Shrink all SmallVecs by 8 bytes (#4951)

We weren't using the "union" cargo feature for the smallvec crate, which
reduces the size of a SmallVec by one machine word. This feature
requires Rust 1.49 but we already require much newer versions.

When using Wasmtime to compile pulldown-cmark from Sightglass, this
saves a decent amount of memory allocations and writes. According to
`valgrind --tool=dhat`:

- 6.2MiB (3.69%) less memory allocated over the program's lifetime
- 0.5MiB (4.13%) less memory allocated at maximum heap size
- 5.5MiB (1.88%) fewer bytes written to
- 0.44% fewer instructions executed

Sightglass reports a statistically significant runtime improvement too:

compilation :: cycles :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 24379323.60 ± 20051394.04 (confidence = 99%)

  shrink-abiarg-0406da67c.so is 1.01x to 1.13x faster than main-be690a468.so!

  [227506364 355007998.78 423280514] main-be690a468.so
  [227686018 330628675.18 406025344] shrink-abiarg-0406da67c.so

compilation :: cycles :: benchmarks/spidermonkey/benchmark.wasm

  Δ = 360151622.56 ± 278294316.90 (confidence = 99%)

  shrink-abiarg-0406da67c.so is 1.01x to 1.07x faster than main-be690a468.so!

  [8709162212 8911001926.44 9535111576] main-be690a468.so
  [5058015392 8550850303.88 9282148438] shrink-abiarg-0406da67c.so

compilation :: cycles :: benchmarks/bz2/benchmark.wasm

  Δ = 6936570.28 ± 6897696.38 (confidence = 99%)

  shrink-abiarg-0406da67c.so is 1.00x to 1.08x faster than main-be690a468.so!

  [155810934 175260571.20 234737344] main-be690a468.so
  [119128240 168324000.92 257451074] shrink-abiarg-0406da67c.so

cranelift/codegen/Cargo.toml

@@ -24,7 +24,7 @@ log = { version = "0.4.6", default-features = false }
 serde = { version = "1.0.94", features = ["derive"], optional = true }
 bincode = { version = "1.2.1", optional = true }
 gimli = { version = "0.26.0", default-features = false, features = ["write"], optional = true }
-smallvec = { version = "1.6.1" }
+smallvec = { version = "1.6.1", features = ["union"] }
 regalloc2 = { version = "0.4.1", features = ["checker"] }
 souper-ir = { version = "2.1.0", optional = true }
 sha2 = { version = "0.9.0", optional = true }

cranelift/egraph/Cargo.toml

@@ -11,7 +11,7 @@ edition = "2021"
 [dependencies]
 cranelift-entity = { path = "../entity", version = "0.89.0" }
 log = { version = "0.4.6", default-features = false }
-smallvec = { version = "1.6.1" }
+smallvec = { version = "1.6.1", features = ["union"] }
 indexmap = { version = "1.9.1" }
 hashbrown = { version = "0.12.2", features = ["raw"] }
 fxhash = "0.2.1"

cranelift/frontend/Cargo.toml

@@ -15,7 +15,7 @@ cranelift-codegen = { path = "../codegen", version = "0.89.0", default-features
 target-lexicon = "0.12"
 log = { version = "0.4.6", default-features = false }
 hashbrown = { version = "0.12", optional = true }
-smallvec = { version = "1.6.1" }
+smallvec = { version = "1.6.1", features = ["union"] }
 
 [features]
 default = ["std"]

cranelift/interpreter/Cargo.toml

@@ -14,7 +14,7 @@ edition = "2021"
 cranelift-codegen = { path = "../codegen", version = "0.89.0" }
 cranelift-entity = { path = "../entity", version = "0.89.0" }
 log = { version = "0.4.8", default-features = false }
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["union"] }
 thiserror = "1.0.15"
 
 [target.x86_64-pc-windows-gnu.dependencies]

cranelift/reader/Cargo.toml

@@ -11,7 +11,7 @@ edition = "2021"
 
 [dependencies]
 cranelift-codegen = { path = "../codegen", version = "0.89.0" }
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["union"] }
 target-lexicon = "0.12"
 
 [badges]

cranelift/wasm/Cargo.toml

@@ -21,7 +21,7 @@ hashbrown = { version = "0.12", optional = true }
 itertools = "0.10.0"
 log = { version = "0.4.6", default-features = false }
 serde = { version = "1.0.94", features = ["derive"], optional = true }
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["union"] }
 
 [dev-dependencies]
 wat = "1.0.47"

fuzz/Cargo.toml

@@ -18,7 +18,7 @@ cranelift-interpreter = { path = "../cranelift/interpreter" }
 cranelift-fuzzgen = { path = "../cranelift/fuzzgen" }
 libfuzzer-sys = { version = "0.4.0", features = ["arbitrary-derive"] }
 target-lexicon = "0.12"
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["union"] }
 wasmtime = { path = "../crates/wasmtime" }
 wasmtime-fuzzing = { path = "../crates/fuzzing" }
 component-test-util = { path = "../crates/misc/component-test-util" }