Re: Unexpected behaviour when catching SIGFPE on FPU-less system

["Kevin D. Kissell" <kevink@paralogos.com>]

Kevin D. Kissell wrote:
> Shane McDonald wrote:
>   
>> In the following chunk of code from cp1emu.c:
>>   
>>     
> [snip]
>   
>> value gets set to an initial value of 0x400, and ctx->fcr31
>> comes in with an initial value of 0x8420.
>> By the time we hit the if statement around the return SIGFPE, ctx->fcr31
>> has been set to 0x8400, not the 0x400 I implied.
>>   
>>     
> Ah, well that would rather change things, and you *would* get an
> exception there.  As written, the code doesn't seem to allow the pending
> exception (.._X) bits to be cleared by the CTC.
>   
>> Nevertheless, that's not the problem.  
>>     
> Maybe it is. 
OK, sorry to have been looking at this in fits and starts, but indeed, I 
submit that the bug is indeed in that ctc_op:  case of the emulator.  
The Cause bits (17:12) are supposed to be writable by that instruction, 
but the CTC1 emulation won't let them be updated by the instruction.  I 
don't have the means to generate, test, and submit a proper patch, but I 
think that actually if you just completely removed lines 387-388:


value &= (FPU_CSR_FLUSH | FPU_CSR_ALL_E | FPU_CSR_ALL_S | 0x03);
ctx->fcr31 &= ~(FPU_CSR_FLUSH | FPU_CSR_ALL_E | FPU_CSR_ALL_S |0x03);

Things would work a good deal better.  At least, it would be a more 
accurate emulation of the architecturally defined FPU.  If I wanted to 
be really, really
pedantic (which I sometimes do), I'd also protect the reserved bits that 
aren't necessarily writable, so we'd nuke those two lines, then have

/* Don't write reserved bits, and convert to ieee library modes */
ctx->fcr31 = (value & ~0x1c0003) | ieee_rm[value & 0x3];

Note that I've changed the existing |= to a direct assignment here.

Hope this helps.

/K.