llvm-project/flang/docs/BijectiveInternalNameUniquing.md

## Bijective Internal Name Uniquing

FIR has a flat namespace.  No two objects may have the same name at
the module level.  (These would be functions, globals, etc.)
This necessitates some sort of encoding scheme to unique
symbols from the front-end into FIR.

Another requirement is
to be able to reverse these unique names and recover the associated
symbol in the symbol table.

Fortran is case insensitive, which allows the compiler to convert the
user's identifiers to all lower case.  Such a universal conversion implies
that all upper case letters are available for use in uniquing.

### Prefix `_Q`

All uniqued names have the prefix sequence `_Q` to indicate the name has
been uniqued.  (Q is chosen because it is a
[low frequency letter](http://pi.math.cornell.edu/~mec/2003-2004/cryptography/subs/frequencies.html)
in English.)

### Scope Building

Symbols can be scoped by the module, submodule, or procedure that contains
that symbol.  After the `_Q` sigil, names are constructed from outermost to
innermost scope as

   * Module name prefixed with `M`
   * Submodule name prefixed with `S`
   * Procedure name prefixed with `F`

Given:
```
    submodule (mod:s1mod) s2mod
      ...
      subroutine sub
        ...
      contains
        function fun
```

The uniqued name of `fun` becomes:
```
    _QMmodSs1modSs2modFsubPfun
```

### Common blocks

   * A common block name will be prefixed with `B`

Given:
```
   common /variables/ i, j
```

The uniqued name of `variables` becomes:
```
    _QBvariables
```

Given:
```
   common i, j
```

The uniqued name in case of `blank common block` becomes:
```
    _QB
```

### Module scope global data

   * A global data entity is prefixed with `E`
   * A global entity that is constant (parameter) will be prefixed with `EC`

Given:
```
    module mod
      integer :: intvar
      real, parameter :: pi = 3.14
    end module
```

The uniqued name of `intvar` becomes:
```
    _QMmodEintvar
```

The uniqued name of `pi` becomes:
```
    _QMmodECpi
```

### Procedures/Subprograms

   * A procedure/subprogram is prefixed with `P`

Given:
```
    subroutine sub
```
The uniqued name of `sub` becomes:
```
    _QPsub
```

### Derived types and related

   * A derived type is prefixed with `T`
   * If a derived type has KIND parameters, they are listed in a consistent
     canonical order where each takes the form `Ki` and where _i_ is the
     compile-time constant value. (All type parameters are integer.)  If _i_
     is a negative value, the prefix `KN` will be used and _i_ will reflect
     the magnitude of the value.

Given:
```
    module mymodule
      type mytype
        integer :: member
      end type
      ...
```
The uniqued name of `mytype` becomes:
```
    _QMmymoduleTmytype
```

Given:
```
    type yourtype(k1,k2)
      integer, kind :: k1, k2
      real :: mem1
      complex :: mem2
    end type
```

The uniqued name of `yourtype` where `k1=4` and `k2=-6` (at compile-time):
```
    _QTyourtypeK4KN6
```

   * A derived type dispatch table is prefixed with `D`.  The dispatch table
     for `type t` would be `_QDTt`
   * A type descriptor instance is prefixed with `C`.  Intrinsic types can
     be encoded with their names and kinds.  The type descriptor for the
     type `yourtype` above would be `_QCTyourtypeK4KN6`.  The type
     descriptor for `REAL(4)` would be `_QCrealK4`.

### Compiler generated names

Compiler generated names do not have to be mapped back to Fortran.  These
names will be prefixed with `_QQ` and followed by a unique compiler
generated identifier. There is, of course, no mapping back to a symbol
derived from the input source in this case as no such symbol exists.