Mesa (master): intel/nir: Lower 8-bit scan/reduce ops to 16-bit

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Module: Mesa
Branch: master
Commit: b98f0d3d7c3b85001382eadd7dcfa3e11de64ca5
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=b98f0d3d7c3b85001382eadd7dcfa3e11de64ca5

Author: Jason Ekstrand <jason at jlekstrand.net>
Date:   Thu Nov  5 23:23:07 2020 -0600

intel/nir: Lower 8-bit scan/reduce ops to 16-bit

We can't really support these directly on any platform.  May as well let
NIR lower them.  The NIR lowering is potentially one more instruction
for scan/reduce ops thanks to not being able to do the B->W conversion
as part of SEL_EXEC.  For imax/imin exclusive scan, it's yet another
instruction thanks to the extra imax/imin NIR has to insert to deal with
the fact that the first live channel will contain the identity value
which, when signed, will cast wrong.  However, it does let us drop some
complexity from our back-end so it's probably worth it.

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7482>

---

 src/intel/compiler/brw_fs_nir.cpp | 42 +++------------------------------------
 src/intel/compiler/brw_nir.c      | 30 ++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 39 deletions(-)

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index 2cbcf4c5f02..38d7540fce0 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -5250,28 +5250,10 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       opcode brw_op = brw_op_for_nir_reduction_op(redop);
       brw_conditional_mod cond_mod = brw_cond_mod_for_nir_reduction_op(redop);
 
-      /* There are a couple of register region issues that make things
-       * complicated for 8-bit types:
-       *
-       *    1. Only raw moves are allowed to write to a packed 8-bit
-       *       destination.
-       *    2. If we use a strided destination, the efficient way to do scan
-       *       operations ends up using strides that are too big to encode in
-       *       an instruction.
-       *
-       * To get around these issues, we just do all 8-bit scan operations in
-       * 16 bits.  It's actually fewer instructions than what we'd have to do
-       * if we were trying to do it in native 8-bit types and the results are
-       * the same once we truncate to 8 bits at the end.
-       */
-      brw_reg_type scan_type = src.type;
-      if (type_sz(scan_type) == 1)
-         scan_type = brw_reg_type_from_bit_size(16, src.type);
-
       /* Set up a register for all of our scratching around and initialize it
        * to reduction operation's identity value.
        */
-      fs_reg scan = bld.vgrf(scan_type);
+      fs_reg scan = bld.vgrf(src.type);
       bld.exec_all().emit(SHADER_OPCODE_SEL_EXEC, scan, src, identity);
 
       bld.emit_scan(brw_op, scan, cluster_size, cond_mod);
@@ -5314,28 +5296,10 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       opcode brw_op = brw_op_for_nir_reduction_op(redop);
       brw_conditional_mod cond_mod = brw_cond_mod_for_nir_reduction_op(redop);
 
-      /* There are a couple of register region issues that make things
-       * complicated for 8-bit types:
-       *
-       *    1. Only raw moves are allowed to write to a packed 8-bit
-       *       destination.
-       *    2. If we use a strided destination, the efficient way to do scan
-       *       operations ends up using strides that are too big to encode in
-       *       an instruction.
-       *
-       * To get around these issues, we just do all 8-bit scan operations in
-       * 16 bits.  It's actually fewer instructions than what we'd have to do
-       * if we were trying to do it in native 8-bit types and the results are
-       * the same once we truncate to 8 bits at the end.
-       */
-      brw_reg_type scan_type = src.type;
-      if (type_sz(scan_type) == 1)
-         scan_type = brw_reg_type_from_bit_size(16, src.type);
-
       /* Set up a register for all of our scratching around and initialize it
        * to reduction operation's identity value.
        */
-      fs_reg scan = bld.vgrf(scan_type);
+      fs_reg scan = bld.vgrf(src.type);
       const fs_builder allbld = bld.exec_all();
       allbld.emit(SHADER_OPCODE_SEL_EXEC, scan, src, identity);
 
@@ -5344,7 +5308,7 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
           * shift of the contents before we can begin.  To make things worse,
           * we can't do this with a normal stride; we have to use indirects.
           */
-         fs_reg shifted = bld.vgrf(scan_type);
+         fs_reg shifted = bld.vgrf(src.type);
          fs_reg idx = bld.vgrf(BRW_REGISTER_TYPE_W);
          allbld.ADD(idx, nir_system_values[SYSTEM_VALUE_SUBGROUP_INVOCATION],
                          brw_imm_w(-1));
diff --git a/src/intel/compiler/brw_nir.c b/src/intel/compiler/brw_nir.c
index 71771a5fc58..282eac338fa 100644
--- a/src/intel/compiler/brw_nir.c
+++ b/src/intel/compiler/brw_nir.c
@@ -672,6 +672,36 @@ lower_bit_size_callback(const nir_instr *instr, UNUSED void *data)
       break;
    }
 
+   case nir_instr_type_intrinsic: {
+      nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+      switch (intrin->intrinsic) {
+      case nir_intrinsic_reduce:
+      case nir_intrinsic_inclusive_scan:
+      case nir_intrinsic_exclusive_scan:
+         /* There are a couple of register region issues that make things
+          * complicated for 8-bit types:
+          *
+          *    1. Only raw moves are allowed to write to a packed 8-bit
+          *       destination.
+          *    2. If we use a strided destination, the efficient way to do
+          *       scan operations ends up using strides that are too big to
+          *       encode in an instruction.
+          *
+          * To get around these issues, we just do all 8-bit scan operations
+          * in 16 bits.  It's actually fewer instructions than what we'd have
+          * to do if we were trying to do it in native 8-bit types and the
+          * results are the same once we truncate to 8 bits at the end.
+          */
+         if (intrin->dest.ssa.bit_size == 8)
+            return 16;
+         return 0;
+
+      default:
+         return 0;
+      }
+      break;
+   }
+
    default:
       return 0;
    }