[Mesa-dev] [PATCH] i965/cse: Don't eliminate instructions with side-effects

[Jason Ekstrand]

On Fri, Aug 8, 2014 at 4:58 PM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> On Friday, August 08, 2014 02:42:27 PM Jason Ekstrand wrote:
> > In particular, this caused problems where atomics operations were getting
> > eliminated.
> >
> > Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
> > ---
> >  src/mesa/drivers/dri/i965/brw_fs_cse.cpp   | 3 ++-
> >  src/mesa/drivers/dri/i965/brw_vec4_cse.cpp | 3 ++-
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
> > index 63d87f9..8cfc6c6 100644
> > --- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
> > +++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
> > @@ -177,7 +177,8 @@ fs_visitor::opt_cse_local(bblock_t *block)
> >     foreach_inst_in_block(fs_inst, inst, block) {
> >        /* Skip some cases. */
> >        if (is_expression(inst) && !inst->is_partial_write() &&
> > -          (inst->dst.file != HW_REG || inst->dst.is_null()))
> > +          (inst->dst.file != HW_REG || inst->dst.is_null()) &&
> > +          !inst->has_side_effects())
> >        {
> >           bool found = false;
> >
> > diff --git a/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
> b/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
> > index 29d2e02..44651b4 100644
> > --- a/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
> > +++ b/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
> > @@ -135,7 +135,8 @@ vec4_visitor::opt_cse_local(bblock_t *block)
> >     foreach_inst_in_block (vec4_instruction, inst, block) {
> >        /* Skip some cases. */
> >        if (is_expression(inst) && !inst->predicate && inst->mlen == 0 &&
> > -          (inst->dst.file != HW_REG || inst->dst.is_null()))
> > +          (inst->dst.file != HW_REG || inst->dst.is_null()) &&
> > +          !inst->has_side_effects())
> >        {
> >           bool found = false;
> >
> >
>
> I was confused at first because operations with side-effects should never
> have been part of the whitelist of opcodes to CSE.  But Matt generalized it
> in 1d97212007ccae, by changing is_expression()'s default case to "return
> inst->is_send_from_grf()".
>

> I think a better patch would be to change that to:
>
>    default:
>       return inst->is_send_from_grf() && !inst->has_side_effects();
>

Right. Good point.  I'll do it that way instead.


>
> It's also worth noting in your commit message that this is not actually
> fixing a current bug, but rather preventing a regression once your patches
> that convert atomics to send-from-GRFs land.
>

Perhaps.  Honestly, I'm not sure why CSE isn't causing problems now unless
it doesn't do CSE on message registers.

--Jason
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