Compiler optimisation

1) printf() does not do the same thing as puts() even if you send a constant not containing any % parameter expansions. Haven't you stil read up on puts() and seen what it does, _besides_ emitting the text string?

Well? I'm waiting. Have you read the documentation for puts yet? Still waiting...

2) You want the fastest code? Converting a sequence of calls put puts() with a loop isn't faster. Many optimizing compilers actually does the reverse. They do loop unrolling where they intentionally duplicate the code inside the loop to reduce the number of loop iterations - sometimes totally unrolling the loop so no loop operation remains.

3) my_byte XOR 1 inverts one bit in a byte, and then tests if the byte result is non-zero. That is not a single-bit operation that may use any bit instructins in the 8051 processor. The ^ may _only_ be used when declaring bit variables, since it is an overload of a standard C operator for XOR. And the standard C operator performs bit operations on full bytes/shorts/ints/long ints and not on a single bit.

Somehow, you have to switch from output mode into input mode. You must pick up the feedback you get in the forum, and not just run along further and further away on the wrong track.

1) printf() does not do the same thing as puts() even if you send a constant not containing any % parameter expansions. Haven't you stil read up on puts() and seen what it does, _besides_ emitting the text string?

Well? I'm waiting. Have you read the documentation for puts yet? Still waiting...

2) You want the fastest code? Converting a sequence of calls put puts() with a loop isn't faster. Many optimizing compilers actually does the reverse. They do loop unrolling where they intentionally duplicate the code inside the loop to reduce the number of loop iterations - sometimes totally unrolling the loop so no loop operation remains.

3) my_byte XOR 1 inverts one bit in a byte, and then tests if the byte result is non-zero. That is not a single-bit operation that may use any bit instructins in the 8051 processor. The ^ may _only_ be used when declaring bit variables, since it is an overload of a standard C operator for XOR. And the standard C operator performs bit operations on full bytes/shorts/ints/long ints and not on a single bit.

Somehow, you have to switch from output mode into input mode. You must pick up the feedback you get in the forum, and not just run along further and further away on the wrong track.

www.cplusplus.com/.../
www.cplusplus.com/.../

Spot the difference!

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

"The ^ may _only_ be used when declaring bit variables"

Here is where i got confused.

Because i had used something like P0^1 so i thought that is the only way to reference a bit.
i do not know why keil chose to use P0^1 to declare bits instead of P0.1
I just used keil only after about 5 years.in fact i do not use MCS51.

Thanks

Sheik mohamed

Yes, it is a very common source of confusion - it caught me out when I first started with C51!

It does seem to be a rather poor choice on Keil's part, and it is certainly not well explained in the manual.

:-(

"I must have used (P0 & 1) instead of (P0 ^ 1)"

No - that is still a whole byte operation!

if you want to use the 8051's single-bit features, then you have to define a single-bit variable:

sbit P0_1 = P0 ^ 1; // Define a single-bit variable

:

if( P0_1 )          // Test the single-bit variable

:

P0_1 = 1;           // Set the single-bit variable

:

P0_1 = 0;           // Clear the single-bit variable

http://www.keil.com/support/man/docs/c51/c51_le_sbit.htm

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...

Yes, I got your point.

But when i use (P0 & 1) the compiler knew that i was testing for bit and it has assembled the right bit instruction for testing bit and not "AND" instruction for testing the whole byte.That is very nice & wise of the compiler.But the compiler did not check if the address is bit-addressable or not.If it had found the address is bit-addressable then it could have assembled more specific "jb" instruction.

Sheik mohamed

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

No, you clearly didn't!

"when i use (P0 & 1) the compiler knew that i was testing for bit"

No, it does not!!

P0 is an 8-bit value;

1 is an integral constant.

In strict ANSI 'C', the integral constant is considered an int, and the 8-bit value would be promoted to an int before doing a bitwise 'AND' of all bits and giving an int result.
Effectively, the expression is:

( P0 & 0x0001 )

Keil C51 gives you the option to disable this promotion, so that the expression becomes just an 8-bit operation.

But the only way to get Keil C51 to operate on a single bit is to use the specific bit operations.

Again, ANSI 'C' bitwise operators have noting to do with the 8051's single-bit operations!

In strict ANSI 'C', the integral constant is considered an int, and the 8-bit value would be promoted to an int before doing a bitwise 'AND' of all bits and giving an int result.

With the integer promotion, the semantics of the operation do not change: the code is still testing for a bit. In this particular case, if the compiler knew that the register is bit-addressable and chose not to ignore this information it could test for this bit directly.

But the only way to get Keil C51 to operate on a single bit is to use the specific bit operations.

Well, that's the point. A smarter compiler will use faster and more compact code constructs where appropriate. This is clearly one of those cases.

Again, ANSI 'C' bitwise operators have noting to do with the 8051's single-bit operations!

I'm not sure what you mean by that. I've seen the Green Hills compiler for Coldfire generate BSET and BCLR instructions when I was using bitwise OR and bitwise AND operators to set or clear bits. Why wouldn't C51 do the same? Especially since the 8051 core does a read-modify-write internally to set or clear bits, so semantics are the same.

Note that the compiler can be smart enough to notice that x & 1 can be seen as a bit operation. It would be a lousy compiler if it couldn't.

But P0 is a volatile 8-bit variable. And the language standard requiers that the compiler does perform the volatile access. So the compiler can't do a single-bit access to a bit variable that just _happens_ to be aliased to one bit of the 8-bit P0 special function register. The compiler _must_ do the full 8-bit read of P0. Then it can use its optimization abilities to convert (x & 1) into a bit operation instead of a full byte-wise and followed by a check of the zero flag.

When you do declare a bit variable P0_0, then the compiler isn't bound by the language standard into having to read the full P0. You have explicitly then told the compiler that you want an access to a single-bit variable, and the compiler may perform such a single-bit access without involving the accumulator.

Remember that it isn't obvious what special actions that may be trigged by accesses to an SFR. In some situations, the byte read may acknowledge an interrupt mechanism. In some situations, the access may control the latching of a 16-bit timer value into two 8-bit SFR.

Keil do not want to add a lot of special code intentionally violating the language standard just because they have considered it "safe" (an assumption since the world is full of 8051 variants with varying "special" features added) to violate the language standard in special situations. And there is no need to add such special code since they have explicitly given you the means to declare bit variables to explicitly tell the compiler to perform just a bit access.