[Mesa-dev] [PATCH v2 35/53] intel/compiler: workaround for SIMD8 half-float MAD in gen < 9

[Iago Toral]

On Wed, 2019-01-02 at 11:46 +0200, Pohjolainen, Topi wrote:
> On Wed, Dec 19, 2018 at 12:51:03PM +0100, Iago Toral Quiroga wrote:
> > Broadwell hardware has a bug that manifests in SIMD8 executions of
> > 16-bit MAD instructions when any of the sources is a Y or W
> > component.
> > We pack these components in the same SIMD register as components X
> > and
> > Z respectively, but starting at offset 16B (so they live in the
> > second
> > half of the register). The problem does not exist in SKL or later.
> > 
> > We work around this issue by moving any such sources to a temporary
> > starting at offset 0B. We want to do this after the main
> > optimization loop
> > to prevent copy-propagation and friends to undo the fix.
> > ---
> >  src/intel/compiler/brw_fs.cpp | 48
> > +++++++++++++++++++++++++++++++++++
> >  src/intel/compiler/brw_fs.h   |  1 +
> >  2 files changed, 49 insertions(+)
> > 
> > diff --git a/src/intel/compiler/brw_fs.cpp
> > b/src/intel/compiler/brw_fs.cpp
> > index 933b0b6ffc4..1343c2f4993 100644
> > --- a/src/intel/compiler/brw_fs.cpp
> > +++ b/src/intel/compiler/brw_fs.cpp
> > @@ -6449,6 +6449,48 @@ fs_visitor::optimize()
> >     validate();
> >  }
> >  
> > +/**
> > + * Broadwell hardware has a bug that manifests in SIMD8 executions
> > of 16-bit
> > + * MAD instructions when any of the sources is a Y or W component.
> > We pack
> > + * these components in the same SIMD register as components X and
> > Z
> > + * respectively, but starting at offset 16B (so they live in the
> > second half
> > + * of the register).
> > + *
> > + * We work around this issue by moving any such sources to a
> > temporary
> > + * starting at offset 0B. We want to do this after the main
> > optimization loop
> > + * to prevent copy-propagation and friends to undo the fix.
> > + */
> > +void
> > +fs_visitor::fixup_hf_mad()
> > +{
> > +   if (devinfo->gen > 8)
> 
> We don't want to run this for gen < 8 either as it would iterate the
> instructions in vain. So just:
> 
>       if (devinfo->gen == 8)

Right, good point.

It should be "if (devinfo->gen != 8) though. I'll fix this, thanks!

> Otherwise:
> 
> Reviewed-by: Topi Pohjolainen <topi.pohjolainen at intel.com>
> 
> > +      return;
> > +
> > +   bool progress = false;
> > +
> > +   foreach_block_and_inst_safe (block, fs_inst, inst, cfg) {
> > +      if (inst->opcode != BRW_OPCODE_MAD ||
> > +          inst->dst.type != BRW_REGISTER_TYPE_HF ||
> > +          inst->exec_size > 8)
> > +         continue;
> > +
> > +      for (int i = 0; i < 3; i++) {
> > +         if (inst->src[i].offset > 0) {
> > +            assert(inst->src[i].type == BRW_REGISTER_TYPE_HF);
> > +            const fs_builder ibld =
> > +               bld.at(block, inst).exec_all().group(inst-
> > >exec_size, 0);
> > +            fs_reg tmp = ibld.vgrf(inst->src[i].type);
> > +            ibld.MOV(tmp, inst->src[i]);
> > +            inst->src[i] = tmp;
> > +            progress = true;
> > +         }
> > +      }
> > +   }
> > +
> > +   if (progress)
> > +      invalidate_live_intervals();
> > +}
> > +
> >  /**
> >   * Three source instruction must have a GRF/MRF destination
> > register.
> >   * ARF NULL is not allowed.  Fix that up by allocating a temporary
> > GRF.
> > @@ -6607,6 +6649,7 @@ fs_visitor::run_vs()
> >     assign_curb_setup();
> >     assign_vs_urb_setup();
> >  
> > +   fixup_hf_mad();
> >     fixup_3src_null_dest();
> >     allocate_registers(8, true);
> >  
> > @@ -6691,6 +6734,7 @@ fs_visitor::run_tcs_single_patch()
> >     assign_curb_setup();
> >     assign_tcs_single_patch_urb_setup();
> >  
> > +   fixup_hf_mad();
> >     fixup_3src_null_dest();
> >     allocate_registers(8, true);
> >  
> > @@ -6725,6 +6769,7 @@ fs_visitor::run_tes()
> >     assign_curb_setup();
> >     assign_tes_urb_setup();
> >  
> > +   fixup_hf_mad();
> >     fixup_3src_null_dest();
> >     allocate_registers(8, true);
> >  
> > @@ -6774,6 +6819,7 @@ fs_visitor::run_gs()
> >     assign_curb_setup();
> >     assign_gs_urb_setup();
> >  
> > +   fixup_hf_mad();
> >     fixup_3src_null_dest();
> >     allocate_registers(8, true);
> >  
> > @@ -6874,6 +6920,7 @@ fs_visitor::run_fs(bool allow_spilling, bool
> > do_rep_send)
> >  
> >        assign_urb_setup();
> >  
> > +      fixup_hf_mad();
> >        fixup_3src_null_dest();
> >        allocate_registers(8, allow_spilling);
> >  
> > @@ -6918,6 +6965,7 @@ fs_visitor::run_cs(unsigned
> > min_dispatch_width)
> >  
> >     assign_curb_setup();
> >  
> > +   fixup_hf_mad();
> >     fixup_3src_null_dest();
> >     allocate_registers(min_dispatch_width, true);
> >  
> > diff --git a/src/intel/compiler/brw_fs.h
> > b/src/intel/compiler/brw_fs.h
> > index 163c0008820..f79f8554fb9 100644
> > --- a/src/intel/compiler/brw_fs.h
> > +++ b/src/intel/compiler/brw_fs.h
> > @@ -103,6 +103,7 @@ public:
> >     void setup_vs_payload();
> >     void setup_gs_payload();
> >     void setup_cs_payload();
> > +   void fixup_hf_mad();
> >     void fixup_3src_null_dest();
> >     void assign_curb_setup();
> >     void calculate_urb_setup();
> > -- 
> > 2.17.1
> > 
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> 
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