Is there any reason to use C instead of C++ for embedded development?

Question

Question

I have two compilers on my hardware C++ and C89

I\'m thinking about using C++ with classes but without polymorphism (to avoid vtables). The main r

Answer 1

My personal preference is C because :

• I know what every line of code is doing (and costs)
• I don't know C++ well enough to know what every line of code is doing (and costs)

Why do people say this? You don't know what every line of C is doing unless you check the asm output. Same goes for C++.

For example, what asm does this innocent statement produce:

a[i] = b[j] * c[k];

It looks fairly innocent, but a gcc based compiler produces this asm for an 8-bit micro

CLRF 0x1f, ACCESS
RLCF 0xfdb, W, ACCESS
ANDLW 0xfe
RLCF 0x1f, F, ACCESS
MOVWF 0x1e, ACCESS
MOVLW 0xf9
MOVF 0xfdb, W, ACCESS
ADDWF 0x1e, W, ACCESS
MOVWF 0xfe9, ACCESS
MOVLW 0xfa
MOVF 0xfdb, W, ACCESS
ADDWFC 0x1f, W, ACCESS
MOVWF 0xfea, ACCESS
MOVFF 0xfee, 0x1c
NOP
MOVFF 0xfef, 0x1d
NOP
MOVLW 0x1
CLRF 0x1b, ACCESS
RLCF 0xfdb, W, ACCESS
ANDLW 0xfe
RLCF 0x1b, F, ACCESS
MOVWF 0x1a, ACCESS
MOVLW 0xfb
MOVF 0xfdb, W, ACCESS
ADDWF 0x1a, W, ACCESS
MOVWF 0xfe9, ACCESS
MOVLW 0xfc
MOVF 0xfdb, W, ACCESS
ADDWFC 0x1b, W, ACCESS
MOVWF 0xfea, ACCESS
MOVFF 0xfee, 0x18
NOP
MOVFF 0xfef, 0x19
NOP
MOVFF 0x18, 0x8
NOP
MOVFF 0x19, 0x9
NOP
MOVFF 0x1c, 0xd
NOP
MOVFF 0x1d, 0xe
NOP
CALL 0x2142, 0
NOP
MOVFF 0x6, 0x16
NOP
MOVFF 0x7, 0x17
NOP
CLRF 0x15, ACCESS
RLCF 0xfdf, W, ACCESS
ANDLW 0xfe
RLCF 0x15, F, ACCESS
MOVWF 0x14, ACCESS
MOVLW 0xfd
MOVF 0xfdb, W, ACCESS
ADDWF 0x14, W, ACCESS
MOVWF 0xfe9, ACCESS
MOVLW 0xfe
MOVF 0xfdb, W, ACCESS
ADDWFC 0x15, W, ACCESS
MOVWF 0xfea, ACCESS
MOVFF 0x16, 0xfee
NOP
MOVFF 0x17, 0xfed
NOP

The number of instructions produced depends massively on:

• The sizes of a, b, and c.
• whether those pointers are stored on the stack or are global
• whether i, j and k are on the stack or are global

This is especially true in the tiny embedded world, where processors are just not set up to handle C. So my answer would be that C and C++ are just as bad as each other, unless you always examine the asm output, in which case they are just as good as each other.

Hugo