MSP430FR6047: What does this assembly sequence do?

Harvey Chapman

Part Number: MSP430FR6047

Tool/software:

The TI compiler generates this sequence quite often that appears to do nothing and the results are always ignored (as best as I can see). Assuming, it has a good reason for generating it, I'd like to understand why this is included. I've seen it just before RETA statements as well as in the middle of functions where the output value and SR bits are completely ignored.

The two registers used vary. This tests the sign bit in r12. If set, r13 = 0xffff. If clear, r13 = 0x0000. The r13 value is never used anywhere. I've even seen CLR.W r13 as the next statement. It's always a test of 0x8000.

; r13 = 0xffff if r12 & 0x8000 else 0x0000

BIT.W     #32768,r12            ; #0x8000
SUBC.W    r13,r13
INV.W     r13

I've found this in many places in my generated assembly. Looking at the C code, I can't seem to identify the pattern that causes this assembly to be generated.

Thanks.

1 month ago

+1 David Schultz 1 month ago

Guru 21985 points

Acts like sign extension of a 16 bit integer to 32 bits. When I test that idea using GCC with:

long __attribute__((noinline)) test(void)
{
  return (int)PAIN;
}

I get:

    c03e:       1c 42 00 02     mov     &0x0200,r12     ;0x0200
    c042:       3c b0 00 80     bit     #-32768,r12     ;#0x8000
    c046:       0d 7d           subc    r13,    r13     ;
    c048:       3d e3           inv     r13             ;

0 Harvey Chapman 1 month ago in reply to David Schultz

Prodigy 80 points

Ah. I see it now. Thank you.

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MSP430FR6047: What does this assembly sequence do?