Re: problems with read-write "asm" operands

[Ben Liblit <liblit at eecs dot berkeley dot edu>]

Sorry to reply to my own posting, but here's a slightly simpler version 
of the same code with nearly the same problem:

	unsigned counters[3];

	int bump(int left, int right)
	{
	  int slot = 0;
	  int le = 0;

	  asm
	    ("cmp %2,%3\n"
	     "setl %b0\n"
	     "setle %b1\n"
	     "add %k1,%k0\n"
	     : "+&q,&q" (slot), "+&q,&q" (le)
	     : "r,g" (left), "g,r" (right)
	     : "cc");

	  ++counters[slot];
	}

The job of the inline assembly code is now just to set "slot" to either 
0, 1, or 2 depending on whether left is <, ==, or > right.  Manipulation 
of the counters array is done in C code, which makes things somewhat 
more readable and more clearly expresses what kinds of side effects can 
occur.

Both "slot" and "le" are zero initialized in the C code, and both are 
declared as read-write ("+") operands.  However....

  - If I compile with "-O9", the register selected for "slot" is
    correctly zero initialized.  The register selected for "le",
    however, is not.  It simply inherits whatever leftover value
    might previously have been in that register.

  - If I compile with "-g", neither register is zero initialized.
    Each simply inherits whatever leftover value might previously have
    been in its selected register.

This is getting mighty strange.  Anybody got a clue?