nak: Fix delay insertion missing WaR

Current code clears register writes after a register read is
encountered, this handles the first WaR but hides the write from the
reads that will succeed the first one. Ignoring subtle WaRaR hazards.
To fix this, we don't clear writes when a register read is encountered.

Thanks to Karol Herbst for finding and distilling this issue.

Signed-off-by: Lorenzo Rossi <git@rossilorenzo.dev>
Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37108>

src/nouveau/compiler/nak/calc_instr_deps.rs

@@ -72,13 +72,15 @@ enum RegReadWrite {
 /// Maps each register read/write to a value
 /// a register can have multiple reads AND multiple writes at the same
 /// point in time if it comes from a merge.
-/// For edits inside a CFG block, a RegUseMap will always be either
-/// empty, with a single write or with one or multiple reads.
+/// For edits inside a CFG block, a RegUseMap will never contain multiple
+/// writes.
 ///
 /// We need to track multiple reads as we don't know which one can cause
 /// the highest latency for the interfering instruction (in RaW).  For the
 /// same reason we might need to track both reads and writes in the case of
 /// a CFG block with multiple successors.
+/// We cannot flush writes after a read operation since we can still
+/// encounter other, slower reads that could interfere with the write.
 #[derive(Clone, PartialEq, Eq, Default)]
 struct RegUseMap<K: Hash + Eq, V> {
     map: FxHashMap<(RegReadWrite, K), V>,
@@ -90,8 +92,6 @@ where
     V: Clone,
 {
     pub fn add_read(&mut self, k: K, v: V) {
-        // Reads wait on previous writes (RaR don't exist)
-        self.map.retain(|k, _v| k.0 != RegReadWrite::Write);
         self.map.insert((RegReadWrite::Read, k), v);
     }