CCS 3.xx How to avoid unwanted casts?

Vitaliy

CCS 3.3.81.6, Code generation tools 4.1.0 DSP 320C5416

unsigned long Sum32(short a,short b)
{
register unsigned long z;
z=(unsigned long)(a*b);
return (z);
}

Get asm:

_Sum32:
FRAME #-2
NOP
STL A,*SP(0)
LD *SP(4),T
SSBX SXM
MPY *SP(0),A ; |114|
LD *(AL),A ; |114| WHAT is THIS ???? i'm need a 32 bit result!
FRAME #2 ; |116|
NOP
NOP
FRET ; |116|

unsigned long SSum32(short a,short b)
{
register unsigned long z;
z=(unsigned long)(a*a);
z+=(unsigned long)(b*b);
return (z);
}

get asm code:

_SSum32:
FRAME #-2
NOP
STL A,*SP(0)
SSBX SXM
SQUR *SP(0),A ; |113|
SQUR *SP(4),B ; |114|
LD *(AL),A ; |113| WHAT is THIS ???? i'm need a 32 bit result!
LD *(BL),B ; |114| WHAT is THIS ???? i'm need a 32 bit result!
ADD B,A ; |114|
FRAME #2 ; |115|
.dwcfa 0x0e, 2
NOP
NOP
FRET ; |115|

How to avoid unwanted casts?

over 15 years ago

0 Archaeologist over 15 years ago

TI__Guru* 84285 points

That's not the right expression for a 32-bit multiply. You must place the cast to unsigned long on one or both of the operands like so: "((unsigned long)a * (unsigned long)b)". See How to Write Multiplies Correctly in C Code

0 Vitaliy over 15 years ago in reply to Archaeologist

Prodigy 250 points

Then will be called an entire function, not fast, for the multiplication of 32bit to 32bit ... Why do I need it, if the multiplication can be done one instruction! This is an obvious mistake.

0 Archaeologist over 15 years ago in reply to Vitaliy

TI__Guru* 84285 points

To make a long story short, change the type of the result to match the signedness of the inputs:

long Sum32(short a, short b)
{ 
    return (long)a * (long)b;
}

If the multiplication really needs to be an unsigned multiplication, use:

unsigned long Sum32(unsigned short a, unsigned short b)
{ 
    return (unsigned long)a * (unsigned long)b;
}

I've submitted performance defect report SDSCM00034307 to investigate why it doesn't match a single MPY if written as a S16xS16->U32.

0 Vitaliy over 15 years ago in reply to Archaeologist

Prodigy 250 points

Thank you! The resulting code is short and without external call. And about S16xS16-> U32 truly incomprehensible situation.

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CCS 3.xx How to avoid unwanted casts?