Re: tdep/2305: IA64: breakpoints on predicated instructions ignorepredicate

["John S. Worley" <jsworley at qwest dot net>]

The following reply was made to PR tdep/2305; it has been noted by GNATS.

From: "John S. Worley" <jsworley@qwest.net>
To: Daniel Jacobowitz <drow@false.org>
Cc: gdb-gnats@sources.redhat.com
Subject: Re: tdep/2305: IA64: breakpoints on predicated instructions ignore
 predicate
Date: Wed, 29 Aug 2007 14:59:37 -0600

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 Daniel -
 
 > Consider:
 >
 > 91	z = b ? x : y;
 > 92	a = b ? c : d;
 >
 > Optimized to set a predicate in line 91, and then four conditional
 > moves (probably in only two bundles).
 > 	  
 > The user clicks on line 92 in their IDE.  GDB should stop on line 92.
 > Stopping on line 91 (i.e., arbitrarily far earlier) is unacceptable.
 >   
     Why? It is well known that optimizers blur the boundaries between 
 lines of code, especially in
 a highly parallel machine like IA64. The fact of your example is that 
 the compiler has recognized
 and eliminated a common subexpression. Consider a variant of your example:
 
     15   int
     16   aaa(int b)
     17   {
     18        int a, z;
     19
     20        a = b;
     21        z = a ? 24 : 15;
     22        a = a ? -4 : -8;
     23
     24        return (a + z + b);
     25   }
 
     Now, the optimizer does a great job with this - it recognizes that 
 it can select a constant value to
 add to 'b' for the return value. In effect, all the assignments have 
 been eliminated. Now, if the user
 sets the breakpoint at line 20, which doesn't really exist, it points to 
 the same arbitrary instruction as
 line 21, e.g., setting a predicate based on b. In effect, the user can 
 set the breakpoint at 20 or 21, but
 not both, and 'step' will skip one line or the other.
 
     Setting a breakpoint at 22 hits the instruction that picks the 
 constant '7' (15 - 8), which is another
 arbitrary choice, since the next instruction (constant '20') is just a 
 much a part of line 22 (or 20 or 21)
 as the previous, e.g., it isn't.
 
     The point here is that stopping at an instruction that "belongs" to 
 line 91 AND 92 (in your example)
  is no more egregious that stopping at an instruction that has no 
 correlation to the 'C' code whatever.
 It's just one of the hard problems of debugging optimized code.
 
     However, to make the choice to stop at code that isn't being 
 logically executed , e.g:
 
     107   if (a < 0) {
     108      b = 14 - a;
     109   } else if (a > 0) {
     110      b = a - 54;
     111   } else {
     112      b = 11;
     113   }
 
     With a breakpoint set at 112, is completely unexpected, confusing, 
 time-consuming, and just plain wrong.
 > It has very little to do with IA-64 and we have discussed it before in
 > ARM and other contexts.
 >   
     It has a lot to do with IA64, since it's parallelism and rich 
 predication calculus are changing the
 rules and challenging conventional wisdoms about assembly programming 
 and compilation. The notion
 that, to paraphrase Stein, "instruction is an instruction is an 
 instruction is an instruction", is untenable.
 > I'm not interested in trading unexpected stops for unexpected failures
 > to stop.  Experience has definitely shown that when we can't get it
 > right, stopping more often is better than less often.
 >
 >   
     Stopping more often is not a better choice, just a simpler one. I 
 suggest there is a need for both
 types of breakpoints, at a minimum because single stepping at the 
 instruction level requires it. I would
 accept the argument that the false stop problem may require action on 
 the user's part, such as a
 IA64-only command like 'breaki' to insert breakpoints that honor the 
 predicate. Is there a  guide for
 adding architecture-dependent commands to GDB?
 
     Regards,
     John Worley
     jsworley@qwest.net
 
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