Re: [RFA] mips: Fix "info registers" output

["Eli Zaretskii" <eliz at is dot elta dot co dot il>]

> Date: Thu, 21 Jun 2001 09:44:18 -0700
> From: Daniel Jacobowitz <dmj+@andrew.cmu.edu>
> 
> Aha!  Let me guess - Irix 6.5 is using the FP registers in 8 byte mode,
> right?

Yes.

> There was indeed a bug on that path of the code, but it doesn't
> actually affect the output.  We have this:
> 
>   raw_buffer[0] = (char *) alloca (REGISTER_RAW_SIZE (FP0_REGNUM));
>   raw_buffer[1] = (char *) alloca (REGISTER_RAW_SIZE (FP0_REGNUM));
>   dbl_buffer = (char *) alloca (2 * REGISTER_RAW_SIZE (FP0_REGNUM));
> 
>   /* Get the data in raw format.  */
>   if (read_relative_register_raw_bytes (regnum, raw_buffer[HI]))
>     error ("can't read register %d (%s)", regnum, REGISTER_NAME (regnum));
> 
> [snip]
> 
>       memcpy (dbl_buffer, raw_buffer[HI], 2 * REGISTER_RAW_SIZE (FP0_REGNUM));
>       flt1 = unpack_double (builtin_type_float,
>                             &raw_buffer[HI][offset], &inv1);
>       doub = unpack_double (builtin_type_double, dbl_buffer, &inv3);
> 
> So we're copying 2 * 8 bytes out of an 8 byte buffer.

I don't think so.  The memcpy part uses raw_buffer[HI] as its address,
but the two buffers whose addresses are in raw_buffer[0] and
raw_buffer[1] are layed out on the stack one after the other.  So you
have enough space there, and memcpy can copy up to 2*8 bytes without
fear.  It's nasty code, but it works.  (No, I didn't write that code ;-)

> The real killer is on the other branch, size == 4:
>       memcpy (dbl_buffer, raw_buffer, 2 * REGISTER_RAW_SIZE (FP0_REGNUM));
> 
> raw_buffer points to 8 bytes, sure enough - but they're both pointers
> to four byte buffers before my patch.  That won't decode.

So why replacing raw_buffer with raw_buffer[HI] in the call to memcpy
isn't all that is needed to fix this?