Merge pull request #3220 from ethereum/IuliaIf

If statement for Iulia / Inline Assembly

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

docs/julia.rst

@@ -15,7 +15,7 @@ future versions of the Solidity compiler will even use JULIA as intermediate
 language. It should also be easy to build high-level optimizer stages for JULIA.
 
 The core components of JULIA are functions, blocks, variables, literals,
-for-loops, switch-statements, expressions and assignments to variables.
+for-loops, if-statements, switch-statements, expressions and assignments to variables.
 
 JULIA is typed, both variables and literals must specify the type with postfix
 notation. The supported types are ``bool``, ``u8``, ``s8``, ``u32``, ``s32``,
@@ -88,6 +88,8 @@ Grammar::
         IdentifierList ':=' Expression
     Expression =
         FunctionCall | Identifier | Literal
+    If =
+        'if' Expression Block
     Switch =
         'switch' Expression Case* ( 'default' Block )?
     Case =
@@ -248,8 +250,14 @@ We will use a destructuring notation for the AST nodes.
         G, L, break
     E(G, L, continue: BreakContinue) =
         G, L, continue
+    E(G, L, <if condition body>: If) =
+        let G0, L0, v = E(G, L, condition)
+        if v is true:
+            E(G0, L0, body)
+        else:
+            G0, L0, regular
     E(G, L, <switch condition case l1:t1 st1 ... case ln:tn stn>: Switch) =
-        E(G, L, switch condition case l1:t1 st1 ... case ln:tn stn default {}) =
+        E(G, L, switch condition case l1:t1 st1 ... case ln:tn stn default {})
     E(G, L, <switch condition case l1:t1 st1 ... case ln:tn stn default st'>: Switch) =
         let G0, L0, v = E(G, L, condition)
         // i = 1 .. n