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Merge pull request #6825 from ethereum/style-fixes-assembly

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[DOCS] Bring code examples inline with style guide in assembly doc
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chriseth 2019-05-27 17:28:12 +02:00 committed by GitHub
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docs/assembly.rst
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@ -104,47 +104,48 @@ efficient code, for example:
pragma solidity >=0.4.16 <0.7.0; pragma solidity >=0.4.16 <0.7.0;
library VectorSum { library VectorSum {
// This function is less efficient because the optimizer currently fails to // This function is less efficient because the optimizer currently fails to
// remove the bounds checks in array access. // remove the bounds checks in array access.
function sumSolidity(uint[] memory _data) public pure returns (uint o_sum) { function sumSolidity(uint[] memory _data) public pure returns (uint sum) {
for (uint i = 0; i < _data.length; ++i) for (uint i = 0; i < _data.length; ++i)
o_sum += _data[i]; sum += _data[i];
} }
// We know that we only access the array in bounds, so we can avoid the check. // We know that we only access the array in bounds, so we can avoid the check.
// 0x20 needs to be added to an array because the first slot contains the // 0x20 needs to be added to an array because the first slot contains the
// array length. // array length.
function sumAsm(uint[] memory _data) public pure returns (uint o_sum) { function sumAsm(uint[] memory _data) public pure returns (uint sum) {
for (uint i = 0; i < _data.length; ++i) { for (uint i = 0; i < _data.length; ++i) {
assembly { assembly {
o_sum := add(o_sum, mload(add(add(_data, 0x20), mul(i, 0x20)))) sum := add(sum, mload(add(add(_data, 0x20), mul(i, 0x20))))
} }
} }
} }
// Same as above, but accomplish the entire code within inline assembly. // Same as above, but accomplish the entire code within inline assembly.
function sumPureAsm(uint[] memory _data) public pure returns (uint o_sum) { function sumPureAsm(uint[] memory _data) public pure returns (uint sum) {
assembly { assembly {
// Load the length (first 32 bytes) // Load the length (first 32 bytes)
let len := mload(_data) let len := mload(_data)
// Skip over the length field. // Skip over the length field.
// //
// Keep temporary variable so it can be incremented in place. // Keep temporary variable so it can be incremented in place.
// //
// NOTE: incrementing _data would result in an unusable // NOTE: incrementing _data would result in an unusable
// _data variable after this assembly block // _data variable after this assembly block
let data := add(_data, 0x20) let data := add(_data, 0x20)
// Iterate until the bound is not met. // Iterate until the bound is not met.
for for
{ let end := add(data, mul(len, 0x20)) } { let end := add(data, mul(len, 0x20)) }
lt(data, end) lt(data, end)
{ data := add(data, 0x20) } { data := add(data, 0x20) }
{ {
o_sum := add(o_sum, mload(data)) sum := add(sum, mload(data))
} }
} }
} }
} }
@ -693,12 +694,13 @@ We consider the runtime bytecode of the following Solidity program::
pragma solidity >=0.4.16 <0.7.0; pragma solidity >=0.4.16 <0.7.0;
contract C { contract C {
function f(uint x) public pure returns (uint y) { function f(uint x) public pure returns (uint y) {
y = 1; y = 1;
for (uint i = 0; i < x; i++) for (uint i = 0; i < x; i++)
y = 2 * y; y = 2 * y;
} }
} }
The following assembly will be generated:: The following assembly will be generated::