On Fri, May 9, 2014 at 9:21 AM, erik quanstrom <quans...@quanstro.net> wrote:
> on 64-bit machines, the unions in the ureg.h can lead to
> internal padding. (power64 avoids this issue because everything
> is 64-bit aligned anyway.) to sidestep the issue, i think
> it might make sense to use #defines. for example, for arm
> the conversion would look something like this:
>
> typedef struct Ureg {
> ulong r0;
> ulong r1;
> ulong r2;
> ulong r3;
> ulong r4;
> ulong r5;
> ulong r6;
> ulong r7;
> ulong r8;
> ulong r9;
> ulong r10;
> ulong r11;
> ulong r12; /* sb */
> ulong r13;
> #define sp r13
> ulong r14;
> #define link r14
> ulong type; /* of exception */
> ulong psr;
> ulong pc; /* interrupted addr */
> } Ureg;
>
> is there any reason not to do this?
Ugh, no! Is there a case where the padding is a problem? Normally
registers belonging to the same union are uniform in size. Those
defines will expand anywhere and not behave as expected:
ulong sp = ureg->sp;
This could lead to all sorts of entertaining problems.
Steve
