# Mu Syntax Here are two valid statements in Mu: ``` increment x y <- increment ``` Understanding when to use one vs the other is the critical idea in Mu. In short, the former increments a value in memory, while the latter increments a value in a register. Most languages start from some syntax and do what it takes to implement it. Mu, however, is designed as a safe way to program in [a regular subset of 32-bit x86 machine code](subx.md), _satisficing_ rather than optimizing for a clean syntax. To keep the mapping to machine code lightweight, Mu exclusively uses statements and most statements map to a single instruction of machine code. Since the x86 instruction set restricts how many memory locations an instruction can use, Mu makes registers explicit as well. Variables must be explicitly mapped to registers; otherwise they live in memory. Statements consist of 3 parts: the operation, optional _inouts_ and optional _outputs_. Outputs come before the operation name and `<-`. Outputs are always registers; memory locations that need to be modified are passed in by reference. So Mu programmers need to make two new categories of decisions: whether to define variables in registers or memory, and whether to put variables to the left or right. There's always exactly one way to write any given operation. In return for this overhead you get a lightweight and future-proof stack. And Mu will provide good error message to support you. Further down, this page enumerates all available primitives in Mu, and [a separate page](http://akkartik.github.io/mu/html/mu_instructions.html) describes how each primitive is translated to machine code. ## Functions and calls Zooming out from single statements, here's a complete sample program in Mu: ex2.mu

Mu programs are lists of functions. Each function has the following form: ``` fn _name_ _inout_ ... -> _output_ ... { _statement_ _statement_ ... } ``` Each function has a header line, and some number of statements, each on a separate line. Headers describe inouts and outputs. Inouts can't be registers, and outputs _must_ be registers. The above program also demonstrates a function call (to the function `do-add`). Function calls look the same as primitive statements: they can return (multiple) outputs in registers, and modify inouts passed in by reference. In addition, there's one more constraint: output registers must match the function header. For example: ``` fn f -> x/eax: int { ... } fn g { a/eax <- f # ok a/ebx <- f # wrong } ``` The function `main` is special; it is where the program starts running. It must always return a single int in register `ebx` (as the exit status of the process). It can also optionally accept an array of strings as input (from the shell command-line). To be precise, `main` must have one of the following two signatures: - `fn main -> x/ebx: int` - `fn main args: (addr array (addr array byte)) -> x/ebx: int` (The name of the output is flexible.) ## Blocks Blocks are useful for grouping related statements. They're delimited by `{` and `}`, both each alone on a line. Blocks can nest: ``` { _statements_ { _more statements_ } } ``` Blocks can be named (with the name ending in a `:` on the same line as the `{`): ``` $name: { _statements_ } ``` Further down we'll see primitive statements for skipping or repeating blocks. Besides control flow, the other use for blocks is... ## Local variables Functions can define new variables at any time with the keyword `var`. There are two variants of the `var` statement, for defining variables in registers or memory. ``` var name: type var name/reg: type <- ... ``` Variables on the stack are never initialized. (They're always implicitly zeroed them out.) Variables in registers are always initialized. Register variables can go in 6 integer registers: `eax`, `ebx`, `ecx`, `edx`, `esi` and `edi`. Floating-point values can also go in 8 other registers: `xmm0`, `xmm1`, `xmm2`, `xmm3`, `xmm4`, `xmm5`, `xmm6` and `xmm7`. Defining a variable in a register either clobbers the previous variable (if it was defined in the same block) or shadows it temporarily (if it was defined in an outer block). Variables exist from their definition until the end of their containing block. Register variables may also die earlier if their register is clobbered by a new variable. Variables on the stack can be of many types (but not `byte`). Variables in integer registers can only contain 32-bit values: `int`, `boolean`, `(addr ...)`. Variables in floating-point registers can only contain values of type `float`. ## Integer primitives Here is the list of arithmetic primitive operations supported by Mu. The name `n` indicates a literal integer rather than a variable, and `var/reg` indicates a variable in a register. ``` var/reg <- increment increment var var/reg <- decrement decrement var var1/reg1 <- add var2/reg2 var/reg <- add var2 add-to var1, var2/reg var/reg <- add n add-to var, n var1/reg1 <- sub var2/reg2 var/reg <- sub var2 sub-from var1, var2/reg var/reg <- sub n sub-from var, n var1/reg1 <- and var2/reg2 var/reg <- and var2 and-with var1, var2/reg var/reg <- and n and-with var, n var1/reg1 <- or var2/reg2 var/reg <- or var2 or-with var1, var2/reg var/reg <- or n or-with var, n var1/reg1 <- xor var2/reg2 var/reg <- xor var2 xor-with var1, var2/reg var/reg <- xor n xor-with var, n var/reg <- copy var2/reg2 copy-to var1, var2/reg var/reg <- copy var2 var/reg <- copy n copy-to var, n compare var1, var2/reg compare var1/reg, var2 compare var/eax, n compare var, n var/reg <- shift-left n var/reg <- shift-right n var/reg <- shift-right-signed n shift-left var, n shift-right var, n shift-right-signed var, n var/reg <- multiply var2 ``` Any statement above that takes a variable in memory can be replaced with a dereference (`*`) of an address variable (of type `(addr ...)`) in a register. (Types can have multiple words, and are wrapped in `()` when they do.) But you can't dereference variables in memory. You have to load them into a register first. Excluding dereferences, the above statements must operate on non-address primitive types: `int` or `boolean`. (Booleans are really just `int`s, and Mu assumes any value but `0` is true.) ## Floating-point primitives These instructions may use the floating-point registers `xmm0` ... `xmm7` (denoted by `/xreg2` or `/xrm32`). They also use integer values on occasion (`/rm32` and `/r32`). They can't take literal floating-point values. ``` var/xreg <- add var2/xreg2 var/xreg <- add var2 var/xreg <- add *var2/reg2 var/xreg <- subtract var2/xreg2 var/xreg <- subtract var2 var/xreg <- subtract *var2/reg2 var/xreg <- multiply var2/xreg2 var/xreg <- multiply var2 var/xreg <- multiply *var2/reg2 var/xreg <- divide var2/xreg2 var/xreg <- divide var2 var/xreg <- divide *var2/reg2 var/xreg <- reciprocal var2/xreg2 var/xreg <- reciprocal var2 var/xreg <- reciprocal *var2/reg2 var/xreg <- square-root var2/xreg2 var/xreg <- square-root var2 var/xreg <- square-root *var2/reg2 var/xreg <- inverse-square-root var2/xreg2 var/xreg <- inverse-square-root var2 var/xreg <- inverse-square-root *var2/reg2 var/xreg <- min var2/xreg2 var/xreg <- min var2 var/xreg <- min *var2/reg2 var/xreg <- max var2/xreg2 var/xreg <- max var2 var/xreg <- max *var2/reg2 Remember, when these instructions use indirect mode, they still use an integer register. Floating-point registers can't hold addresses. Most instructions operate exclusively on integer or floating-point operands. The only exceptions are the instructions for converting between integers and floating-point numbers. var/xreg <- convert var2/reg2 var/xreg <- convert var2 var/xreg <- convert *var2/reg2 var/reg <- convert var2/xreg2 var/reg <- convert var2 var/reg <- convert *var2/reg2 There are no instructions accepting floating-point literals. To obtain integer literals in floating-point registers, copy them to general-purpose registers and then convert them to floating-point. One pattern you may have noticed above is that the floating-point instructions above always write to registers. The only exceptions are `copy` instructions, which can write to memory locations. var/xreg <- copy var2/xreg2 copy-to var1, var2/xreg var/xreg <- copy var2 var/xreg <- copy *var2/reg2 Floating-point comparisons always put a register on the left-hand side: compare var1/xreg1, var2/xreg2 compare var1/xreg1, var2 ``` ## Operating on individual bytes A special-case is variables of type 'byte'. Mu is a 32-bit platform so for the most part only supports types that are multiples of 32 bits. However, we do want to support strings in ASCII and UTF-8, which will be arrays of bytes. Since most x86 instructions implicitly load 32 bits at a time from memory, variables of type 'byte' are only allowed in registers, not on the stack. Here are the possible statements for reading bytes to/from memory: ``` var/reg <- copy-byte var2/reg2 # var: byte, var2: byte var/reg <- copy-byte *var2/reg2 # var: byte, var2: (addr byte) copy-byte-to *var1/reg1, var2/reg2 # var1: (addr byte), var2: byte ``` In addition, variables of type 'byte' are restricted to (the lowest bytes of) just 4 registers: eax, ecx, edx and ebx. ## Primitive jumps There are two kinds of jumps, both with many variations: `break` and `loop`. `break` instructions jump to the end of the containing block. `loop` instructions jump to the beginning of the containing block. All jumps can take an optional label starting with '$': ``` loop $foo ``` This instruction jumps to the beginning of the block called $foo. The corresponding `break` jumps to the end of the block. Either jump statement must lie somewhere inside such a block. Jumps are only legal to containing blocks. (Use named blocks with restraint; jumps to places far away can get confusing.) There are two unconditional jumps: ``` loop loop label break break label ``` The remaining jump instructions are all conditional. Conditional jumps rely on the result of the most recently executed `compare` instruction. (To keep programs easy to read, keep compare instructions close to the jump that uses them.) ``` break-if-= break-if-= label break-if-!= break-if-!= label ``` Inequalities are similar, but have unsigned and signed variants. For simplicity, always use signed integers; use the unsigned variants only to compare addresses. ``` break-if-< break-if-< label break-if-> break-if-> label break-if-<= break-if-<= label break-if->= break-if->= label break-if-addr< break-if-addr< label break-if-addr> break-if-addr> label break-if-addr<= break-if-addr<= label break-if-addr>= break-if-addr>= label ``` Similarly, conditional loops: ``` loop-if-= loop-if-= label loop-if-!= loop-if-!= label loop-if-< loop-if-< label loop-if-> loop-if-> label loop-if-<= loop-if-<= label loop-if->= loop-if->= label loop-if-addr< loop-if-addr< label loop-if-addr> loop-if-addr> label loop-if-addr<= loop-if-addr<= label loop-if-addr>= loop-if-addr>= label ``` ## Addresses Passing objects by reference requires the `address` operation, which returns an object of type `addr`. ``` var/reg: (addr T) <- address var2: T ``` Here `var2` can't live in a register. ## Array operations Mu arrays are size-prefixed so that operations on them can check bounds as necessary at run-time. The `length` statement returns the number of elements in an array. ``` var/reg: int <- length arr/reg: (addr array T) ``` The `index` statement takes an `addr` to an `array` and returns an `addr` to one of its elements, that can be read from or written to. ``` var/reg: (addr T) <- index arr/reg: (addr array T), n var/reg: (addr T) <- index arr: (array T sz), n ``` The index can also be a variable in a register, with a caveat: ``` var/reg: (addr T) <- index arr/reg: (addr array T), idx/reg: int var/reg: (addr T) <- index arr: (array T sz), idx/reg: int ``` The caveat: the size of T must be 1, 2, 4 or 8 bytes. The x86 instruction set has complex addressing modes that can index into an array in a single instruction in these situations. For types in general you'll need to split up the work, performing a `compute-offset` before the `index`. ``` var/reg: (offset T) <- compute-offset arr: (addr array T), idx/reg: int # arr can be in reg or mem var/reg: (offset T) <- compute-offset arr: (addr array T), idx: int # arr can be in reg or mem ``` The `compute-offset` statement returns a value of type `(offset T)` after performing any necessary bounds checking. Now the offset can be passed to `index` as usual: ``` var/reg: (addr T) <- index arr/reg: (addr array T), idx/reg: (offset T) ``` ## Compound types Primitive types can be combined together using the `type` keyword. For example: ``` type point { x: int y: int } ``` Mu programs are currently sequences of `fn` and `type` definitions. To access within a compound type, use the `get` instruction. There are two forms. You need either a variable of the type itself (say `T`) in memory, or a variable of type `(addr T)` in a register. ``` var/reg: (addr T_f) <- get var/reg: (addr T), f var/reg: (addr T_f) <- get var: T, f ``` The `f` here is the field name from the `type` definition, and `T_f` must match the type of `f` in the `type` definition. For example, some legal instructions for the definition of `point` above: ``` var a/eax: (addr int) <- get p, x var a/eax: (addr int) <- get p, y ``` ## Handles for safe access to the heap We've seen the `addr` type, but it's intended to be short-lived. In particular, you can't save `addr` values inside compound `type`s. To do that you need a "fat pointer" called a `handle` that is safe to keep around for extended periods and ensures it's used safely without corrupting the heap and causing security issues or hard-to-debug misbehavior. To actually _use_ a `handle`, we have to turn it into an `addr` first using the `lookup` statement. ``` var y/reg: (addr T) <- lookup x ``` Now operate on the `addr` as usual, safe in the knowledge that you can later recover any writes to its payload from `x`. It's illegal to continue to use this `addr` after a function that reclaims heap memory. You have to repeat the lookup from the `handle`. (Luckily Mu doesn't implement reclamation yet.) Having two kinds of addresses takes some getting used to. Do we pass in variables by value, by `addr` or by `handle`? In inputs or outputs? Here are 3 rules: * Functions that need to look at the payload should accept an `(addr ...)`. * Functions that need to treat a handle as a value, without looking at its payload, should accept a `(handle ...)`. Helpers that save handles into data structures are a common example. * Functions that need to allocate memory should accept an `(addr handle ...)`. Try to avoid mixing these use cases. If you have a variable `src` of type `(handle ...)`, you can save it inside a compound type like this (provided the types match): ``` var dest/reg: (addr handle T_f) <- get var: (addr T), f copy-handle src, dest ``` Or this: ``` var dest/reg: (addr handle T) <- index arr: (addr array handle T), n copy-handle src, dest ``` To create handles to non-array types, use `allocate`: ``` var x: (addr handle T) ... initialize x ... allocate x ``` To create handles to array types, use `populate`: ``` var x: (addr handle array T) ... initialize x ... populate x, 3 # array of 3 T's ``` You can copy handles to another variable on the stack like this: ``` var x: (handle T) # ..some code initializing x.. var y/eax: (addr handle T) <- address ... copy-handle x, y ``` ## Conclusion Anything not allowed here is forbidden. At least until you modify mu.subx. Please [contact me](mailto:ak@akkartik.com) or [report issues](https://github.com/akkartik/mu/issues) when you encounter a missing or misleading error message.