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Runtime library bug?

Question asked by DonMilne on Jul 14, 2011
Latest reply on Jul 18, 2011 by StuartS

I recently upgraded to v5, update 9.1.


The compiler emits a jump to a function "__spswitch32" whenever it comes across a C switch statement with more than 3 labels. I was able to find the source of this function: "<VDSP>\Blackfin\lib\src\libc\runtime\spswitch32.asm".  As expected it consists of a loop which searches a table for the expression value, then jumps to an address.


Anyway I noticed something odd about that function.


The implementation uses a hardware loop, and jumps out of the hardware loop to a label ".found" when the expression value is found. The assembly source contains the following at the label mentioned :-



      // LC0 = 0;  ???

      P1 = P4 + (P3<<2);

      P1 = [P1];                // lab = labels[mid];

      (P5:3) = [SP++];

      JUMP (P1);



This puzzled me.  Why is that "LC0=0" line commented out?  The syntax is actually illegal (you can't "load immediate" into LC0), but still the intent was clear:  the code breaks out of hardware loop with LC0!=0, i.e. with the CPU thinking its still in a loop, hence the programmer basically wanted to "turn the loop machine off".   Is it a bug that he didn't?