pan/va: Add SEG_ADD/SEG_SUB operation

[Alyssa: Fixes to the instruction.]

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15588>

src/panfrost/bifrost/valhall/ISA.xml

@@ -1823,6 +1823,21 @@
     <saturate/>
   </group>
 
+  <group name="SEG_ADD" title="Segment addition" dests="1" opcode2="6" unit="CVT">
+    <desc>
+      Similar to SHADDX, but especially used for loading offsets into
+      WLS. Usually this is only required for atomic operations, which cannot
+      directly use wls_pointer as an address.
+
+      .neg indicates SEG_SUB instead.
+    </desc>
+    <ins name="SEG_ADD.u64" opcode="0x1A3"/>
+    <mod name="neg" start="38" size="1"/>
+    <mod name="preserve_null" start="39" size="1"/>
+    <src>A</src>
+    <src widen="true">B</src>
+  </group>
+
   <group name="SHADDX" title="Shift, extend, and 64-bit add" dests="1" opcode2="7" unit="CVT">
     <desc>
       Sign or zero extend B to 64-bits, left-shift by `shift`, and add the

src/panfrost/bifrost/valhall/valhall.c.py

@@ -78,6 +78,7 @@ SKIP = set([
         "IDP.v4s8",
         "IDP.v4u8",
         "FATAN_ASSIST.f32",
+        "SEG_ADD.u64",
         "TEX_DUAL",
         "TODO.VAR_TEX",
     ])