feat(tvix/eval): Store string context alongside data r/7591

Previously, Nix strings were represented as a Box (within Value)
pointing to a tuple of an optional context, and another Box pointing to
the actual string allocation itself. This is pretty inefficient, both in
terms of memory usage (we use 48 whole bytes for a None context!) and in
terms of the extra indirection required to get at the actual data. It
was necessary, however, because with native Rust DSTs if we had
something like `struct NixString(Option<NixContext>, BStr)` we could
only pass around *fat* pointers to that value (with the length in the
pointer) and that'd make Value need to be bigger (which is a waste of
both memory and cache space, since that memory would be unused for all
other Values).

Instead, this commit implements *manual* allocation of a packed string
representation, with the length *in the allocation* as a field past the
context. This requires a big old pile of unsafe Rust, but the payoff is
clear:

    hello outpath  time:   [882.18 ms 897.16 ms 911.23 ms]
                   change: [-15.143% -13.819% -12.500%] (p = 0.00 < 0.05)
                   Performance has improved.

Fortunately this change can be localized entirely within
value/string.rs, since we were abstracting things out nicely.

Change-Id: Ibf56dd16c9c503884f64facbb7f0ac596463efb6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10852
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: aspen <root@gws.fyi>

1 files changed, 5 insertions, 7 deletions

diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 095bc269d497..e8e27e5968de 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -47,7 +47,7 @@ pub enum Value {
     Bool(bool),
     Integer(i64),
     Float(f64),
-    String(Box<NixString>),
+    String(NixString),
 
     #[serde(skip)]
     Path(Box<PathBuf>),
@@ -192,7 +192,7 @@ where
     T: Into<NixString>,
 {
     fn from(t: T) -> Self {
-        Self::String(Box::new(t.into()))
+        Self::String(t.into())
     }
 }
 
@@ -333,9 +333,7 @@ impl Value {
             let value = if let Some(v) = vals.pop() {
                 v.force(co, span.clone()).await?
             } else {
-                return Ok(Value::String(Box::new(NixString::new_context_from(
-                    context, result,
-                ))));
+                return Ok(Value::String(NixString::new_context_from(context, result)));
             };
             let coerced: Result<BString, _> = match (value, kind) {
                 // coercions that are always done
@@ -700,7 +698,7 @@ impl Value {
     /// everytime you want a string.
     pub fn to_str(&self) -> Result<NixString, ErrorKind> {
         match self {
-            Value::String(s) if !s.has_context() => Ok((**s).clone()),
+            Value::String(s) if !s.has_context() => Ok((*s).clone()),
             Value::Thunk(thunk) => Self::to_str(&thunk.value()),
             other => Err(type_error("contextless strings", other)),
         }
@@ -711,7 +709,7 @@ impl Value {
         NixString,
         "contextful string",
         Value::String(s),
-        (**s).clone()
+        (*s).clone()
     );
     gen_cast!(to_path, Box<PathBuf>, "path", Value::Path(p), p.clone());
     gen_cast!(to_attrs, Box<NixAttrs>, "set", Value::Attrs(a), a.clone());