[Mesa-dev] [PATCH] i965/fs: Don't disable SIMD16 when using the pixel interpolator

Neil Roberts neil at linux.intel.com
Thu Jul 2 11:41:50 PDT 2015

There was a comment saying that in SIMD16 mode the pixel interpolator
returns coords interleaved 8 channels at a time and that this requires
extra work to support. However, this interleaved format is exactly
what the PLN instruction requires so I don't think anything needs to
be done to support it apart from removing the line to disable it and
to ensure that the message lengths for the send message are correct.

I am more convinced that this is correct because as it says in the
comment this interleaved output is identical to what is given in the
thread payload. The code generated to apply the plane equation to
these coordinates is identical on SIMD16 and SIMD8 except that the
dispatch width is larger which implies no special unmangling is

Perhaps the confusion stems from the fact that the description of the
PLN instruction in the IVB PRM seems to imply that the src1 inputs are
not interleaved so it wouldn't work. However, in the HSW and BDW PRMs,
the pseudo-code is different and looks like it expects the interleaved
format. Mesa doesn't seem to generate different code on IVB to
uninterleave the payload registers and everything is working so I can
only assume that the PRM is wrong.

I tested the interpolateAt tests on HSW and did a full Piglit run on
IVB on there were no regressions.

I've CC'd Chris Forbes because according to git-annotate he wrote the
original comment so he might know something I don't.

 src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
index 59081ea..717e597 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
@@ -1461,12 +1461,6 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
    case nir_intrinsic_interp_var_at_centroid:
    case nir_intrinsic_interp_var_at_sample:
    case nir_intrinsic_interp_var_at_offset: {
-      /* in SIMD16 mode, the pixel interpolator returns coords interleaved
-       * 8 channels at a time, same as the barycentric coords presented in
-       * the FS payload. this requires a bit of extra work to support.
-       */
-      no16("interpolate_at_* not yet supported in SIMD16 mode.");
       fs_reg dst_xy = bld.vgrf(BRW_REGISTER_TYPE_F, 2);
       /* For most messages, we need one reg of ignored data; the hardware
@@ -1531,7 +1525,7 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
                            bld.SEL(offset(src, i), itemp, fs_reg(7)));
-            mlen = 2;
+            mlen = 2 * dispatch_width / 8;
             inst = bld.emit(FS_OPCODE_INTERPOLATE_AT_PER_SLOT_OFFSET, dst_xy, src,
@@ -1543,7 +1537,8 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       inst->mlen = mlen;
-      inst->regs_written = 2; /* 2 floats per slot returned */
+      /* 2 floats per slot returned */
+      inst->regs_written = 2 * dispatch_width / 8;
       inst->pi_noperspective = instr->variables[0]->var->data.interpolation ==

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