There is a hardware restriction where <0,1,0>:HF in Align16 doesn't replicate
a single 16-bit channel, but instead it replicates a full 32-bit channel.
---
.../compiler/brw_fs_combine_constants.cpp | 24 +++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)
diff --git a/src/intel/compiler/brw_fs_combine_constants.cpp
b/src/intel/compiler/brw_fs_combine_constants.cpp
index 54017e5668b..56e414d3f4e 100644
--- a/src/intel/compiler/brw_fs_combine_constants.cpp
+++ b/src/intel/compiler/brw_fs_combine_constants.cpp
@@ -301,7 +301,26 @@ fs_visitor::opt_combine_constants()
*/
exec_node *n = (imm->inst ? imm->inst :
imm->block->last_non_control_flow_inst()->next);
- const fs_builder ibld = bld.at(imm->block, n).exec_all().group(1, 0);
+
+ /* Prior to gen9 we also have to deal with this restriction:
+ *
+ * "In Align16 mode, the channel selects and channel enables apply to a
+ * pair of half-floats, because these parameters are defined for DWord
+ * elements ONLY. This is applicable when both source and destination
+ * are half-floats."
+ *
+ * This means that when we emit a 3-src instruction such as MAD or LRP,
+ * for which we use Align16, if we need to promote an HF constant to a
+ * register we need to be aware that the <0,1,0>:HF region would still
+ * read 2 HF slots and not not replicate the single one like we want.
+ * We fix this by populating both HF slots with the constant we need to
+ * read.
+ */
+ const uint32_t width =
+ devinfo->gen < 9 &&
+ imm->type == BRW_REGISTER_TYPE_HF &&
+ (!imm->inst || imm->inst->is_3src(devinfo)) ? 2 : 1;
+ const fs_builder ibld = bld.at(imm->block, n).exec_all().group(width, 0);
reg = retype(reg, imm->type);
if (imm->type == BRW_REGISTER_TYPE_F) {
@@ -314,7 +333,8 @@ fs_visitor::opt_combine_constants()
imm->subreg_offset = reg.offset;
/* Keep offsets 32-bit aligned since we are mixing 32-bit and 16-bit
- * constants into the same register
+ * constants into the same register (and we are writing 32-bit slots
+ * prior to gen9 for HF constants anyway).
*
* TODO: try to pack pairs of HF constants into each 32-bit slot
*/
--
2.17.1
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