[Mesa-dev] [PATCH] intel/fs: Use a pure vertical stride for large register strides

[Jason Ekstrand]

Register strides higher than 4 are uncommon but they can happen.  For
instance, if you have a 64-bit extract_u8 operation, we turn that into
UB -> UQ MOV with a source stride of 8.  Our previous calculation would
try to generate a stride of <32;8,8>:ub which is invalid because the
maximum horizontal stride is 4.  To solve this problem, we instead use a
stride of <8;1,0>.  As noted in the comment, this does not work as a
destination but that's ok as very few things actually generate that
stride.

Cc: mesa-stable at lists.freedesktop.org
---
 src/intel/compiler/brw_fs_generator.cpp | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/intel/compiler/brw_fs_generator.cpp b/src/intel/compiler/brw_fs_generator.cpp
index 46f9a33..a557f80 100644
--- a/src/intel/compiler/brw_fs_generator.cpp
+++ b/src/intel/compiler/brw_fs_generator.cpp
@@ -90,9 +90,18 @@ brw_reg_from_fs_reg(const struct gen_device_info *devinfo, fs_inst *inst,
           *       different execution size when the number of components
           *       written to each destination GRF is not the same.
           */
-         const unsigned width = MIN2(reg_width, phys_width);
-         brw_reg = brw_vecn_reg(width, brw_file_from_reg(reg), reg->nr, 0);
-         brw_reg = stride(brw_reg, width * reg->stride, width, reg->stride);
+         if (reg->stride > 4) {
+            /* For registers with an exceptionally large stride, we use a
+             * width of 1 and only use the vertical stride.  This only works
+             * for sources since destinations require hstride == 1.
+             */
+            brw_reg = brw_vec1_reg(brw_file_from_reg(reg), reg->nr, 0);
+            brw_reg = stride(brw_reg, reg->stride, 1, 0);
+         } else {
+            const unsigned width = MIN2(reg_width, phys_width);
+            brw_reg = brw_vecn_reg(width, brw_file_from_reg(reg), reg->nr, 0);
+            brw_reg = stride(brw_reg, width * reg->stride, width, reg->stride);
+         }
 
          if (devinfo->gen == 7 && !devinfo->is_haswell) {
             /* From the IvyBridge PRM (EU Changes by Processor Generation, page 13):
-- 
2.5.0.400.gff86faf