[Mesa-dev] [PATCH 3/4] intel/compiler: Fix lower_conversions for 8-bit types.

[Iago Toral Quiroga]

From: Jose Maria Casanova Crespo <jmcasanova at igalia.com>

For 8-bit types the execution type is word. A byte raw MOV has 16-bit
execution type and 8-bit destination and it shouldn't be considered
a conversion case. So there is no need to change alignment and enter
in lower_conversions for these instructions.

Fixes a regresion in the piglit test "glsl-fs-shader-stencil-export"
that is introduced with this patch from the Vulkan shaderInt16 series:
'i965/compiler: handle conversion to smaller type in the lowering
pass for that'. The problem is caused because there is already a case
in the driver that injects Byte instructions like this:

mov(8)          g127<1>UB       g2<32,8,4>UB

And the aforementioned pass was not accounting for the special
handling of the execution size of Byte instructions. This patch
fixes this.

v2: (Jason Ekstrand)
   - Simplify is_byte_raw_mov, include reference to PRM and not
   consider B <-> UB conversions as raw movs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106393
---
 src/intel/compiler/brw_fs_lower_conversions.cpp | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/intel/compiler/brw_fs_lower_conversions.cpp b/src/intel/compiler/brw_fs_lower_conversions.cpp
index f95b39d3e8..f6c936cf21 100644
--- a/src/intel/compiler/brw_fs_lower_conversions.cpp
+++ b/src/intel/compiler/brw_fs_lower_conversions.cpp
@@ -43,6 +43,24 @@ supports_type_conversion(const fs_inst *inst) {
    }
 }
 
+/* From the SKL PRM Vol 2a, "Move":
+ *
+ *    "A mov with the same source and destination type, no source modifier,
+ *    and no saturation is a raw move. A packed byte destination region (B
+ *    or UB type with HorzStride == 1 and ExecSize > 1) can only be written
+ *    using raw move."
+ */
+static bool
+is_byte_raw_mov (const fs_inst *inst)
+{
+   return type_sz(inst->dst.type) == 1 &&
+      inst->opcode == BRW_OPCODE_MOV &&
+      inst->src[0].type == inst->dst.type &&
+      !inst->saturate &&
+      !inst->src[0].negate &&
+      !inst->src[0].abs;
+}
+
 bool
 fs_visitor::lower_conversions()
 {
@@ -54,7 +72,8 @@ fs_visitor::lower_conversions()
       bool saturate = inst->saturate;
 
       if (supports_type_conversion(inst)) {
-         if (type_sz(inst->dst.type) < get_exec_type_size(inst)) {
+         if (type_sz(inst->dst.type) < get_exec_type_size(inst) &&
+             !is_byte_raw_mov(inst)) {
             /* From the Broadwell PRM, 3D Media GPGPU, "Double Precision Float to
              * Single Precision Float":
              *
-- 
2.14.1