Tool/software: TI C/C++ Compiler
I have used this loop construct with an 8-bit loop counter. If the value for count is passed as 0 I would expect the loop to execute 256 times. It actually executes 65536 times when the optimiser is turned on (-O2).
static uint8_t buffer[256];
void do_loop(uint8_t count)
{
uint8_t *p = buffer;
do *p++ = 0;
while (--count != 0);
}
Looking at the list file I'm seeing instructions like this (only code lines and comments retained):
285 ;*****************************************************************************
286 ;* FUNCTION NAME: do_loop *
287 ;* *
288 ;* Regs Modified : SP,SR,r12,r15 *
289 ;* Regs Used : SP,SR,r12,r15 *
290 ;* Local Frame Size : 0 Args + 0 Auto + 0 Save = 0 byte *
291 ;*****************************************************************************
292 do_loop:
293 ;* --------------------------------------------------------------------------*
294 ;* r12 assigned to $O$L1
295 ;* r12 assigned to count
302 ;* r15 assigned to p
311 000000 4C4C MOV.B r12,r12 ; []
313 000002 008F! MOVX.A #buffer+0,r15 ; [] |44|
000004 0000
314 ;* --------------------------------------------------------------------------*
315 ;* BEGIN LOOP $C$L1
316 ;*
317 ;* Loop source line : 45
318 ;* Loop closing brace source line : 48
319 ;* Known Minimum Trip Count : 1
320 ;* Known Maximum Trip Count : 256
321 ;* Known Max Trip Count Factor : 1
322 ;* --------------------------------------------------------------------------*
323 000006 $C$L1:
325 000006 00AF ADDA #1,r15 ; [] |47|
000008 0001
326 00000a 43CF MOV.B #0,-1(r15) ; [] |47|
00000c FFFF
328 00000e 831C SUB.W #1,r12 ; [] |49|
329 000010 23FA JNE $C$L1 ; [] |49|
330 ; [] |49|
331 ;* --------------------------------------------------------------------------*
336 000012 0110 RETA ; []
So even though the loop counter is initially loaded as 8-bit, the decrement is on a 16-bit variable. If the initial value is 0 it gets decremented to 0xffff instead of 0xff as would be expected.
It looks a lot like the instruction on line 328 should be SUB.B instead of SUB.W
Changing the test to while ((--count & 0xff) != 0) makes no difference either.
This was compiled using Compiler 16.9.8.LTS. I have also tried using 18.1.4.LTS and the result is the same.
-vmspx --data_model=restricted -O2 --use_hw_mpy=F5 --include_path="C:/ti/ccsv8/ccs_base/msp430/include" --include_path="C:/ti/ccsv8/tools/compiler/ti-cgt-msp430_16.9.8.LTS/include" --advice:power=all --advice:hw_config=all --define=__MSP430FR5994__ --define=_MPU_ENABLE --printf_support=nofloat --diag_warning=225 --diag_wrap=off --display_error_number --silicon_errata=CPU21 --silicon_errata=CPU22 --silicon_errata=CPU40 --asm_listing
