Texas Instruments TMS320C2x, Руководство пользователя

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What Kind of Optimization Is Being Performed?
3-19 Optimizing Your Code
Example 3 − 3. Data-Flow Optimizations
simp(int j)
{
int a = 3;
int b = (j * a) + (j * 2);
int c = (j << a);
int d = (j >> 3) + (j << b);
call(a,b,c,d);
...
}
TMS320C2x/C2xx/C5x C Compiler Output:
_simp:
. . .
*****************************************************
* b = j * 5;
*****************************************************
LARK AR2,−3+LF1 ; AR2 = &j
MAR *0+
LT * ; t = *AR2
MPYK 5 ; p = t * 5
ADRK 4−LF1 ; AR2 = &b
SPL * ; *AR2 = p
*****************************************************
* call(3, b, j << 3, (j >> 3) + (j << b));
*****************************************************
LT * ; t = *AR2 (b)
SBRK 4−LF1 ; AR2 = &j
LACT * ,AR1 ; ACC = j << b
SACL * ,AR2 ; save off ACC on TOS (top of stack)
SSXM ; need sign extension for right shift
LAC * ,12,AR1 ; high ACC = j >> 3
ADD * ,15 ; add TOS to high ACC
SACH *+,1,AR2 ; stack high ACC
LAC * ,3,AR1 ; ACC = j << 3
SACL *+,AR2 ; stack ACC
ADRK 4−LF1 ; AR2 = &b
LAC * ,AR1 ; ACC = b
SACL *+ ; stack ACC
CALLD _call ; call begins
LACK 3 ; ACC = 3
SACL *+ ; stack ACC
*** CALL _call OCCURS ; call occurs
. . .
The constant 3, assigned to a, is copy propagated to all uses of a; a becomes a dead variable
and is eliminated. The sum of multiplying j by 3 (a) and 2 is simplified into b = j * 5, which is recog-
nized as a common subexpression. The assignments to c and d are dead and are replaced with
their expressions. These optimizations are performed across jumps.