Compiler optimisation

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Ok,

In that case atleast the compiler could even have different versions of printf and use the one which is appropriate for the current project.

That is if i have never used float inside printf in my project then float to string part of the printf library is not neccessary.

The comipler/linker during optimization can decide which printf library would be suitable for my project.

"That is if i have never used float inside printf in my project then float to string part of the printf library is not neccessary"

That is exactly what happens already!

Have you now understood why the code that you thought did a single-bit test does not actually do a single-bit test?

Note that it isn't trivial for the linker to analyze object files and try to figure out which of several printf() functions to use.

Remember that not all printf() calls needs to look like:

printf("formatting string",param,param,...);

You can also have:

void function(const char* fmt) {
    printf(fmt,int1,int2);
}

and you can have:

char fmt[100];
sprintf(fmt,"xxx",...);
printf(fmt,...);

The linker runs at link time. It doesn't know what happens at run time. A program could have multiple sets of strings, to allow it to print the same messages in english, italian, german, ...

And remember that printf() and sprintf() shares the same background "engine", so it isn't enough to look at all printf() calls.

The only one who really knows everything about your program - or is expected to - is you.

Not to continue the the discussion about the C language.

But a look at the 8051 architecture. Please note, that
there are port access instructions that read the latch
and others that read the port pin. You will find this
in every description of standard 8051.

What does that mean? This means, that under certain
circumstances (please think about that yourself) you
will have

 if (P0 ^ 1) // the if-condition is FALSE
   P0.0 = 0;

and

 if (_testbit(P0.0)) // the if-condition is TRUE.

So, already on the hardware level, these both are NOT the same!

Hello All,

Thank you all for keeping patience with me.

I agree that all my allegations were complete wrong.

1.when i use printf with no var_args then the compiler should call puts instead of printf.

puts Adds extra linefeed to the string so it cannot be used instead of printf.So i was wrong here.

But i think the compiler could be supplied with different versions of printf and let the user decide which printf version is best for him.This way compiler & linker need not struggle to find the best printf.

The optimiser should have used a counter and repeated the following block

    MOV     R3,#0FFH
    MOV     R2,#HIGH ?SC_0
    MOV     R1,#LOW ?SC_0
    LCALL   _puts

2.The Compiler must find and remove the duplicate constant strings

No the compiler allocates the string only once.it was again my mistake

3.Bit Test instructions are not used when i actually test for the bit

I must have used (P0 & 1) instead of (P0 ^ 1) again my mistake.

But when i use (P0 & 1) the compiler understood my intention of bit testing but it has assembled it in a different way.

#include <REGX51.H>
#include <stdio.h>


void main(void)
{
        if(P0 & 1)
        {
                                if(P0_1)
                                {
                P1 = 10;
                                }
        }

}
             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
                                           ; SOURCE LINE # 7
0000 E580              MOV     A,P0
0002 30E006            JNB     ACC.0,?C0003
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
0005 308103            JNB     P0_1,?C0003
                                           ; SOURCE LINE # 10
                                           ; SOURCE LINE # 11
0008 75900A            MOV     P1,#0AH
                                           ; SOURCE LINE # 12
                                           ; SOURCE LINE # 13
                                           ; SOURCE LINE # 15
000B         ?C0003:
000B 22                RET
             ; FUNCTION main (END)

where it could have simply put "JNB P0_1" instead of "mov a,P0" & "jnb ACC.0"

But overall if you compile a source file using an old version of the compiler and again with the new version of the compiler the produced hex file is byte to byte same.

Why the compiler or its optimizer has not improved in reducing the code & data size for many years.

Once again thank for all your patience

S.Sheik mohamed

That's all very well to say - but what features would go in the "small" printf?

Every user would have their own opinion of what is an "important" feature to have in printf, and what is a "luxury".

In the end, it's probably easiest for the compiler to provide a "full" printf, and, if the user has specific requirements, then they code their own according to those requirements.

The Keil C51 tools have been around for many years - decades, in fact - so, obviously, all the easy optimisations were done years ago.

However, since Keil was acquired by ARM, one might tend to suspect that the non-ARM tools might be receiving less focus...

Simple,

One with full functionality.
Another one with floating point support removed.I think this will reduce the code dramatically.

In majority of the application the float may not be neccessary.

Sheik mohamed