Subject: /lib/c2 fails to remove redundant 'tst' and 'branch's
Index:	lib/c21.c 2.11BSD

Description:
	/lib/c2 (the "optimizer") fails to remove redundant 'tst'
	instructions (see #1 in the Repeat-By section).  This was
	due to c2's not treating the 'tst' instruction as being
	capable of setting the condition codes.

	also, /lib/c2 does not collapse adjacent 'branches' to the
	same label after a 'tst' (see #2 below).  In general branches
	can not be merged.  However, following a 'tst' instruction
	it is known that the C and V condition code bits are 0 and
	some branches can be merged.

	overall the savings are worth the small increase in compilation
	time.  recompiling the kernel using the new optimizer saved
	2 clicks (128 bytes) of text in the kernel itself and 1 click
	(64 bytes) in the networking portion (/netnix).

Repeat-By:
	Look at the generated code for this test program (it's not
	contrived, this occurs in sys/kern_clock.c).  The multiple
	branch situation occurs even more frequently than the nested
	'if'.

	int x;

	main()
	 {
	 if (x)
	    if (x > 0)
	       exit();
	 }

	OLD Optimizer
	-------------
_main:
	tst	_x
	jeq	L3
	tst	_x		#1
	jmi	L3
	jeq	L3		#2
	jsr	pc,_exit
L3:	jmp	cret

	NEW Optimizer
	------------
_main:
	tst	_x
	jle	L3
	jsr	pc,_exit
L3:	jmp	cret

	A saving of 8 bytes!

Fix:
	Apply the following patch to 'c21.c', recompile and install c2.

----------------------------------------------------------------------
*** /usr/src/lib/c2/c21.c.old	Fri Oct  4 08:39:10 1991
--- /usr/src/lib/c2/c21.c	Sun Oct 20 16:15:12 1991
***************
*** 229,235 ****
--- 229,254 ----
  				p = p->back;
  				nchange++;
  			}
+ /*
+  * If the instruction prior to the conditional branch was a 'tst' then
+  * save the condition code status.  The C construct:
+  * 		if (x) 
+  *		   if (x > 0)
+  * generates "tst _x; jeq ...; tst _x; jmi ...;jeq ...".  The code below removes
+  * the second "tst _x", leaving "tst _x; jeq ...;jmi ...; jeq ...".
+ */
+ 			if (p->back->op == TST) {
+ 				singop(p->back);
+ 				setcc(regs[RT1]);
+ 				break;
+ 			}
  		}
+ /*
+  * If the previous instruction was also a conditional branch then
+  * attempt to merge the two into a single branch.
+ */
+ 		if (p->back->op == CBR)
+ 			fixupbr(p);
  	case CFCC:
  		ccloc[0] = 0;
  		continue;
***************
*** 242,247 ****
--- 261,316 ----
  	}
  	}
  }
+ 
+ /*
+  * This table is used to convert two branches to the same label after a 
+  * 'tst' (which clears the C and V condition codes) into a single branch.
+  * Entries which translate to JBR could eventually cause the 'tst' instruction 
+  * to be eliminated as well, but that can wait for now.  There are unused or
+  * impossible combinations ('tst' followed by 'jlo' for example.  since
+  * 'tst' clears C it makes little sense to 'jlo/bcs') in the table, it 
+  * would have cost more in code to remove them than the entries themselves.
+  *
+  * Example:  "tst _x; jmi L3; jeq L3".  Find the row for 'jmi', then look
+  * at the column for 'jeq', the resulting "opcode" is 'jle'.
+ */
+ 	char	brtable[12][12] = {
+ 	/* jeq  jne  jle  jge  jlt  jgt  jlo  jhi  jlos jhis jpl  jmi */
+ /* jeq */ {JEQ ,JBR ,JLE ,JGE ,JLE ,JGE ,JEQ ,JBR ,JEQ ,JBR ,JGE ,JLE},
+ /* jne */ {JBR ,JNE ,JBR ,JBR ,JNE ,JNE ,JNE ,JNE ,JBR ,JBR ,JBR ,JNE},
+ /* jle */ {JLE ,JBR ,JLE ,JBR ,JLE ,JBR ,JLE ,JBR ,JLE ,JBR ,JBR ,JLE},
+ /* jge */ {JGE ,JBR ,JBR ,JGE ,JBR ,JGE ,JGE ,JBR ,JGE ,JBR ,JGE ,JBR},
+ /* jlt */ {JLE ,JNE ,JLE ,JBR ,JLT ,JNE ,JLT ,JBR ,JLE ,JBR ,JBR ,JLT},
+ /* jgt */ {JGE ,JNE ,JBR ,JGE ,JNE ,JGT ,JGT ,JGT ,JBR ,JGE ,JGE ,JNE},
+ /* jlo */ {JEQ ,JNE ,JLE ,JGE ,JLT ,JGT ,JLO ,JHI ,JLOS,JHIS,JPL ,JMI},
+ /* jhi */ {JBR ,JNE ,JBR ,JBR ,JNE ,JNE ,JNE ,JNE ,JBR ,JBR ,JBR ,JNE},
+ /* jlos*/ {JEQ ,JBR ,JLE ,JGE ,JLE ,JGE ,JLE ,JBR ,JEQ ,JBR ,JGE ,JLE},
+ /* jhis*/ {JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR ,JBR},
+ /* jpl */ {JGE ,JBR ,JBR ,JGE ,JBR ,JGE ,JGE ,JBR ,JGE ,JBR ,JGE ,JBR},
+ /* jmi */ {JLE ,JNE ,JLE ,JBR ,JLT ,JNE ,JLT ,JNE ,JLE ,JLT ,JBR ,JLT}
+ 	  };
+ 
+ fixupbr(p)
+ 	register struct node *p;
+ {
+ 	register struct node *p1, *p2;
+ 	int op;
+ 
+ 	p1 = p->back;
+ 	p2 = p1->back;
+ 	if (p->labno != p1->labno)
+ 		return;
+ 	if (p2->op != TST) {
+ 		if (p2->op == CBR && p2->back->op == TST)
+ 			goto ok;
+ 		return;
+ 	}
+ ok:	p->subop = brtable[p->subop][p1->subop];
+ 	nchange++;
+ 	nredunj++;
+ 	p2->forw = p;
+ 	p->back = p1->back;
+ 	}
  
  jumpsw()
  {