bench: add benchmarks to show off string perf improvements

Change-Id: I6d032706e1e8b6fa39502276ac2018cb69de6ce5

bench/bench.sh

@@ -50,6 +50,10 @@ cases=(
     [rebuild]="{BUILD}/bin/nix $flake_args eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'"
     [rebuild-lh]="GC_INITIAL_HEAP_SIZE=10g {BUILD}/bin/nix eval $flake_args --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'"
     [parse]="{BUILD}/bin/nix $flake_args eval -f bench/nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix"
+    [substr100k]="{BUILD}/bin/nix eval --no-eval-cache -f bench/substring.nix --apply 'f: f.bench 100000'"
+    [substr200k]="{BUILD}/bin/nix eval --no-eval-cache -f bench/substring.nix --apply 'f: f.bench 200000'"
+    [substr300k]="{BUILD}/bin/nix eval --no-eval-cache -f bench/substring.nix --apply 'f: f.bench 300000'"
+    [substr400k]="{BUILD}/bin/nix eval --no-eval-cache -f bench/substring.nix --apply 'f: f.bench 400000'"
 )
 
 defaultBenches=(

bench/substring.nix

@@ -0,0 +1,19 @@
+# Tests stringLength and substring performance
+with builtins;
+{
+  bench =
+    baseStrLen:
+    let
+      # a big string
+      baseStr = concatStringsSep "" (genList (_: "x") baseStrLen);
+      # a way to force the values
+      sum = ns: foldl' (acc: n: acc + n) 0 ns;
+      forceOpTimes =
+        range: # Int
+        op: # Int -> Int
+        sum (genList (ix: op ix) range);
+
+      benchOp = ix: stringLength (substring ix 1 baseStr);
+    in
+    forceOpTimes baseStrLen benchOp;
+}