Add .slice(0, x) and .slice(x, y.length) bytecode optimisations

Author: rkalis

packages/cashc/test/cashproof/slice.equiv

@@ -8,3 +8,7 @@
 # x.split(10)[1].split(15)[0]
 OP_10 OP_SPLIT OP_NIP OP_15 OP_SPLIT OP_DROP
 ;
+
+# Slice optimisation
+OP_0 OP_SPLIT OP_NIP <=> ;
+OP_SIZE OP_SPLIT OP_DROP <=> ;

packages/cashc/test/generation/fixtures.ts

@@ -878,4 +878,31 @@ export const fixtures: Fixture[] = [
       updatedAt: '',
     },
   },
+  {
+    fn: 'slice_optimised.cash',
+    artifact: {
+      contractName: 'Slice',
+      constructorInputs: [{ name: 'data', type: 'bytes32' }],
+      abi: [{ name: 'spend', inputs: [] }],
+      bytecode: '14 OP_SPLIT OP_DROP OP_0 14 OP_NUM2BIN OP_EQUAL',
+      debug: {
+        bytecode: '01147f750001148087',
+        sourceMap: '3:34:3:36;:20::37:1;;4:31:4:32:0;:23::33:1;;:8::35',
+        logs: [],
+        requires: [
+          {
+            ip: 8,
+            line: 4,
+            message: undefined,
+          },
+        ],
+      },
+      source: fs.readFileSync(new URL('../valid-contract-files/slice_optimised.cash', import.meta.url), { encoding: 'utf-8' }),
+      compiler: {
+        name: 'cashc',
+        version,
+      },
+      updatedAt: '',
+    },
+  },
 ];

packages/cashc/test/valid-contract-files/slice_optimised.cash

@@ -0,0 +1,6 @@
+contract Slice(bytes32 data) {
+    function spend() {
+        bytes pkh = data.slice(0, 20);
+        require(pkh == bytes20(0));
+    }
+}

packages/utils/src/cashproof-optimisations.ts

@@ -121,4 +121,8 @@ OP_15 OP_NIP                           <=> OP_DROP OP_15;
 OP_16 OP_NIP                           <=> OP_DROP OP_16;
 
 OP_2 OP_PICK OP_SWAP OP_2 OP_PICK OP_NIP <=> OP_DROP OP_2DUP;
+
+# .slice(0, x) optimisation & .slice(x, y.length) optimisation
+OP_0 OP_SPLIT OP_NIP                   <=> ;
+OP_SIZE OP_SPLIT OP_DROP               <=> ;
 `;

packages/utils/src/optimisations.ts

@@ -114,6 +114,10 @@ const provableOptimisations = [
   ['OP_16 OP_NIP', 'OP_DROP OP_16'],
 
   ['OP_2 OP_PICK OP_SWAP OP_2 OP_PICK OP_NIP', 'OP_DROP OP_2DUP'],
+
+  // .slice(0, x) optimisation & .slice(x, y.length) optimisation
+  ['OP_0 OP_SPLIT OP_NIP', ''],
+  ['OP_SIZE OP_SPLIT OP_DROP', ''],
 ] as [string, string][];
 
 const unprovableOptimisations = [

website/docs/releases/release-notes.md

@@ -6,7 +6,8 @@ title: Release Notes
 
 #### cashc compiler
 - :sparkles: Add `.slice(start, end)` operator for bytes and strings.
-- :sparkles: Add bounded bytes typing and bounds checking for `.split()`
+- :sparkles: Add bounded bytes typing and bounds checking for `.split()` (includes checking for negative indices)
+- :racehorse: Add optimisation for `.slice(0, x)` and `.slice(x, y.length)` (also applies to `.split(0)[1]`)
 - :bug: Disallow incorrect bounded bytes typing when using `.split()`
 
 #### CashScript SDK