[Mesa-dev] [PATCH v2 061/103] i965/vec4/nir: do not emit 64-bit MAD

[Iago Toral Quiroga]

RepCtrl=1 does not work with 64-bit operands so we need to use RepCtrl=0.

In that situation, the regioning generated for the sources seems to be
equivalent to <4,4,1>:DF, so it will only work for components XY, which
means that we have to move any other swizzle to a temporary so that we can
source from channel X (or Y) in MAD and we also need to split the instruction
(we are already scalarizing DF instructions but there is room for
improvement and with MAD would be more restricted in that area)

Also, it seems that MAD operations like this only write proper output for
channels X and Y, so writes to Z and W also need to be done to a temporary
using channels X/Y and then move that to channels Z or W of the actual dst.

As a result the code we produce for native 64-bit MAD instructions is rather
bad, and much worse than just emitting MUL+ADD. For reference, a simple case
of a fully scalarized dvec4 MAD operation requires 15 instructions if we use
native MAD and 8 instructions if we emit ADD+MUL instead. There are some
improvements that we can do to the emission of MAD that might bring the
instruction count down in some cases, but it comes at the expense of a more
complex implementation so it does not seem worth it, at least initially.

This patch makes translation of NIR's 64-bit FMMA instructions produce MUL+ADD
instead of MAD. Currently, there is nothing else in the vec4 backend that emits
MAD instructions, so this is sufficient and it helps optimization passes see
MUL+ADD from the get go.
---
 src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
index 1da7c85..0b8c808 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
@@ -1896,12 +1896,19 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
       break;
 
    case nir_op_ffma:
-      op[0] = fix_3src_operand(op[0]);
-      op[1] = fix_3src_operand(op[1]);
-      op[2] = fix_3src_operand(op[2]);
+      if (type_sz(dst.type) == 8) {
+         dst_reg mul_dst = dst_reg(this, glsl_type::dvec4_type);
+         emit(MUL(mul_dst, op[1], op[0]));
+         inst = emit(ADD(dst, src_reg(mul_dst), op[2]));
+         inst->saturate = instr->dest.saturate;
+      } else {
+         op[0] = fix_3src_operand(op[0]);
+         op[1] = fix_3src_operand(op[1]);
+         op[2] = fix_3src_operand(op[2]);
 
-      inst = emit(MAD(dst, op[2], op[1], op[0]));
-      inst->saturate = instr->dest.saturate;
+         inst = emit(MAD(dst, op[2], op[1], op[0]));
+         inst->saturate = instr->dest.saturate;
+      }
       break;
 
    case nir_op_flrp:
-- 
2.7.4