i965/fs: add support for ir_*_interpolate_at_* expressions

SIMD8-only for now.

V5: - Fix style complaints
    - Move prototype to be with other oddball emit functions
    - Use unreachable() instead of assert() where possible

V6: - Describe what is happening with the clamping
    - Add reg_width to make some expressions clearer

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

2 files changed, 150 insertions, 2 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
index 1d3f9d0cdf1..6f169dcf4c8 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.h
+++ b/src/mesa/drivers/dri/i965/brw_fs.h
@@ -439,6 +439,8 @@ public:
    void emit_untyped_surface_read(unsigned surf_index, fs_reg dst,
                                   fs_reg offset);
 
+   void emit_interpolate_expression(ir_expression *ir);
+
    bool try_rewrite_rhs_to_dst(ir_assignment *ir,
 			       fs_reg dst,
 			       fs_reg src,
diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 8e8affabd40..90bf3fa3384 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -344,6 +344,133 @@ fs_visitor::try_emit_mad(ir_expression *ir)
    return true;
 }
 
+static int
+pack_pixel_offset(float x)
+{
+   /* Clamp upper end of the range to +7/16. See explanation in non-constant
+    * offset case below. */
+   int n = MIN2((int)(x * 16), 7);
+   return n & 0xf;
+}
+
+void
+fs_visitor::emit_interpolate_expression(ir_expression *ir)
+{
+   /* 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.");
+
+   ir_dereference * deref = ir->operands[0]->as_dereference();
+   ir_swizzle * swiz = NULL;
+   if (!deref) {
+      /* the api does not allow a swizzle here, but the varying packing code
+       * may have pushed one into here.
+       */
+      swiz = ir->operands[0]->as_swizzle();
+      assert(swiz);
+      deref = swiz->val->as_dereference();
+   }
+   assert(deref);
+   ir_variable * var = deref->variable_referenced();
+   assert(var);
+
+   /* 1. collect interpolation factors */
+
+   fs_reg dst_x = fs_reg(this, glsl_type::get_instance(ir->type->base_type, 2, 1));
+   fs_reg dst_y = dst_x;
+   dst_y.reg_offset++;
+
+   /* for most messages, we need one reg of ignored data; the hardware requires mlen==1
+    * even when there is no payload. in the per-slot offset case, we'll replace this with
+    * the proper source data. */
+   fs_reg src = fs_reg(this, glsl_type::float_type);
+   int mlen = 1;     /* one reg unless overriden */
+   int reg_width = dispatch_width / 8;
+   fs_inst *inst;
+
+   switch (ir->operation) {
+   case ir_unop_interpolate_at_centroid:
+      inst = emit(FS_OPCODE_INTERPOLATE_AT_CENTROID, dst_x, src, fs_reg(0u));
+      break;
+
+   case ir_binop_interpolate_at_sample: {
+      ir_constant *sample_num = ir->operands[1]->as_constant();
+      assert(sample_num || !"nonconstant sample number should have been lowered.");
+
+      unsigned msg_data = sample_num->value.i[0] << 4;
+      inst = emit(FS_OPCODE_INTERPOLATE_AT_SAMPLE, dst_x, src, fs_reg(msg_data));
+      break;
+   }
+
+   case ir_binop_interpolate_at_offset: {
+      ir_constant *const_offset = ir->operands[1]->as_constant();
+      if (const_offset) {
+         unsigned msg_data = pack_pixel_offset(const_offset->value.f[0]) |
+                            (pack_pixel_offset(const_offset->value.f[1]) << 4);
+         inst = emit(FS_OPCODE_INTERPOLATE_AT_SHARED_OFFSET, dst_x, src,
+                     fs_reg(msg_data));
+      } else {
+         /* pack the operands: hw wants offsets as 4 bit signed ints */
+         ir->operands[1]->accept(this);
+         src = fs_reg(this, glsl_type::ivec2_type);
+         fs_reg src2 = src;
+         for (int i = 0; i < 2; i++) {
+            fs_reg temp = fs_reg(this, glsl_type::float_type);
+            emit(MUL(temp, this->result, fs_reg(16.0f)));
+            emit(MOV(src2, temp));  /* float to int */
+
+            /* Clamp the upper end of the range to +7/16. ARB_gpu_shader5 requires
+             * that we support a maximum offset of +0.5, which isn't representable
+             * in a S0.4 value -- if we didn't clamp it, we'd end up with -8/16,
+             * which is the opposite of what the shader author wanted.
+             *
+             * This is legal due to ARB_gpu_shader5's quantization rules:
+             *
+             * "Not all values of <offset> may be supported; x and y offsets may
+             * be rounded to fixed-point values with the number of fraction bits
+             * given by the implementation-dependent constant
+             * FRAGMENT_INTERPOLATION_OFFSET_BITS"
+             */
+
+            fs_inst *inst = emit(BRW_OPCODE_SEL, src2, src2, fs_reg(7));
+            inst->conditional_mod = BRW_CONDITIONAL_L; /* min(src2, 7) */
+
+            src2.reg_offset++;
+            this->result.reg_offset++;
+         }
+
+         mlen = 2 * reg_width;
+         inst = emit(FS_OPCODE_INTERPOLATE_AT_PER_SLOT_OFFSET, dst_x, src,
+                     fs_reg(0u));
+      }
+      break;
+   }
+
+   default:
+      unreachable("not reached");
+   }
+
+   inst->mlen = mlen;
+   inst->regs_written = 2 * reg_width; /* 2 floats per slot returned */
+   inst->pi_noperspective = var->determine_interpolation_mode(key->flat_shade) ==
+         INTERP_QUALIFIER_NOPERSPECTIVE;
+
+   /* 2. emit linterp */
+
+   fs_reg res(this, ir->type);
+   this->result = res;
+
+   for (int i = 0; i < ir->type->vector_elements; i++) {
+      int ch = swiz ? ((*(int *)&swiz->mask) >> 2*i) & 3 : i;
+      emit(FS_OPCODE_LINTERP, res,
+           dst_x, dst_y,
+           fs_reg(interp_reg(var->data.location, ch)));
+      res.reg_offset++;
+   }
+}
+
 void
 fs_visitor::visit(ir_expression *ir)
 {
@@ -355,9 +482,22 @@ fs_visitor::visit(ir_expression *ir)
 
    if (try_emit_saturate(ir))
       return;
-   if (ir->operation == ir_binop_add) {
+
+   /* Deal with the real oddball stuff first */
+   switch (ir->operation) {
+   case ir_binop_add:
       if (try_emit_mad(ir))
-	 return;
+         return;
+      break;
+
+   case ir_unop_interpolate_at_centroid:
+   case ir_binop_interpolate_at_offset:
+   case ir_binop_interpolate_at_sample:
+      emit_interpolate_expression(ir);
+      return;
+
+   default:
+      break;
    }
 
    for (operand = 0; operand < ir->get_num_operands(); operand++) {
@@ -815,6 +955,12 @@ fs_visitor::visit(ir_expression *ir)
       inst = emit(BRW_OPCODE_SEL, this->result, op[1], op[2]);
       inst->predicate = BRW_PREDICATE_NORMAL;
       break;
+
+   case ir_unop_interpolate_at_centroid:
+   case ir_binop_interpolate_at_offset:
+   case ir_binop_interpolate_at_sample:
+      unreachable("already handled above");
+      break;
    }
 }