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Integral Promotion

P Maresh

We are currently in the process of evaluating Keil tools for possible purchase. Using the evaluation package, I have been checking some common functions. One of them is a simple bit test macro suitable for a large variable where speed is not important.

#define BTST(VAR, BITPOS) ((VAR) & (1<<(BITPOS)))

I used this in the following manner

unsigned long temp;
unsigned char shift;

BTST(temp, shift);

The Keil C51 compiler generates the following:


MOV A,#01H
MOV R6,#00H
MOV R0,shift?143
INC R0
SJMP ?C0015
?C0014:
CLR C
RLC A
XCH A,R6
RLC A
XCH A,R6
?C0015:
DJNZ R0,?C0014
MOV R7,A
MOV A,R7
ANL A,temp?144+03H
MOV R7,A


Note that even though I have ANSI promotions enabled, it does not promote the unsigned character to a long. It appears to promote it to an integer (using A and R6 for the data), but then never uses the MSB (R6) when performing the AND operation.

I have even tried this:
BTST(temp, (unsigned long) shift);

and I get the same results.

I have used this macro with many other compilers with complete success, following what I understand about integral promotion in ANSI C.

There seem to be several threads about bit-fields and the like, but none seemed to deal with integers or longs or shift operations.

Any thoughts?

Thanks much in advance,

Peter

Parents
  • 0 in reply to Jon Ward

    Firstly, I apologize for including only a code snippet that could not be directly plugged into the compiler. Of course, the details I omitted were critical, thanks for showing me how to get this to work.

    The result that I was storing into was character length. The compiler was optimizing out the unneeded bytes. I hadn't even noticed this (I was thinking the result was a long) until your message made me look over the code again.

    I would also like to say that although the result of this exercise was a stupid mistake on my part, it wasn't a total waste of anyone's time as it has shown the support group on this message board to be execellent, which is also a critical part of evaluating these tools we are considering purchasing.

    I look forward to posting more stupid questions...

    Thanks Mark and Jon,
    Peter

Reply
  • 0 in reply to Jon Ward

    Firstly, I apologize for including only a code snippet that could not be directly plugged into the compiler. Of course, the details I omitted were critical, thanks for showing me how to get this to work.

    The result that I was storing into was character length. The compiler was optimizing out the unneeded bytes. I hadn't even noticed this (I was thinking the result was a long) until your message made me look over the code again.

    I would also like to say that although the result of this exercise was a stupid mistake on my part, it wasn't a total waste of anyone's time as it has shown the support group on this message board to be execellent, which is also a critical part of evaluating these tools we are considering purchasing.

    I look forward to posting more stupid questions...

    Thanks Mark and Jon,
    Peter

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