5485 - promote SubX to top-level

1 files changed, 0 insertions, 84 deletions

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-== Goal
-
-A memory-safe language with a simple translator to x86 that can be feasibly written in x86.
-
-== Definitions of terms
-
-Memory-safe: it should be impossible to:
-  a) create a pointer out of arbitrary data, or
-  b) to access heap memory after it's been freed.
-
-Simple: do all the work in a 2-pass translator:
-  Pass 1: check each instruction's types in isolation.
-  Pass 2: emit code for each instruction in isolation.
-
-== Implications
-
-=> Each instruction matches a pattern and yields a template to emit.
-=> There's a 1-to-1 mapping between instructions in the source language and x86 machine code.
-  Zero runtime.
-=> Programmers have to decide how to use registers.
-=> Translator can't insert any instructions that write to registers. (We don't know if a register is in use.)
-
-== Lessons from Mu
-
-1. For easy bounds checking, never advance pointers to arrays or heap allocations. No pointer arithmetic.
-2. Store the array length with the array.
-3. Store an allocation id with heap allocations. Allocation id goes monotonically up, never gets reused. When it wraps around to zero the program panics.
-4. Heap pointers also carry around allocation id.
-5. When dereferencing a heap pointer, first ensure its alloc id matches the alloc id of the payload. This ensures some other copy of the pointer didn't get freed (and potentially reused)
-
-== Problem 1
-
-How to index into an array?
-
-  The array has a length that needs to be checked.
-  Its elements have a type T.
-  The base will be in memory, either on the stack or the heap.
-  The index may be in the register, stack or heap.
-
-That's too much work to do in a single instruction.
-
-So arrays have to take multiple steps. And we have to guard against the steps
-being misused in unsafe ways.
-
-To index into an array with elements of type T, starting with the size of the
-array in bytes:
-
-  step 1: get the offset the index is at
-    <reg offset> : (index T) <- index <reg/mem idx> : int, <literal> : (size T)
-  step 2: convert the array to address-of-element
-    <reg x> : (address T) <- advance <reg/mem A> : (array T), <reg offset> : (index T)
-    implicitly compares the offset with the size, panic if greater
-    =>
-      compare <reg offset> : (index T), <reg/mem> : (array T)
-      jge panic
-  step 3: use the address to the element
-    ...
-
-(index T) is a special type. You can do only two things with it:
-  - pass it to the advance instruction
-  - convert it to a number (but no converting back)
-
-(address T) is a short-term pointer. You can't store addresses in structs, you
-can't define global variables of that type, and you can't pass the type to the
-memory allocator to save to the heap. You also can't store addresses in the
-stack, because you may encounter a free before you end the function.
-
-Maybe we'll also forbid any sort of copy of address types. Only place you can
-store an (address T) is the register you saved to. To copy you need a handle
-to a heap allocation.
-
-Still not entirely protected against temporal issues. But pretty close.
-
-== Problem 2
-
-How to dereference a heap allocation?
-
-== List of types
-
-int 
-char
-(address _)   X  
-(array _)
-(handle _)