Code enlargement

[Frank Klemm <pfk at fuchs dot offl dot uni-jena dot de>]

C-Code:
		...1
		if ( x == 0 ) {
		   ...2
		}
		...3
		return;

gcc2.95:
		...1
		cmpl $0,x
		jne  lbl
		...2
	lbl:	...3
		ret


gcc3.0 tends to code this like (also with -Os):
		...1
		cmpl $0,x
		je  lbl1
	lbl2:	...3
		ret

	lbl1:	...2				\ code
		jmp lbl2			/ junks

Problems:
	- code is at least 2 byte longer (2 jumps instead of 1 jump)

	- code often is 9 byte longer (distance becomes larger than
	  -128...+127 bytes, so a 'Jcc long' (6 byte) and 'JMP long' 
	  (5 byte) is needed.

Solutions:
	- avoid this trick with '-Os', may be also with '-O2', because speed
	  increase is very little, zero or below zero. Brnach prediction
	  seems to work very well.

	- avoid trick especially if jump distance is too long compared with
	  the nontricky code.

	- Insert the code junks as early as possible (after every jmp this
	  is possible, or before the function start). But be carefully to
	  avoid other code increases inside the function

	- if code is small and target is Pentium Pro, II, III, 4, Duron, Athlon
          often a conditional-mov works well (especially if the conditional
          expression is highly inpredictable).

	  if ( expect (x>12,0) )		// condition is seldom true
	  if ( expect (x>12,1) )		// condition is very often true

	  The current expect() syntax don't allow to describe a condition
	  which is high unpredictable so that jumps should be avoided and cmov's
	  should be used if possible.

-- 
Frank Klemm