On Thu, Aug 10, 2023 at 09:35:18AM +0300, Michael Tokarev wrote:
> 09.08.2023 23:34, Stafford Horne пишет:
> > The architecture specification calls for the EPCR to be set to "Address
> > of next not executed instruction" when there is a floating point
> > exception (FPE). This was not being done, so fix it by using the same
> > pattern as syscall. Also, we move this logic down to be done for
> > instructions not in the delay slot as called for by the architecture
> > manual.
> >
> > Without this patch FPU exceptions will loop, as the exception handling
> > will always return back to the failed floating point instruction.
> >
> > This was not noticed in earlier testing because:
> >
> > 1. The compiler usually generates code which clobbers the input operand
> > such as:
> >
> > lf.div.s r19,r17,r19
> >
> > 2. The target will store the operation output before to the register
> > before handling the exception. So an operation such as:
> >
> > float a = 100.0f;
> > float b = 0.0f;
> > float c = a / b; /* lf.div.s r19,r17,r19 */
> >
> > Will first execute:
> >
> > 100 / 0 -> Store inf to c (r19)
> > -> triggering divide by zero exception
> > -> handle and return
> >
> > Then it will execute:
> >
> > 100 / inf -> Store 0 to c (no exception)
> >
> > To confirm the looping behavior and the fix I used the following:
> >
> > float fpu_div(float a, float b) {
> > float c;
> > asm volatile("lf.div.s %0, %1, %2"
> > : "+r" (c)
> > : "r" (a), "r" (b));
> > return c;
> > }
>
> Is it a -stable material? It applies cleanly to 8.0 and 7.2.
> Or maybe it is not needed on older versions, not being noticed before?
I would say no, it will work on 8.0 an 7.2 but this code path is not very useful
withouth the other 8.1 Floating Point Exception handling updates.
-Stafford
