nak/ra: Handle parallel copies as a special case

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

src/nouveau/compiler/nak_assign_regs.rs

@@ -177,7 +177,7 @@ impl RegAllocator {
         self.file
     }
 
-    fn reg_is_used(&self, reg: u32) -> bool {
+    pub fn reg_is_used(&self, reg: u32) -> bool {
         self.used.get(reg.try_into().unwrap())
     }
 
@@ -850,7 +850,7 @@ impl AssignRegsBlock {
         dsts_killed: &KillSet,
         pcopy: &mut OpParCopy,
     ) -> Option<Box<Instr>> {
-        match &instr.op {
+        match &mut instr.op {
             Op::Undef(undef) => {
                 if let Dst::SSA(ssa) = undef.dst {
                     assert!(ssa.comps() == 1);
@@ -893,6 +893,51 @@ impl AssignRegsBlock {
 
                 None
             }
+            Op::ParCopy(pcopy) => {
+                for (_, src) in pcopy.dsts_srcs.iter_mut() {
+                    if let SrcRef::SSA(src_vec) = src.src_ref {
+                        debug_assert!(src_vec.comps() == 1);
+                        let src_ssa = &src_vec[0];
+                        src.src_ref = self.get_scalar(*src_ssa).into();
+                    }
+                }
+
+                self.ra.free_killed(srcs_killed);
+
+                // Try to propagate sources if possible
+                for (dst, src) in pcopy.dsts_srcs.iter_mut() {
+                    let SrcRef::Reg(src_reg) = src.src_ref else {
+                        continue;
+                    };
+                    debug_assert!(src_reg.comps() == 1);
+
+                    let Dst::SSA(dst_vec) = dst else {
+                        continue;
+                    };
+                    debug_assert!(dst_vec.comps() == 1);
+                    let dst_ssa = &dst_vec[0];
+
+                    if dst_ssa.file() != src_reg.file() {
+                        continue;
+                    }
+
+                    let ra = &mut self.ra[src_reg.file()];
+                    if !ra.reg_is_used(src_reg.base_idx()) {
+                        ra.assign_reg(*dst_ssa, src_reg.base_idx());
+                        *dst = src_reg.into();
+                    }
+                }
+
+                for (dst, _) in pcopy.dsts_srcs.iter_mut() {
+                    if let Dst::SSA(dst_vec) = dst {
+                        debug_assert!(dst_vec.comps() == 1);
+                        *dst = self.alloc_scalar(ip, sum, dst_vec[0]).into();
+                    }
+                }
+
+                self.ra.free_killed(dsts_killed);
+                Some(instr)
+            }
             _ => {
                 for file in self.ra.values_mut() {
                     instr_assign_regs_file(