>
> Incidentally, these fixes open up more possibilities for copy propagation
> which uncovered new bugs in copy-propagation. The folowing patches address
> each of these new issues.

Oh man, that sucks...

> ---
>  .../drivers/dri/i965/brw_fs_copy_propagation.cpp    | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> index 5fae10f..23df877 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> @@ -329,6 +329,15 @@ can_take_stride(fs_inst *inst, unsigned arg, unsigned stride,
>     return true;
>  }
>  
> +static inline bool
> +region_match(fs_reg src, unsigned regs_read,
> +             fs_reg dst, unsigned regs_written)

How about 'region_contained_in(dst, regs_write, src, regs_read)'? (I
personally wouldn't mind 'region_match' but
'write_region_contains_read_region' sounds a bit too long for my taste).

> +{
> +   return src.reg_offset >= dst.reg_offset &&
> +          (src.reg_offset + regs_read) <= (dst.reg_offset + regs_written) &&
> +          src.subreg_offset >= dst.subreg_offset;

This works under the assumption that src.subreg_offset is strictly less
than the reg_offset unit -- Which *should* be the case unless we've
messed up that restriction in some place (we have in the past :P).  To
be on the safe side you could do something like following, if you like:

|   return (src.reg_offset * REG_SIZE + src.subreg_offset >=
|           dst.reg_offset * REG_SIZE + dst.subreg_offset) &&
|          src.reg_offset + regs_read <= dst.reg_offset + regs_written;

With the above taken into account:

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

> +}
> +
>  bool
>  fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
>  {
> @@ -351,10 +360,8 @@ fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
>     /* Bail if inst is reading a range that isn't contained in the range
>      * that entry is writing.
>      */
> -   if (inst->src[arg].reg_offset < entry->dst.reg_offset ||
> -       (inst->src[arg].reg_offset * 32 + inst->src[arg].subreg_offset +
> -        inst->regs_read(arg) * inst->src[arg].stride * 32) >
> -       (entry->dst.reg_offset + entry->regs_written) * 32)
> +   if (!region_match(inst->src[arg], inst->regs_read(arg),
> +                     entry->dst, entry->regs_written))
>        return false;
>  
>     /* we can't generally copy-propagate UD negations because we
> @@ -554,10 +561,8 @@ fs_visitor::try_constant_propagate(fs_inst *inst, acp_entry *entry)
>        /* Bail if inst is reading a range that isn't contained in the range
>         * that entry is writing.
>         */
> -      if (inst->src[i].reg_offset < entry->dst.reg_offset ||
> -          (inst->src[i].reg_offset * 32 + inst->src[i].subreg_offset +
> -           inst->regs_read(i) * inst->src[i].stride * 32) >
> -          (entry->dst.reg_offset + entry->regs_written) * 32)
> +      if (!region_match(inst->src[i], inst->regs_read(i),
> +                        entry->dst, entry->regs_written))
>           continue;
>  
>        fs_reg val = entry->src;
> -- 
> 2.5.0
>
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> mesa-dev at lists.freedesktop.org
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