Re: V4 core speculative execution?

> I am having a problem with code that works on 68K and V3 Coldfires not
> working on V4. I find it hard to believe, but it looks like the V4 core
> partially executes code that is branched around. Here is a distilled
> example of the sort of code that fails, not pretty but I think it is
> valid C: -
>
> typedef void (*f_ty)(void);         // a function pointer
>
> // A union of an integer and a pointer, initialised to an integer that
> is not a valid address
> union {
>     int x;
>     f_ty  f;
> } p = { 0x55555555 };
>
> // This is the sort of sequence that fails
> {
>        dword d = p.x;
>         f_ty f = p.f;
>         if(d != 0x55555555)
>              f();
>  }
>
> This compiles to: -
>
>      movea.l  _p,a0
>      cmpa.l  #0x55555555
>      beq.s    skip
>      jsr       (a0)
> skip
>
> So a0 should contain all 5's, which is not a valid address in my map,
> but the branch should prevent it being used as an address. What actually
> happends seems to depend on the code alignment, etc.
>
> It can simply run past the code OK.
>
> It can give an XLB interrupt with 55555554 in the address capture register.
>
> It can lock up the processor. When I hit break in my BDM debugger the PC
> is a couple of instructions past this block.
>
> If I set a breakpoint anywhere in the program the code runs OK,
> regardless of whether the breakpoint is triggered.
>
> So it looks like the core sometimes does a pre-fetch on 55555555, even
> though it will not be executed. My questions are: -
>
> Is this a known bug/feature in the V4 core?
> Why does it sometimes lock the processor rather than giving an XLB
> interrupt.
> Why does having a breakpoint active make it work?
> Is that a workaround or do I have to find all the places in my code that
> this could happen an insert NOPs or something. A complete nightmare
> because I have a whole state machien architecture that relies on being
> able to contain addresses and some flags in the same 32 bit value!
>
> Chris
>
>
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