https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65082
Bug ID: 65082
Summary: Wasted cycles when using a register based varible
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: NickParker at Eaton dot com
gcc version 4.8.0 20130306 (experimental) (GCC)
Was just playing around and found this. When using a register based variable,
the compiler misses an obvious optimisation.
Notice in code below the addition does not take place 'in place' and is instead
performed in scratch/temporary registers and then shifted back to "phaseAccPh".
Why not just add directly to "phaseAccPh" since in this case it IS register
based already. It seems that GCC "thinks" that the variable is still in SRAM
or something else.....
Nick.
c code:
---------------------------------------------------------
register uint16_t phaseAccPh asm ("r4");
uint16_t phaseAccFr;
phaseAccPh += phaseAccFr;
asm code:
---------------------------------------------------------
40:pll.c **** void pllExec(void)
41:pll.c **** {
15 .loc 1 41 0
16 .cfi_startproc
17 /* prologue: function */
18 /* frame size = 0 */
19 /* stack size = 0 */
20 .L__stack_usage = 0
42:pll.c **** int16_t mix_output_s2;
43:pll.c **** phaseAccPh += phaseAccFr;
21 .loc 1 43 0
22 0000 E091 0000 lds r30,phaseAccFr
23 0004 F091 0000 lds r31,phaseAccFr+1
24 0008 E40D add r30,r4
25 000a F51D adc r31,r5
26 000c 2F01 movw r4,r30
