intel/fs: Prevent emission of IR instructions not aligned to their own execution size.

This can occur during payload setup of SIMD-split send message
instructions, which can lead to the emission of header setup
instructions with a non-zero channel group and fixed SIMD width.  Such
instructions could end up using undefined channel enable signals
except they don't care since they're always marked force_writemask_all.

Not known to affect correctness of any workload at this point, but it
would be trivial to back-port to stable if something comes up.

Reported-by: Sagar Ghuge <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Tested-by: Sagar Ghuge <[email protected]>

1 files changed, 17 insertions, 3 deletions

diff --git a/src/intel/compiler/brw_fs_builder.h b/src/intel/compiler/brw_fs_builder.h
index 0cafaf50e56..4846820722c 100644
--- a/src/intel/compiler/brw_fs_builder.h
+++ b/src/intel/compiler/brw_fs_builder.h
@@ -114,11 +114,25 @@ namespace brw {
       fs_builder
       group(unsigned n, unsigned i) const
       {
-         assert(force_writemask_all ||
-                (n <= dispatch_width() && i < dispatch_width() / n));
          fs_builder bld = *this;
+
+         if (n <= dispatch_width() && i < dispatch_width() / n) {
+            bld._group += i * n;
+         } else {
+            /* The requested channel group isn't a subset of the channel group
+             * of this builder, which means that the resulting instructions
+             * would use (potentially undefined) channel enable signals not
+             * specified by the parent builder.  That's only valid if the
+             * instruction doesn't have per-channel semantics, in which case
+             * we should clear off the default group index in order to prevent
+             * emitting instructions with channel group not aligned to their
+             * own execution size.
+             */
+            assert(force_writemask_all);
+            bld._group = 0;
+         }
+
          bld._dispatch_width = n;
-         bld._group += i * n;
          return bld;
       }