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authorchriseth <chris@ethereum.org>2017-11-21 20:36:41 +0800
committerchriseth <chris@ethereum.org>2017-11-22 23:25:24 +0800
commit6dbc34e16ee8bda0e156ccb20a3fb8cb6ff52c92 (patch)
treec7e5c438ef7517293c08f4e0516f8c38c9a7c5c6 /docs
parentff229ab05a43d113bcd2de79f25caf52017c3486 (diff)
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If statement for Iulia / inline assembly.
Diffstat (limited to 'docs')
-rw-r--r--docs/assembly.rst24
1 files changed, 21 insertions, 3 deletions
diff --git a/docs/assembly.rst b/docs/assembly.rst
index 00bfb388..c233985b 100644
--- a/docs/assembly.rst
+++ b/docs/assembly.rst
@@ -26,6 +26,7 @@ arising when writing manual assembly by the following features:
* access to external variables: ``function f(uint x) { assembly { x := sub(x, 1) } }``
* labels: ``let x := 10 repeat: x := sub(x, 1) jumpi(repeat, eq(x, 0))``
* loops: ``for { let i := 0 } lt(i, x) { i := add(i, 1) } { y := mul(2, y) }``
+* if statements: ``if slt(x, 0) { x := sub(0, x) }``
* switch statements: ``switch x case 0 { y := mul(x, 2) } default { y := 0 }``
* function calls: ``function f(x) -> y { switch x case 0 { y := 1 } default { y := mul(x, f(sub(x, 1))) } }``
@@ -400,7 +401,7 @@ Labels
Another problem in EVM assembly is that ``jump`` and ``jumpi`` use absolute addresses
which can change easily. Solidity inline assembly provides labels to make the use of
jumps easier. Note that labels are a low-level feature and it is possible to write
-efficient assembly without labels, just using assembly functions, loops and switch instructions
+efficient assembly without labels, just using assembly functions, loops, if and switch instructions
(see below). The following code computes an element in the Fibonacci series.
.. code::
@@ -523,6 +524,21 @@ is performed by replacing the variable's value on the stack by the new value.
=: v // instruction style assignment, puts the result of sload(10) into v
}
+If
+--
+
+The if statement can be used for conditionally executing code.
+There is no "else" part, consider using "switch" (see below) if
+you need multiple alternatives.
+
+.. code::
+
+ {
+ if eq(value, 0) { revert(0, 0) }
+ }
+
+The curly braces for the body are required.
+
Switch
------
@@ -622,7 +638,7 @@ Things to Avoid
---------------
Inline assembly might have a quite high-level look, but it actually is extremely
-low-level. Function calls, loops and switches are converted by simple
+low-level. Function calls, loops, ifs and switches are converted by simple
rewriting rules and after that, the only thing the assembler does for you is re-arranging
functional-style opcodes, managing jump labels, counting stack height for
variable access and removing stack slots for assembly-local variables when the end
@@ -669,7 +685,7 @@ for the Solidity compiler. In this form, it tries to achieve several goals:
3. Control flow should be easy to detect to help in formal verification and optimization.
In order to achieve the first and last goal, assembly provides high-level constructs
-like ``for`` loops, ``switch`` statements and function calls. It should be possible
+like ``for`` loops, ``if`` and ``switch`` statements and function calls. It should be possible
to write assembly programs that do not make use of explicit ``SWAP``, ``DUP``,
``JUMP`` and ``JUMPI`` statements, because the first two obfuscate the data flow
and the last two obfuscate control flow. Furthermore, functional statements of
@@ -875,6 +891,7 @@ Grammar::
FunctionalAssemblyAssignment |
AssemblyAssignment |
LabelDefinition |
+ AssemblyIf |
AssemblySwitch |
AssemblyFunctionDefinition |
AssemblyFor |
@@ -891,6 +908,7 @@ Grammar::
IdentifierList = Identifier ( ',' Identifier)*
AssemblyAssignment = '=:' Identifier
LabelDefinition = Identifier ':'
+ AssemblyIf = 'if' FunctionalAssemblyExpression AssemblyBlock
AssemblySwitch = 'switch' FunctionalAssemblyExpression AssemblyCase*
( 'default' AssemblyBlock )?
AssemblyCase = 'case' FunctionalAssemblyExpression AssemblyBlock