Importing a bunch of pages from old websites.

[website_subgeniuskitty.com] / data / development / pdp-11 / software_development / modern_unix / gcc_bugs.md

# Overview #

Although GCC claims to support PDP-11 targets there are some bugs that must be
worked around both in GCC and the GNU assembler.

# GNU binutils Bugs #

## Problem Description ##

One of the addressing modes supported by the PDP-11 is 'index deferred',
represented by `@X(Rn)`. This operand indicates that `Rn` contains a pointer
which should be dereferenced and the result added to `X` to generate a new
pointer to the final location. For example, consider the following four values,
one stored in a register and the other three in memory. Then `@2(R1)` is the
value `222`.

    R1: 1000
    1000: 2000
    2000: 111
    2002: 222

Similarly, `@0(R1)` is the value `111`. In most PDP-11 assemblers, including
DEC's MACRO-11 assembler, the string `@(Rn)` is an alias to `@0(Rn)`. But when
the GNU assembler encounters `@(Rn)` it assembles it as though it were `(Rn)`,
a single level of indirection instead of two levels!

If we're only writing assembly then we can work around this bug by always using
the form `@0(Rn)`. But what if we're writing C and using GCC to compile it?
Consider the following C code example, taken directly from some stack-based
debugger code written for the PDP-11.

    uint16_t ** csp = (uint16_t **) 070000;
    *csp = (uint16_t *) 060000;
    **csp = 0;

When GCC transpiles this to assembly it generates code of the form `@(Rn)` when
assigning a value to `**csp` thus causing the value `0` to overwrite the value
`060000` at `*csp` if GNU `as` is used to assemble the code.

## Solution Description ##

The following patch, tested on GNU binutils 2.28, fixes the bug. Since it
overloads the `operand->code` variable to pass unrelated state information to
`parse_reg()` I haven't submitted it for inclusion in GNU binutils. Once I'm
done bug-hunting in my toolchain I will clean up all the fixes and submit them
in one bundle.

    --- tc-pdp11.c	2017-06-24 22:33:00.260210000 -0700
    +++ tc-pdp11.c.fixed	2017-06-24 22:32:12.455205000 -0700
    @@ -431,6 +431,9 @@
     {
       LITTLENUM_TYPE literal_float[2];
     
    +  /* Store the value (if any) passed by parse_op_noreg() before parse_reg() overwrites it. */
    +  int deferred = operand->code;
    +
       str = skip_whitespace (str);
     
       switch (*str)
    @@ -451,6 +454,15 @@
     	  operand->code |= 020;
     	  str++;
     	}
    +      /*
    +       * This catches the case where @(Rn) is interpreted as (Rn) rather than @0(Rn)
    +       */
    +      else if (deferred)
    +        {
    +          operand->additional = 1;
    +          operand->word = 0;
    +          operand->code |= 060;
    +        }
           else
     	{
     	  operand->code |= 010;
    @@ -581,6 +593,12 @@
     
       if (*str == '@' || *str == '*')
         {
    +      /*
    +       * operand->code is overwritten by parse_reg() inside parse_op_no_deferred()
    +       * We use it to temporarily catch the alias @(Rn) -> @0(Rn) since
    +       *   parse_op_no_deferred() starts at str+1 and thus misses the '@'.
    +       */
    +      operand->code |= 010;
           str = parse_op_no_deferred (str + 1, operand);
           if (operand->error)
    return str;