feat(tvix/eval): Don't emit OpForce for non-thunk constants r/7463

In the compiler, skip emitting an OpForce if the last op was an
OpConstant for a non-thunk constant. This gives a small (~1% on my
machine) perf boost, eg when evaluating hello.outPath:

    ❯ hyperfine \
        "./before --no-warnings -E '(import <nixpkgs> {}).hello.outPath'" \
        "./after --no-warnings -E '(import <nixpkgs> {}).hello.outPath'"
    Benchmark 1: ./before --no-warnings -E '(import <nixpkgs> {}).hello.outPath'
      Time (mean ± σ):      1.151 s ±  0.022 s    [User: 1.003 s, System: 0.151 s]
      Range (min … max):    1.123 s …  1.184 s    10 runs

    Benchmark 2: ./after --no-warnings -E '(import <nixpkgs> {}).hello.outPath'
      Time (mean ± σ):      1.140 s ±  0.022 s    [User: 0.989 s, System: 0.152 s]
      Range (min … max):    1.115 s …  1.175 s    10 runs

    Summary
      ./after --no-warnings -E '(import <nixpkgs> {}).hello.outPath' ran
        1.01 ± 0.03 times faster than ./before --no-warnings -E '(import <nixpkgs> {}).hello.outPath'

Change-Id: I2105fd431d4bad699087907e16c789418e9a4062
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10714
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

3 files changed, 26 insertions, 0 deletions

diff --git a/tvix/eval/src/chunk.rs b/tvix/eval/src/chunk.rs
index b6fc6be5b9..f1a35a6ce1 100644
--- a/tvix/eval/src/chunk.rs
+++ b/tvix/eval/src/chunk.rs
@@ -70,6 +70,11 @@ impl Chunk {
         self.spans[0].span
     }
 
+    /// Return a reference to the last op in the chunk, if any
+    pub fn last_op(&self) -> Option<&OpCode> {
+        self.code.last()
+    }
+
     /// Pop the last operation from the chunk and clean up its tracked
     /// span. Used when the compiler backtracks.
     pub fn pop_op(&mut self) {
@@ -90,6 +95,11 @@ impl Chunk {
         ConstantIdx(idx)
     }
 
+    /// Return a reference to the constant at the given [`ConstantIdx`]
+    pub fn get_constant(&self, constant: ConstantIdx) -> Option<&Value> {
+        self.constants.get(constant.0)
+    }
+
     // Span tracking implementation
 
     fn push_span(&mut self, span: codemap::Span, start: usize) {
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 60f5666ea9..0fa3d41393 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -1457,6 +1457,15 @@ impl Compiler<'_> {
     }
 
     fn emit_force<N: ToSpan>(&mut self, node: &N) {
+        if let Some(&OpCode::OpConstant(c)) = self.chunk().last_op() {
+            if !self.chunk().get_constant(c).unwrap().is_thunk() {
+                // Optimization: Don't emit a force op for non-thunk constants, since they don't
+                // need one!
+                // TODO: this is probably doable for more ops (?)
+                return;
+            }
+        }
+
         self.push_op(OpCode::OpForce, node);
     }
 
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index df663dd34c..e9b5098342 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -724,6 +724,13 @@ impl Value {
     gen_is!(is_attrs, Value::Attrs(_));
     gen_is!(is_catchable, Value::Catchable(_));
 
+    /// Returns `true` if the value is a [`Thunk`].
+    ///
+    /// [`Thunk`]: Value::Thunk
+    pub fn is_thunk(&self) -> bool {
+        matches!(self, Self::Thunk(..))
+    }
+
     /// Compare `self` against other using (fallible) Nix ordering semantics.
     ///
     /// The function is intended to be used from within other generator