Move multiproof hashes allocation under condition to avoid zero-length alloc (#6092)

Co-authored-by: ernestognw <[email protected]>

Author: Snezhkko

contracts/utils/cryptography/MerkleProof.sol

@@ -217,26 +217,25 @@ library MerkleProof {
             revert MerkleProofInvalidMultiproof();
         }
 
-        // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
-        // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
-        bytes32[] memory hashes = new bytes32[](proofFlagsLen);
-        uint256 leafPos = 0;
-        uint256 hashPos = 0;
-        uint256 proofPos = 0;
-        // At each step, we compute the next hash using two values:
-        // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
-        //   get the next hash.
-        // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
-        //   `proof` array.
-        for (uint256 i = 0; i < proofFlagsLen; i++) {
-            bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
-            bytes32 b = proofFlags[i]
-                ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
-                : proof[proofPos++];
-            hashes[i] = Hashes.commutativeKeccak256(a, b);
-        }
-
         if (proofFlagsLen > 0) {
+            // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
+            // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
+            bytes32[] memory hashes = new bytes32[](proofFlagsLen);
+            uint256 leafPos = 0;
+            uint256 hashPos = 0;
+            uint256 proofPos = 0;
+            // At each step, we compute the next hash using two values:
+            // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
+            //   get the next hash.
+            // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
+            //   `proof` array.
+            for (uint256 i = 0; i < proofFlagsLen; i++) {
+                bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
+                bytes32 b = proofFlags[i]
+                    ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
+                    : proof[proofPos++];
+                hashes[i] = Hashes.commutativeKeccak256(a, b);
+            }
             if (proofPos != proof.length) {
                 revert MerkleProofInvalidMultiproof();
             }
@@ -305,26 +304,25 @@ library MerkleProof {
             revert MerkleProofInvalidMultiproof();
         }
 
-        // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
-        // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
-        bytes32[] memory hashes = new bytes32[](proofFlagsLen);
-        uint256 leafPos = 0;
-        uint256 hashPos = 0;
-        uint256 proofPos = 0;
-        // At each step, we compute the next hash using two values:
-        // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
-        //   get the next hash.
-        // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
-        //   `proof` array.
-        for (uint256 i = 0; i < proofFlagsLen; i++) {
-            bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
-            bytes32 b = proofFlags[i]
-                ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
-                : proof[proofPos++];
-            hashes[i] = hasher(a, b);
-        }
-
         if (proofFlagsLen > 0) {
+            // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
+            // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
+            bytes32[] memory hashes = new bytes32[](proofFlagsLen);
+            uint256 leafPos = 0;
+            uint256 hashPos = 0;
+            uint256 proofPos = 0;
+            // At each step, we compute the next hash using two values:
+            // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
+            //   get the next hash.
+            // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
+            //   `proof` array.
+            for (uint256 i = 0; i < proofFlagsLen; i++) {
+                bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
+                bytes32 b = proofFlags[i]
+                    ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
+                    : proof[proofPos++];
+                hashes[i] = hasher(a, b);
+            }
             if (proofPos != proof.length) {
                 revert MerkleProofInvalidMultiproof();
             }
@@ -391,26 +389,25 @@ library MerkleProof {
             revert MerkleProofInvalidMultiproof();
         }
 
-        // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
-        // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
-        bytes32[] memory hashes = new bytes32[](proofFlagsLen);
-        uint256 leafPos = 0;
-        uint256 hashPos = 0;
-        uint256 proofPos = 0;
-        // At each step, we compute the next hash using two values:
-        // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
-        //   get the next hash.
-        // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
-        //   `proof` array.
-        for (uint256 i = 0; i < proofFlagsLen; i++) {
-            bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
-            bytes32 b = proofFlags[i]
-                ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
-                : proof[proofPos++];
-            hashes[i] = Hashes.commutativeKeccak256(a, b);
-        }
-
         if (proofFlagsLen > 0) {
+            // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
+            // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
+            bytes32[] memory hashes = new bytes32[](proofFlagsLen);
+            uint256 leafPos = 0;
+            uint256 hashPos = 0;
+            uint256 proofPos = 0;
+            // At each step, we compute the next hash using two values:
+            // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
+            //   get the next hash.
+            // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
+            //   `proof` array.
+            for (uint256 i = 0; i < proofFlagsLen; i++) {
+                bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
+                bytes32 b = proofFlags[i]
+                    ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
+                    : proof[proofPos++];
+                hashes[i] = Hashes.commutativeKeccak256(a, b);
+            }
             if (proofPos != proof.length) {
                 revert MerkleProofInvalidMultiproof();
             }
@@ -479,26 +476,25 @@ library MerkleProof {
             revert MerkleProofInvalidMultiproof();
         }
 
-        // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
-        // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
-        bytes32[] memory hashes = new bytes32[](proofFlagsLen);
-        uint256 leafPos = 0;
-        uint256 hashPos = 0;
-        uint256 proofPos = 0;
-        // At each step, we compute the next hash using two values:
-        // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
-        //   get the next hash.
-        // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
-        //   `proof` array.
-        for (uint256 i = 0; i < proofFlagsLen; i++) {
-            bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
-            bytes32 b = proofFlags[i]
-                ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
-                : proof[proofPos++];
-            hashes[i] = hasher(a, b);
-        }
-
         if (proofFlagsLen > 0) {
+            // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
+            // `xxx[xxxPos++]`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
+            bytes32[] memory hashes = new bytes32[](proofFlagsLen);
+            uint256 leafPos = 0;
+            uint256 hashPos = 0;
+            uint256 proofPos = 0;
+            // At each step, we compute the next hash using two values:
+            // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
+            //   get the next hash.
+            // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
+            //   `proof` array.
+            for (uint256 i = 0; i < proofFlagsLen; i++) {
+                bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
+                bytes32 b = proofFlags[i]
+                    ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
+                    : proof[proofPos++];
+                hashes[i] = hasher(a, b);
+            }
             if (proofPos != proof.length) {
                 revert MerkleProofInvalidMultiproof();
             }

scripts/generate/templates/MerkleProof.js

@@ -138,26 +138,25 @@ function processMultiProof${suffix}(${formatArgsMultiline(
         revert MerkleProofInvalidMultiproof();
     }
 
-    // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
-    // \`xxx[xxxPos++]\`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
-    bytes32[] memory hashes = new bytes32[](proofFlagsLen);
-    uint256 leafPos = 0;
-    uint256 hashPos = 0;
-    uint256 proofPos = 0;
-    // At each step, we compute the next hash using two values:
-    // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
-    //   get the next hash.
-    // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
-    //   \`proof\` array.
-    for (uint256 i = 0; i < proofFlagsLen; i++) {
-        bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
-        bytes32 b = proofFlags[i]
-            ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
-            : proof[proofPos++];
-        hashes[i] = ${hash ?? DEFAULT_HASH}(a, b);
-    }
-
     if (proofFlagsLen > 0) {
+        // The xxxPos values are "pointers" to the next value to consume in each array. All accesses are done using
+        // \`xxx[xxxPos++]\`, which return the current value and increment the pointer, thus mimicking a queue's "pop".
+        bytes32[] memory hashes = new bytes32[](proofFlagsLen);
+        uint256 leafPos = 0;
+        uint256 hashPos = 0;
+        uint256 proofPos = 0;
+        // At each step, we compute the next hash using two values:
+        // - a value from the "main queue". If not all leaves have been consumed, we get the next leaf, otherwise we
+        //   get the next hash.
+        // - depending on the flag, either another value from the "main queue" (merging branches) or an element from the
+        //   \`proof\` array.
+        for (uint256 i = 0; i < proofFlagsLen; i++) {
+            bytes32 a = leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++];
+            bytes32 b = proofFlags[i]
+                ? (leafPos < leavesLen ? leaves[leafPos++] : hashes[hashPos++])
+                : proof[proofPos++];
+            hashes[i] = ${hash ?? DEFAULT_HASH}(a, b);
+        }
         if (proofPos != proof.length) {
             revert MerkleProofInvalidMultiproof();
         }