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Hello,
1.when i use printf with no var_args then the compiler should call puts instead of printf.
ex1:
#include <REGX51.H> #include <stdio.h> void main(void) { printf("This must call puts instead of printf"); }
Program Size: data=30.1 xdata=0 code=1103
ex2:
#include <REGX51.H> #include <stdio.h> void main(void) { puts("This must call puts instead of printf"); }
Program Size: data=9.0 xdata=0 code=168
The above code links the printf function from the library which is huge(produces 1103 bytes).But the compiler can use puts when there is no var_args given which is much smaller than printf(produces 168 bytes).
2.The Compiler must find and remove the duplicate constant strings
ex3:
#include <REGX51.H> #include <stdio.h> void main(void) { puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); puts("This string gets duplicated as many time as i use it"); }
Program Size: data=9.0 xdata=0 code=334
ex4:
#include <REGX51.H> #include <stdio.h> void main(void) { puts("This string gets duplicated as many time as i use it"); }
Program Size: data=9.0 xdata=0 code=183
3.Bit Test instructions are not used when i actually test for the bit
ex5:
#include <REGX51.H> #include <stdio.h> void main(void) { if(P0^1) { P1 = 10; } } ASSEMBLY LISTING OF GENERATED OBJECT CODE ; FUNCTION main (BEGIN) ; SOURCE LINE # 6 ; SOURCE LINE # 7 ; SOURCE LINE # 8 0000 E580 MOV A,P0 0002 6401 XRL A,#01H 0004 6003 JZ ?C0002 ; SOURCE LINE # 9 ; SOURCE LINE # 10 0006 75900A MOV P1,#0AH ; SOURCE LINE # 11 ; SOURCE LINE # 13 0009 ?C0002: 0009 22 RET ; FUNCTION main (END)
In the above assembly output it should have used a single instruction JNB instead of three MOV,XRL and JZ.This is very basic anybody would object the assembly code produced.
I have not used the compiler much.But the compiler needs a look by the programmers at keil.
The above programs were all compiled with compiler optimisation level set to 9 & favour speed.
About 5 years back i compiled a c51 source code using keil. Now i recompiled the same source code with the latest compiler from keil and compared the two output .hex files. Unfortunately it produced exactly the same output.Here i was expecting some code and data size reduction as the compiler must be capable of optimising more.
It seems there was no improvement on the compiler side.
It is not a complaint but in the interest of improving the compiler.
regards,
S.Sheik mohamed
The Above produces the following assembly code
ASSEMBLY LISTING OF GENERATED OBJECT CODE ; FUNCTION main (BEGIN) ; SOURCE LINE # 4 ; SOURCE LINE # 5 ; SOURCE LINE # 6 0000 7BFF MOV R3,#0FFH 0002 7A00 R MOV R2,#HIGH ?SC_0 0004 7900 R MOV R1,#LOW ?SC_0 ; SOURCE LINE # 7 ; SOURCE LINE # 8 ; SOURCE LINE # 9 ; SOURCE LINE # 10 ; SOURCE LINE # 11 ; SOURCE LINE # 12 ; SOURCE LINE # 13 ; SOURCE LINE # 14 ; SOURCE LINE # 15 ; SOURCE LINE # 16 ; SOURCE LINE # 17 ; SOURCE LINE # 18 ; SOURCE LINE # 19 ; SOURCE LINE # 20 ; SOURCE LINE # 21 ; SOURCE LINE # 22 0006 120000 R LCALL L?0002 ; SOURCE LINE # 23 ; SOURCE LINE # 24 ; SOURCE LINE # 25 ; SOURCE LINE # 26 ; SOURCE LINE # 27 ; SOURCE LINE # 28 ; SOURCE LINE # 29 ; SOURCE LINE # 30 ; SOURCE LINE # 31 ; SOURCE LINE # 32 ; SOURCE LINE # 33 ; SOURCE LINE # 34 ; SOURCE LINE # 35 ; SOURCE LINE # 36 ; SOURCE LINE # 37 ; SOURCE LINE # 38 0009 120000 R LCALL L?0002 000C 020000 E LJMP _puts ; SOURCE LINE # 39 000F L?0002: 000F 120000 E LCALL _puts 0012 7BFF MOV R3,#0FFH 0014 7A00 R MOV R2,#HIGH ?SC_0 0016 7900 R MOV R1,#LOW ?SC_0 0018 120000 E LCALL _puts 001B 7BFF MOV R3,#0FFH 001D 7A00 R MOV R2,#HIGH ?SC_0 001F 7900 R MOV R1,#LOW ?SC_0 0021 120000 E LCALL _puts 0024 7BFF MOV R3,#0FFH 0026 7A00 R MOV R2,#HIGH ?SC_0 0028 7900 R MOV R1,#LOW ?SC_0 002A 120000 E LCALL _puts 002D 7BFF MOV R3,#0FFH 002F 7A00 R MOV R2,#HIGH ?SC_0 C51 COMPILER V9.02 T 12/12/2010 06:18:02 PAGE 3 0031 7900 R MOV R1,#LOW ?SC_0 0033 120000 E LCALL _puts 0036 7BFF MOV R3,#0FFH 0038 7A00 R MOV R2,#HIGH ?SC_0 003A 7900 R MOV R1,#LOW ?SC_0 003C 120000 E LCALL _puts 003F 7BFF MOV R3,#0FFH 0041 7A00 R MOV R2,#HIGH ?SC_0 0043 7900 R MOV R1,#LOW ?SC_0 0045 120000 E LCALL _puts 0048 7BFF MOV R3,#0FFH 004A 7A00 R MOV R2,#HIGH ?SC_0 004C 7900 R MOV R1,#LOW ?SC_0 004E 120000 E LCALL _puts 0051 7BFF MOV R3,#0FFH 0053 7A00 R MOV R2,#HIGH ?SC_0 0055 7900 R MOV R1,#LOW ?SC_0 0057 120000 E LCALL _puts 005A 7BFF MOV R3,#0FFH 005C 7A00 R MOV R2,#HIGH ?SC_0 005E 7900 R MOV R1,#LOW ?SC_0 0060 120000 E LCALL _puts 0063 7BFF MOV R3,#0FFH 0065 7A00 R MOV R2,#HIGH ?SC_0 0067 7900 R MOV R1,#LOW ?SC_0 0069 120000 E LCALL _puts 006C 7BFF MOV R3,#0FFH 006E 7A00 R MOV R2,#HIGH ?SC_0 0070 7900 R MOV R1,#LOW ?SC_0 0072 120000 E LCALL _puts 0075 7BFF MOV R3,#0FFH 0077 7A00 R MOV R2,#HIGH ?SC_0 0079 7900 R MOV R1,#LOW ?SC_0 007B 120000 E LCALL _puts 007E 7BFF MOV R3,#0FFH 0080 7A00 R MOV R2,#HIGH ?SC_0 0082 7900 R MOV R1,#LOW ?SC_0 0084 120000 E LCALL _puts 0087 7BFF MOV R3,#0FFH 0089 7A00 R MOV R2,#HIGH ?SC_0 008B 7900 R MOV R1,#LOW ?SC_0 008D 120000 E LCALL _puts 0090 7BFF MOV R3,#0FFH 0092 7A00 R MOV R2,#HIGH ?SC_0 0094 7900 R MOV R1,#LOW ?SC_0 0096 120000 E LCALL _puts 0099 7BFF MOV R3,#0FFH 009B 7A00 R MOV R2,#HIGH ?SC_0 009D 7900 R MOV R1,#LOW ?SC_0 009F 22 RET ; FUNCTION main (END)
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
I am not accusing,complaining or under estimating any one either at keil or in this forum I think only when we discuss we can improve.
If by anyway i hurted anybody's heart please forgive me!!
No, it does not - because the code that you have written does not test for a bit!
As already explained to you, the code you have written contains an expression using the ANSI Standard exclusive-OR operator.
Perhaps you are confusing the meaning of "bitwise", as used by the ANSI Standard, with the individual bit operations of the 8051...?
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.
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.
No the compiler allocates the string only once.it was again my mistake
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
"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.
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.
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!