nak: Add a dead-code pass

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

src/nouveau/compiler/nak.rs

@@ -6,6 +6,7 @@
 mod nak_from_nir;
 mod nak_ir;
 mod nak_opt_copy_prop;
+mod nak_opt_dce;
 mod nir;
 
 use nak_bindings::*;
@@ -40,5 +41,9 @@ pub extern "C" fn nak_compile_shader(
 
     println!("NAK IR:\n{}", &s);
 
+    s.opt_dce();
+
+    println!("NAK IR:\n{}", &s);
+
     std::ptr::null_mut()
 }

src/nouveau/compiler/nak_ir.rs

@@ -595,6 +595,13 @@ impl Instr {
     pub fn num_srcs(&self) -> usize {
         self.srcs().len()
     }
+
+    pub fn can_eliminate(&self) -> bool {
+        match self.op {
+            Opcode::FS_OUT | Opcode::EXIT | Opcode::AST(_) => false,
+            _ => true,
+        }
+    }
 }
 
 pub struct MetaInstr {

src/nouveau/compiler/nak_opt_dce.rs

@@ -0,0 +1,86 @@
+/*
+ * Copyright © 2022 Collabora, Ltd.
+ * SPDX-License-Identifier: MIT
+ */
+
+use crate::nak_ir::*;
+
+use std::collections::HashSet;
+
+struct DeadCodePass {
+    live_ssa: HashSet<SSAValue>,
+}
+
+impl DeadCodePass {
+    pub fn new() -> DeadCodePass {
+        DeadCodePass {
+            live_ssa: HashSet::new(),
+        }
+    }
+
+    fn mark_ssa_live(&mut self, ssa: &SSAValue) {
+        self.live_ssa.insert(*ssa);
+    }
+
+    fn mark_instr_live(&mut self, instr: &Instr) {
+        for src in instr.srcs() {
+            if let Src::SSA(ssa) = src {
+                self.mark_ssa_live(ssa);
+            }
+        }
+    }
+
+    fn is_dst_live(&self, dst: &Dst) -> bool {
+        match dst {
+            Ref::SSA(ssa) => self.live_ssa.get(ssa).is_some(),
+            Ref::Zero => false,
+            _ => panic!("Invalid SSA destination"),
+        }
+    }
+
+    fn is_instr_live(&self, instr: &Instr) -> bool {
+        if !instr.can_eliminate() {
+            return true;
+        }
+
+        for dst in instr.dsts() {
+            if self.is_dst_live(dst) {
+                return true;
+            }
+        }
+
+        false
+    }
+
+    pub fn run(&mut self, f: &mut Function) {
+        let mut has_any_dead = false;
+
+        for b in f.blocks.iter().rev() {
+            for instr in b.instrs.iter().rev() {
+                if self.is_instr_live(instr) {
+                    self.mark_instr_live(instr);
+                } else {
+                    has_any_dead = true;
+                }
+            }
+        }
+
+        if has_any_dead {
+            f.map_instrs(&|instr: Instr| -> Instr {
+                if self.is_instr_live(&instr) {
+                    instr
+                } else {
+                    Instr::new_noop()
+                }
+            })
+        }
+    }
+}
+
+impl Shader {
+    pub fn opt_dce(&mut self) {
+        for f in &mut self.functions {
+            DeadCodePass::new().run(f);
+        }
+    }
+}