⚡️ Optimize toString and add comment (ethereum#256)

Author: Vectorized

.gas-snapshot

@@ -1,9 +1,9 @@
 Base64Test:testBase64DecodeSentenceGas() (gas: 3673)
 Base64Test:testBase64DecodeShortStringGas() (gas: 906)
 Base64Test:testBase64EncodeDecode(bytes) (runs: 256, μ: 16948, ~: 12932)
-Base64Test:testBase64EncodeDecodeAltModes(bytes) (runs: 256, μ: 520084, ~: 390549)
+Base64Test:testBase64EncodeDecodeAltModes(bytes) (runs: 256, μ: 515224, ~: 371734)
 Base64Test:testBase64EncodeEmptyString() (gas: 953)
-Base64Test:testBase64EncodeFileSafeAndNoPadding(bytes,bool,bool) (runs: 256, μ: 12275, ~: 6360)
+Base64Test:testBase64EncodeFileSafeAndNoPadding(bytes,bool,bool) (runs: 256, μ: 12293, ~: 6360)
 Base64Test:testBase64EncodeSentence() (gas: 4877)
 Base64Test:testBase64EncodeShortStrings() (gas: 8594)
 Base64Test:testBase64EncodeToStringWithDoublePadding() (gas: 1723)
@@ -284,72 +284,72 @@ LibStringTest:testFromAddressToHexString() (gas: 3098)
 LibStringTest:testFromAddressToHexStringChecksumed() (gas: 35075)
 LibStringTest:testFromAddressToHexStringChecksumedDifferential(uint256) (runs: 256, μ: 422930, ~: 637248)
 LibStringTest:testFromAddressToHexStringWithLeadingZeros() (gas: 3099)
-LibStringTest:testHexStringNoPrefixVariants(uint256,uint256) (runs: 256, μ: 412476, ~: 219066)
+LibStringTest:testHexStringNoPrefixVariants(uint256,uint256) (runs: 256, μ: 412369, ~: 219152)
 LibStringTest:testStringConcat() (gas: 7381)
-LibStringTest:testStringConcat(string,string) (runs: 256, μ: 404934, ~: 187709)
+LibStringTest:testStringConcat(string,string) (runs: 256, μ: 410123, ~: 629052)
 LibStringTest:testStringConcatOriginal() (gas: 7981)
 LibStringTest:testStringDirectReturn() (gas: 8105)
 LibStringTest:testStringDirectReturn(string) (runs: 256, μ: 3647, ~: 3687)
 LibStringTest:testStringEndsWith() (gas: 2807)
-LibStringTest:testStringEndsWith(uint256) (runs: 256, μ: 397134, ~: 205901)
+LibStringTest:testStringEndsWith(uint256) (runs: 256, μ: 395390, ~: 205901)
 LibStringTest:testStringEq(string,string) (runs: 256, μ: 1552, ~: 1557)
 LibStringTest:testStringEscapeHTML() (gas: 11935)
-LibStringTest:testStringEscapeHTML(uint256) (runs: 256, μ: 448693, ~: 635936)
+LibStringTest:testStringEscapeHTML(uint256) (runs: 256, μ: 448423, ~: 635936)
 LibStringTest:testStringEscapeJSON() (gas: 16943)
-LibStringTest:testStringEscapeJSONHexEncode() (gas: 323619)
+LibStringTest:testStringEscapeJSONHexEncode() (gas: 323592)
 LibStringTest:testStringIndexOf() (gas: 17230)
-LibStringTest:testStringIndexOf(uint256) (runs: 256, μ: 393636, ~: 212810)
+LibStringTest:testStringIndexOf(uint256) (runs: 256, μ: 393539, ~: 212810)
 LibStringTest:testStringIndicesOf() (gas: 19437)
-LibStringTest:testStringIndicesOf(uint256) (runs: 256, μ: 444346, ~: 635652)
+LibStringTest:testStringIndicesOf(uint256) (runs: 256, μ: 444444, ~: 635652)
 LibStringTest:testStringLastIndexOf() (gas: 23210)
-LibStringTest:testStringLastIndexOf(uint256) (runs: 256, μ: 427117, ~: 632447)
+LibStringTest:testStringLastIndexOf(uint256) (runs: 256, μ: 428735, ~: 632447)
 LibStringTest:testStringLowerDifferential() (gas: 3332571)
 LibStringTest:testStringLowerDifferential(string) (runs: 256, μ: 11106, ~: 9271)
 LibStringTest:testStringLowerOriginal() (gas: 1739)
 LibStringTest:testStringPackAndUnpackOne() (gas: 740053)
 LibStringTest:testStringPackAndUnpackOne(string) (runs: 256, μ: 387595, ~: 185319)
-LibStringTest:testStringPackAndUnpackOneDifferential(string) (runs: 256, μ: 360993, ~: 184794)
+LibStringTest:testStringPackAndUnpackOneDifferential(string) (runs: 256, μ: 362722, ~: 184794)
 LibStringTest:testStringPackAndUnpackTwo() (gas: 809512)
 LibStringTest:testStringPackAndUnpackTwo(string,string) (runs: 256, μ: 411817, ~: 629628)
-LibStringTest:testStringPackAndUnpackTwoDifferential(string,string) (runs: 256, μ: 405235, ~: 185956)
+LibStringTest:testStringPackAndUnpackTwoDifferential(string,string) (runs: 256, μ: 408694, ~: 628047)
 LibStringTest:testStringRepeat() (gas: 9107)
-LibStringTest:testStringRepeat(string,uint256) (runs: 256, μ: 413645, ~: 627968)
+LibStringTest:testStringRepeat(string,uint256) (runs: 256, μ: 410257, ~: 413035)
 LibStringTest:testStringRepeatOriginal() (gas: 13235)
-LibStringTest:testStringReplace(uint256) (runs: 256, μ: 456871, ~: 637465)
+LibStringTest:testStringReplace(uint256) (runs: 256, μ: 453106, ~: 637465)
 LibStringTest:testStringReplaceLong() (gas: 9803)
 LibStringTest:testStringReplaceMedium() (gas: 8505)
 LibStringTest:testStringReplaceShort() (gas: 17530)
 LibStringTest:testStringRuneCount() (gas: 2501341)
 LibStringTest:testStringRuneCountDifferential(string) (runs: 256, μ: 9426, ~: 9432)
 LibStringTest:testStringSlice() (gas: 17767)
-LibStringTest:testStringSlice(uint256) (runs: 256, μ: 425536, ~: 633417)
+LibStringTest:testStringSlice(uint256) (runs: 256, μ: 427119, ~: 634413)
 LibStringTest:testStringSplit() (gas: 20287)
-LibStringTest:testStringSplit(uint256) (runs: 256, μ: 424564, ~: 260249)
+LibStringTest:testStringSplit(uint256) (runs: 256, μ: 425199, ~: 260249)
 LibStringTest:testStringStartsWith() (gas: 2566)
-LibStringTest:testStringStartsWith(uint256) (runs: 256, μ: 417689, ~: 629966)
+LibStringTest:testStringStartsWith(uint256) (runs: 256, μ: 419440, ~: 630464)
 LibStringTest:testStringUpperDifferential() (gas: 3308750)
 LibStringTest:testStringUpperDifferential(string) (runs: 256, μ: 11127, ~: 9292)
 LibStringTest:testStringUpperOriginal() (gas: 1759)
-LibStringTest:testToHexStringFixedLengthInsufficientLength() (gas: 3351)
-LibStringTest:testToHexStringFixedLengthPositiveNumberLong() (gas: 4152)
-LibStringTest:testToHexStringFixedLengthPositiveNumberShort() (gas: 926)
-LibStringTest:testToHexStringFixedLengthUint256Max() (gas: 4132)
-LibStringTest:testToHexStringFixedLengthZeroRightPadded(uint256,uint256) (runs: 256, μ: 9483, ~: 5412)
-LibStringTest:testToHexStringPositiveNumber() (gas: 818)
-LibStringTest:testToHexStringUint256Max() (gas: 3522)
-LibStringTest:testToHexStringZero() (gas: 753)
-LibStringTest:testToHexStringZeroRightPadded(uint256) (runs: 256, μ: 2063, ~: 1211)
-LibStringTest:testToStringPositiveNumber() (gas: 903)
-LibStringTest:testToStringPositiveNumberBrutalized() (gas: 627388)
-LibStringTest:testToStringSignedDifferential(int256) (runs: 256, μ: 429287, ~: 627373)
-LibStringTest:testToStringSignedGas() (gas: 8473)
-LibStringTest:testToStringSignedMemory(int256) (runs: 256, μ: 404183, ~: 190308)
-LibStringTest:testToStringSignedOriginalGas() (gas: 11059)
-LibStringTest:testToStringUint256Max() (gas: 6804)
-LibStringTest:testToStringUint256MaxBrutalized() (gas: 639165)
-LibStringTest:testToStringZero() (gas: 646)
-LibStringTest:testToStringZeroBrutalized() (gas: 626892)
-LibStringTest:testToStringZeroRightPadded(uint256) (runs: 256, μ: 411089, ~: 626802)
+LibStringTest:testToHexStringFixedLengthInsufficientLength() (gas: 3343)
+LibStringTest:testToHexStringFixedLengthPositiveNumberLong() (gas: 4144)
+LibStringTest:testToHexStringFixedLengthPositiveNumberShort() (gas: 918)
+LibStringTest:testToHexStringFixedLengthUint256Max() (gas: 4124)
+LibStringTest:testToHexStringFixedLengthZeroRightPadded(uint256,uint256) (runs: 256, μ: 9443, ~: 5385)
+LibStringTest:testToHexStringPositiveNumber() (gas: 817)
+LibStringTest:testToHexStringUint256Max() (gas: 3521)
+LibStringTest:testToHexStringZero() (gas: 752)
+LibStringTest:testToHexStringZeroRightPadded(uint256) (runs: 256, μ: 2062, ~: 1210)
+LibStringTest:testToStringPositiveNumber() (gas: 852)
+LibStringTest:testToStringPositiveNumberBrutalized() (gas: 627286)
+LibStringTest:testToStringSignedDifferential(int256) (runs: 256, μ: 428451, ~: 627351)
+LibStringTest:testToStringSignedGas() (gas: 8397)
+LibStringTest:testToStringSignedMemory(int256) (runs: 256, μ: 402037, ~: 189063)
+LibStringTest:testToStringSignedOriginalGas() (gas: 10983)
+LibStringTest:testToStringUint256Max() (gas: 5569)
+LibStringTest:testToStringUint256MaxBrutalized() (gas: 636695)
+LibStringTest:testToStringZero() (gas: 643)
+LibStringTest:testToStringZeroBrutalized() (gas: 626886)
+LibStringTest:testToStringZeroRightPadded(uint256) (runs: 256, μ: 410649, ~: 626799)
 MerkleProofLibTest:testVerifyMultiProof(bool,bool,bool,bool,bytes32) (runs: 256, μ: 413960, ~: 632782)
 MerkleProofLibTest:testVerifyMultiProofForHeightOneTree(bool,bool,bool,bool,bool,bool[]) (runs: 256, μ: 31951, ~: 30979)
 MerkleProofLibTest:testVerifyMultiProofForHeightTwoTree(bool,bool,bool,bool,bool,bytes32) (runs: 256, μ: 5656, ~: 5562)

src/utils/LibString.sol

@@ -30,22 +30,21 @@ library LibString {
             // The maximum value of a uint256 contains 78 digits (1 byte per digit), but
             // we allocate 0xa0 bytes to keep the free memory pointer 32-byte word aligned.
             // We will need 1 word for the trailing zeros padding, 1 word for the length,
-            // and 3 words for a maximum of 78 digits. Total: 5 * 0x20 = 0xa0.
-            let m := add(mload(0x40), 0xa0)
+            // and 3 words for a maximum of 78 digits.
+            str := add(mload(0x40), 0x80)
             // Update the free memory pointer to allocate.
-            mstore(0x40, m)
-            // Assign the `str` to the end.
-            str := sub(m, 0x20)
+            mstore(0x40, add(str, 0x20))
             // Zeroize the slot after the string.
             mstore(str, 0)
 
             // Cache the end of the memory to calculate the length later.
             let end := str
 
+            let w := not(0) // Tsk.
             // We write the string from rightmost digit to leftmost digit.
             // The following is essentially a do-while loop that also handles the zero case.
             for { let temp := value } 1 {} {
-                str := sub(str, 1)
+                str := add(str, w) // `sub(str, 1)`.
                 // Write the character to the pointer.
                 // The ASCII index of the '0' character is 48.
                 mstore8(str, add(48, mod(temp, 10)))
@@ -72,6 +71,8 @@ library LibString {
         }
         /// @solidity memory-safe-assembly
         assembly {
+            // We still have some spare memory space on the left,
+            // as we have allocated 3 words (96 bytes) for up to 78 digits.
             let length := mload(str) // Load the string length.
             mstore(str, 0x2d) // Store the '-' character.
             str := sub(str, 1) // Move back the string pointer by a byte.
@@ -116,11 +117,9 @@ library LibString {
             // for the digits, 0x02 bytes for the prefix, and 0x20 bytes for the length.
             // We add 0x20 to the total and round down to a multiple of 0x20.
             // (0x20 + 0x20 + 0x02 + 0x20) = 0x62.
-            let m := add(start, and(add(shl(1, length), 0x62), not(0x1f)))
+            str := add(start, and(add(shl(1, length), 0x42), not(0x1f)))
             // Allocate the memory.
-            mstore(0x40, m)
-            // Assign the `str` to the end.
-            str := sub(m, 0x20)
+            mstore(0x40, add(str, 0x20))
             // Zeroize the slot after the string.
             mstore(str, 0)