Calculating constants at compile time, not run time

For the source file being compiled when this diagnostic is seen ...

Gregory Helton said:

the compiler is giving me warnings recommending that I not use floating point math

... please follow the directions in the article How to Submit a Compiler Test Case.

Thanks and regards,

-George

Gregory Helton said:

#define TIMB1_CORE_RESET_CCR0 (unsigned short)(round(CORE_RESET_TIME / SMCLK_PERIOD))

That references the round() function. I'm not sure if the compiler constant evaluation would be able to eliminate the function call.

What happens if you instead try:

#define TIMB1_CORE_RESET_CCR0 (unsigned short)((CORE_RESET_TIME / SMCLK_PERIOD) + 0.5f)

Chester Gillon said:

That references the round() function. I'm not sure if the compiler constant evaluation would be able to eliminate the function call.

Attached are two example projects, both compiled using TI MSP430 compiler v21.6.0 using Optimisation Level 0 (Register Optimisations).

1. MSP430F2418_compiler_constants_round uses the original TIMB1_CORE_RESET_CCR0 macro definition which uses the round() function:

#include <msp430.h> 
#include <math.h>

#define XTAL_FREQ 10000000UL
#define CORE_RESET_TIME (float)(0.000050)

#define MCLK_DIV 8UL
#define SMCLK_DIV 8UL
#define ACLK_DIV 256UL

#define MCLK_FREQ (XTAL_FREQ / MCLK_DIV)
#define SMCLK_FREQ (XTAL_FREQ / SMCLK_DIV)
#define ACLK_FREQ (XTAL_FREQ / ACLK_DIV)

#define ACLK_PERIOD ( (float)1/(float)ACLK_FREQ )
#define SMCLK_PERIOD ( (float)1/(float)SMCLK_FREQ )

#define TIMB1_CORE_RESET_CCR0 (unsigned short)(round(CORE_RESET_TIME / SMCLK_PERIOD))

/**
 * main.c
 */
int main(void)
{
	WDTCTL = WDTPW | WDTHOLD;	// stop watchdog timer
	
	return TIMB1_CORE_RESET_CCR0;
}

When compile the above:

a. Get "remark #1531-D: (ULP 5.2) Detected floating point operation(s). Recommend moving them to RAM during run time or not using as these are processing/power intensive" reported 52 times.

b. The linker map file shows floating point library functions are being linked from the compiler run time library, e.g. round, floor, __mspabi_addd, __mspabi_fixdu

c. The linker map shows the total size is 3276 (code), 64 (ro data), 80 (rw data)

d. The assembler listing shows for the return TIMB1_CORE_RESET_CCR0 statement functions are being called:

	.dwpsn	file "../main.c",line 27,column 2,is_stmt,isa 0
        MOV.W     #0,r12                ; [] |27| 
        MOV.W     #57344,r13            ; [] |27| 
        MOV.W     #16383,r14            ; [] |27| 
        MOV.W     #16463,r15            ; [] |27| 
$C$DW$5	.dwtag  DW_TAG_TI_branch

        CALLA     #round                ; [] |27| 
                                          ; [] |27| 
$C$DW$6	.dwtag  DW_TAG_TI_branch

        CALLA     #__mspabi_fixdu       ; [] |27| 
                                          ; [] |27| 
	.dwpsn	file "../main.c",line 28,column 1,is_stmt,isa 0
$C$DW$7	.dwtag  DW_TAG_TI_branch

        RETA      ; [] 

2. MSP430F2418_compiler_constants_cast uses a cast in the TIMB1_CORE_RESET_CCR0 macro definition:

#include <msp430.h> 

#define XTAL_FREQ 10000000UL
#define CORE_RESET_TIME 0.000050

#define MCLK_DIV 8UL
#define SMCLK_DIV 8UL
#define ACLK_DIV 256UL

#define MCLK_FREQ (XTAL_FREQ / MCLK_DIV)
#define SMCLK_FREQ (XTAL_FREQ / SMCLK_DIV)
#define ACLK_FREQ (XTAL_FREQ / ACLK_DIV)

#define ACLK_PERIOD ( (double)1/(double)ACLK_FREQ )
#define SMCLK_PERIOD ( (double)1/(double)SMCLK_FREQ )

#define TIMB1_CORE_RESET_CCR0 (unsigned short)((CORE_RESET_TIME / SMCLK_PERIOD) + 0.5)

/**
 * main.c
 */
int main(void)
{
	WDTCTL = WDTPW | WDTHOLD;	// stop watchdog timer
	
	return TIMB1_CORE_RESET_CCR0;
}

When compile the above:

a. The compiler no longer reports "remark #1531-D: (ULP 5.2) Detected floating point operation(s)."

b. The linker map doesn't show floating point library function being linked.

c. The linker map shows the total size is 50 (code), 64 (ro data), 80 (rw data). I.e. significant size reduction due to not pulling in floating point library functions.

d. The assembler listing shows for the return TIMB1_CORE_RESET_CCR0 statement the compiler uses a constant value 63 calculated at compile time, and there are no function calls:

        MOV.W     #63,r12               ; [] |26| 
	.dwpsn	file "../main.c",line 27,column 1,is_stmt,isa 0
$C$DW$3	.dwtag  DW_TAG_TI_branch

        RETA      ; [] 

Note that since the calculation for TIMB1_CORE_RESET_CCR0 ends up rounding the value 62.5 to an integer, I had to change the macro to use all double-precision calculations to ensure the rounded integer was 63, rather than 62.

MSP430F2418_compiler_constants_round.zip

MSP430F2418_compiler_constants_cast.zip