[Mesa-dev] [PATCH 09/23] i965/fs: don't copy propagate from a larger type if the stride is not 1

[Samuel Iglesias Gonsálvez]

From: Iago Toral Quiroga <itoral at igalia.com>

Because the stride is in units for the type, if we copy-propagate from
a another instruction using a larger type, then we need to make sure
that the source in that instruction, the one we will be copy-propagating
from, sources consecutive elements, otherwise, when sourced using a
a smaller type, the actual elements read would change.

Prevents that we turn this:
mov(8) vgrf3+2.0:DF, vgrf11<0>:DF
load_payload(8) vgrf15:UD, ..., vgrf3+2.0<0>:D, vgrf3+3.0<0>:D

Into:
mov(8) vgrf3+2.0:DF, vgrf11<0>:DF
load_payload(8) vgrf15:UD, ..., vgrf11<0>:D, vgrf11<0>:D

In the original instructions, vgrf3+2.0<0>:D reads a replicated 64-bit
value, while the result after copy propagation only reads the first
32-bit of the value.
---
 src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
index 9fc06cb..f98ab41 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
@@ -400,6 +400,15 @@ fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
    if (type_sz(entry->dst.type) < type_sz(inst->src[arg].type))
       return false;
 
+   /* Bail if the instruction type is larger than the execution type of the
+    * copy and the stride of the source we would be copy propagating from
+    * has a stride other than 1. Otherwise, since the stride is in units of
+    * the type, we would be changing the region effectively sourced.
+    */
+   if (type_sz(entry->dst.type) > type_sz(inst->src[arg].type) &&
+       entry->src.stride != 1)
+      return false;
+
    /* Bail if the result of composing both strides cannot be expressed
     * as another stride. This avoids, for example, trying to transform
     * this:
-- 
2.5.0