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, 3 insertions, 8 deletions

diff --git a/tvix/eval/src/value/attrs.rs b/tvix/eval/src/value/attrs.rs
index c9f04b54cc57..33259c8058eb 100644
--- a/tvix/eval/src/value/attrs.rs
+++ b/tvix/eval/src/value/attrs.rs
@@ -366,7 +366,7 @@ impl NixAttrs {
             let key = stack_slice.pop().unwrap();
 
             match key {
-                Value::String(ks) => set_attr(&mut attrs, *ks, value)?,
+                Value::String(ks) => set_attr(&mut attrs, ks, value)?,
 
                 Value::Null => {
                     // This is in fact valid, but leads to the value
@@ -422,13 +422,8 @@ impl IntoIterator for NixAttrs {
 fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
     let (name_idx, value_idx) = {
         match (&slice[2], &slice[0]) {
-            (Value::String(s1), Value::String(s2)) if (**s1 == *NAME_S && **s2 == *VALUE_S) => {
-                (3, 1)
-            }
-
-            (Value::String(s1), Value::String(s2)) if (**s1 == *VALUE_S && **s2 == *NAME_S) => {
-                (1, 3)
-            }
+            (Value::String(s1), Value::String(s2)) if (*s1 == *NAME_S && *s2 == *VALUE_S) => (3, 1),
+            (Value::String(s1), Value::String(s2)) if (*s1 == *VALUE_S && *s2 == *NAME_S) => (1, 3),
 
             // Technically this branch lets type errors pass,
             // but they will be caught during normal attribute