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Denton Liu 2016-05-11 15:55:07 -04:00
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commit 51fe1bd9c2
1 changed files with 97 additions and 97 deletions
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194
docs/control-structures.rst
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@ -246,8 +246,8 @@ arising when writing manual assembly by the following features:
We now want to describe the inline assembly language in detail.
.. warning::
Inline assembly is still a relatively new feature and might change if it does not prove useful,
so please try to keep up to date.
Inline assembly is still a relatively new feature and might change if it does not prove useful,
so please try to keep up to date.
Example
-------
@ -258,23 +258,23 @@ idea is that assembly libraries will be used to enhance the language in such way
.. code::
library GetCode {
function at(address _addr) returns (bytes o_code) {
assembly {
// retrieve the size of the code, this needs assembly
let size := extcodesize(_addr)
// allocate output byte array - this could also be done without assembly
// by using o_code = new bytes(size)
o_code := mload(0x40)
// new "memory end" including padding
mstore(0x40, add(o_code, and(add(add(size, 0x20), 0x1f), not(0x1f))))
// store length in memory
mstore(o_code, size)
// actually retrieve the code, this needs assembly
extcodecopy(_addr, add(o_code, 0x20), 0, size)
}
}
}
library GetCode {
function at(address _addr) returns (bytes o_code) {
assembly {
// retrieve the size of the code, this needs assembly
let size := extcodesize(_addr)
// allocate output byte array - this could also be done without assembly
// by using o_code = new bytes(size)
o_code := mload(0x40)
// new "memory end" including padding
mstore(0x40, add(o_code, and(add(add(size, 0x20), 0x1f), bnot(0x1f))))
// store length in memory
mstore(o_code, size)
// actually retrieve the code, this needs assembly
extcodecopy(_addr, add(o_code, 0x20), 0, size)
}
}
}
Inline assemmbly could also be beneficial in cases where the optimizer fails to produce
efficient code. Please be aware that assembly is much more difficult to write because
@ -283,24 +283,24 @@ you really know what you are doing.
.. code::
library VectorSum {
// This function is less efficient because the optimizer currently fails to
// remove the bounds checks in array access.
function sumSolidity(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i)
o_sum += _data[i];
}
// 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
// array length.
function sumAsm(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i) {
assembly {
library VectorSum {
// This function is less efficient because the optimizer currently fails to
// remove the bounds checks in array access.
function sumSolidity(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i)
o_sum += _data[i];
}
// 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
// array length.
function sumAsm(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i) {
assembly {
o_sum := mload(add(add(_data, 0x20), i))
}
}
}
}
}
}
Syntax
------
@ -487,7 +487,7 @@ Strings are stored left-aligned and cannot be longer than 32 bytes.
.. code::
assembly { 2 3 add "abc" and }
assembly { 2 3 add "abc" and }
Functional Style
-----------------
@ -497,7 +497,7 @@ adding `3` to the contents in memory at position `0x80` would be
.. code::
3 0x80 mload add 0x80 mstore
3 0x80 mload add 0x80 mstore
As it is often hard to see what the actual arguments for certain opcodes are,
Solidity inline assembly also provides a "functional style" notation where the same code
@ -505,7 +505,7 @@ would be written as follows
.. code::
mstore(0x80, add(mload(0x80), 3))
mstore(0x80, add(mload(0x80), 3))
Functional style and instructional style can be mixed, but any opcode inside a
functional style expression has to return exactly one stack slot (most of the opcodes do).
@ -536,18 +536,18 @@ It is planned that the stack height changes can be specified in inline assembly.
.. code::
contract c {
uint b;
function f(uint x) returns (uint r) {
assembly {
b pop // remove the offset, we know it is zero
sload
x
mul
=: r // assign to return variable r
}
}
}
contract c {
uint b;
function f(uint x) returns (uint r) {
assembly {
b pop // remove the offset, we know it is zero
sload
x
mul
=: r // assign to return variable r
}
}
}
Labels
------
@ -558,19 +558,19 @@ jumps easier. The following code computes an element in the Fibonacci series.
.. code::
{
let n := calldataload(4)
let a := 1
let b := a
loop:
jumpi(loopend, eq(n, 0))
a add swap1
n := sub(n, 1)
jump(loop)
loopend:
mstore(0, a)
return(0, 0x20)
}
{
let n := calldataload(4)
let a := 1
let b := a
loop:
jumpi(loopend, eq(n, 0))
a add swap1
n := sub(n, 1)
jump(loop)
loopend:
mstore(0, a)
return(0, 0x20)
}
Please note that automatically accessing stack variables can only work if the
assembler knows the current stack height. This fails to work if the jump source
@ -583,19 +583,19 @@ will have a wrong impression about the stack height at label `two`:
.. code::
{
jump(two)
one:
// Here the stack height is 1 (because we pushed 7),
// but the assembler thinks it is 0 because it reads
// from top to bottom.
// Accessing stack variables here will lead to errors.
jump(three)
two:
7 // push something onto the stack
jump(one)
three:
}
{
jump(two)
one:
// Here the stack height is 1 (because we pushed 7),
// but the assembler thinks it is 0 because it reads
// from top to bottom.
// Accessing stack variables here will lead to errors.
jump(three)
two:
7 // push something onto the stack
jump(one)
three:
}
Declaring Assembly-Local Variables
@ -610,19 +610,19 @@ be just `0`, but it can also be a complex functional-style expression.
.. code::
contract c {
function f(uint x) returns (uint b) {
assembly {
let v := add(x, 1)
mstore(0x80, v)
{
let y := add(sload(v), 1)
b := y
} // y is "deallocated" here
b := add(b, v)
} // v is "deallocated" here
}
}
contract c {
function f(uint x) returns (uint b) {
assembly {
let v := add(x, 1)
mstore(0x80, v)
{
let y := add(sload(v), 1)
b := y
} // y is "deallocated" here
b := add(b, v)
} // v is "deallocated" here
}
}
Assignments
@ -640,12 +640,12 @@ For both ways, the colon points to the name of the variable.
.. code::
assembly {
let v := 0 // functional-style assignment as part of variable declaration
let g := add(v, 2)
sload(10)
=: v // instruction style assignment, puts the result of sload(10) into v
}
assembly {
let v := 0 // functional-style assignment as part of variable declaration
let g := add(v, 2)
sload(10)
=: v // instruction style assignment, puts the result of sload(10) into v
}
Things to Avoid
@ -681,6 +681,6 @@ arrays are pointers to memory arrays. The length of a dynamic array is stored at
first slot of the array and then only the array elements follow.
.. warning::
Statically-sized memory arrays do not have a length field, but it will be added soon
to allow better convertibility between statically- and dynamically-sized arrays, so
please do not rely on that.
Statically-sized memory arrays do not have a length field, but it will be added soon
to allow better convertibility between statically- and dynamically-sized arrays, so
please do not rely on that.