[fpc-pascal] position of External: SIGFPE

[Jonas Maebe]

On 25 Mar 2013, at 01:55, Xiangrong Fang wrote:

> Why the "External: SIGFPE" error not reported on the line it was  
> generated?
> e.g. In the following code:
>
> 1: if SomeCondition then begin
> 2:   Y := exp(Y);
> 3: end;
> 4: d := Data[X];
>
> The SIGFPE is generated on line 2, because Y is too big. But while the
> program is running in IDE, it told me that line 4 is generating  
> SIGFPE?

Because that is how the 80x86 architecture works. Back when the  
original 8086 came out, the FPU was a separate co-processor you had to  
plug in. It worked in parallel with the main cpu (so the cpu didn't  
wait for the co-processor to finish with an instruction before  
continuing), so when it wanted to raise an exception it waited for the  
next FPU instruction to be dispatched rather than do so immediately  
(so the exception would always be raised in the same place, rather  
than at a random address). The FPU had no clue about the address of  
the original instruction, and hence could not report it.

We are now several decades later, but for compatibility reasons the  
FPU on the x86 processors still uses the same logic. It is possible to  
get the exception at the place where it happened by inserting fwait  
instructions after every fpu operation. Those tell the cpu to stop  
executing until the FPU has finished executing the previous  
instruction. That obviously slows down the code significantly. FPC has  
no option to do this.


Jonas
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