[sdcc-devel] [ sdcc-Bugs-813572 ] unmatched switch causes jump to a random location

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Bugs item #813572, was opened at 2003-09-27 09:51
Message generated for change (Comment added) made by epetrich
You can respond by visiting: 
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Category: msc51(8051) target
>Group: non bugs
>Status: Closed
>Resolution: Rejected
Priority: 5
Submitted By: Stas Sergeev (stsp)
>Assigned to: Erik Petrich (epetrich)
Summary: unmatched switch causes jump to a random location

Initial Comment:
Hi.



There seem to be a bug in sdcc which causes a jump

to a random location when the "switch" without a

"default" case is unmatched.



Attached is a test program. Also here it is:

---

#include <at89S8252.h>	/* special function register

declarations */



int main()

{

  char a, b;

  b = SBUF;

  switch (b) {

    case 1: a = 'a';

	    break;

    case 2: a = 'b';

	    break;

  }

  return a;

}

---



The code it generates, looks as follows:

---

;switch.c:7: switch (b) {

;     genMinus

;     genMinusDec

	dec	r2

;     genJumpTab

	mov	a,r2

	add	a,acc

	add	a,r2

	mov	dptr,#00106$

	jmp	@a+dptr

00106$:

	ljmp	00101$

	ljmp	00102$

	C$switch.c$8$2$2 ==.

;switch.c:8: case 1: a = 'a';

---



Now, as "b" contains some garbage and there is no a

"default" case, the "switch" must be bypassed.

But instead it is being performed, and the ljmp

misses a jump table with an unpredictable results.

----------------------------------------------------------------------

>Comment By: Erik Petrich (epetrich)
Date: 2003-09-27 11:15

Message:
Logged In: YES 
user_id=635249

Unless I am overlooking your point, the compiler appears to

work as designed.



If you specify --nojtbound, the compiler omits the code that

verifies that the switched value is within the range of

cases handled in the switch statement. Essentially, by using

--nojtbound you are making a promise to the compiler that

you have already (by some other means) ensured that your

value is within the range of cases given; in exchange, the

compiler generates faster and shorter code. Your sample test

case breaks this promise and so will fail when b<1 or b>2.



Perhaps the option's name has tricked you? Whereas most of

the --no_x_ options turn OFF an optimization, the

--nojtbound option turns one ON.



----------------------------------------------------------------------

Comment By: Stas Sergeev (stsp)
Date: 2003-09-27 10:11

Message:
Logged In: YES 
user_id=501371

Compile the test-case on a latest sdcc from CVS with the

following command line:

sdcc -mmcs51 --nojtbound --noinduction --noloopreverse

--xram-size 0 --code-size 8192 --debug --no-peep switch.c



----------------------------------------------------------------------

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