[PATCH 1/1] vkd3d-shader/spirv: Implement MAD in two operations if flagged as precise.

[Conor McCarthy]

From: Conor McCarthy

---
 libs/vkd3d-shader/spirv.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/libs/vkd3d-shader/spirv.c b/libs/vkd3d-shader/spirv.c
index 5403e5443..06408919a 100644
--- a/libs/vkd3d-shader/spirv.c
+++ b/libs/vkd3d-shader/spirv.c
@@ -1447,6 +1447,20 @@ static uint32_t vkd3d_spirv_build_op_isub(struct vkd3d_spirv_builder *builder,
             SpvOpISub, result_type, operand0, operand1);
 }

+static uint32_t vkd3d_spirv_build_op_fadd(struct vkd3d_spirv_builder *builder,
+        uint32_t result_type, uint32_t operand0, uint32_t operand1)
+{
+    return vkd3d_spirv_build_op_tr2(builder, &builder->function_stream,
+            SpvOpFAdd, result_type, operand0, operand1);
+}
+
+static uint32_t vkd3d_spirv_build_op_fmul(struct vkd3d_spirv_builder *builder,
+        uint32_t result_type, uint32_t operand0, uint32_t operand1)
+{
+    return vkd3d_spirv_build_op_tr2(builder, &builder->function_stream,
+            SpvOpFMul, result_type, operand0, operand1);
+}
+
 static uint32_t vkd3d_spirv_build_op_fdiv(struct vkd3d_spirv_builder *builder,
         uint32_t result_type, uint32_t operand0, uint32_t operand1)
 {
@@ -7202,8 +7216,24 @@ static void spirv_compiler_emit_ext_glsl_instruction(struct spirv_compiler *comp
     for (i = 0; i < instruction->src_count; ++i)
         src_id[i] = spirv_compiler_emit_load_src(compiler, &src[i], dst->write_mask);

-    val_id = vkd3d_spirv_build_op_ext_inst(builder, type_id,
-            instr_set_id, glsl_inst, src_id, instruction->src_count);
+    if (instruction->handler_idx == VKD3DSIH_MAD && (instruction->flags & VKD3DSI_PRECISE_XYZW))
+    {
+        /* The HLSL docs state: "If components of a mad instruction are tagged as precise, the
+         * hardware must execute a mad instruction or the exact equivalent, and it cannot split
+         * it into a multiply followed by an add."
+         * But DXIL.rst states the opposite: "Floating point multiply & add. This operation is
+         * not fused for "precise" operations."
+         * Windows drivers seem to conform with the latter, for SM 4-5 and SM 6. */
+        val_id = vkd3d_spirv_build_op_fmul(builder, type_id, src_id[0], src_id[1]);
+        vkd3d_spirv_build_op_decorate(builder, val_id, SpvDecorationNoContraction, NULL, 0);
+        val_id = vkd3d_spirv_build_op_fadd(builder, type_id, val_id, src_id[2]);
+        vkd3d_spirv_build_op_decorate(builder, val_id, SpvDecorationNoContraction, NULL, 0);
+    }
+    else
+    {
+        val_id = vkd3d_spirv_build_op_ext_inst(builder, type_id,
+                instr_set_id, glsl_inst, src_id, instruction->src_count);
+    }

if (instruction->handler_idx == VKD3DSIH_FIRSTBIT_HI
             || instruction->handler_idx == VKD3DSIH_FIRSTBIT_SHI)