Type Promotion in C
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I understand that the shifts take place in registers but then the evaluated values have to be stored back into their respective variables, which are of type short and char, which cannot hold 4-byte values.
Ralph_2 wrote:
but then the evaluated values have to be stored back into their respective variables
Nope. Why do you think that?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Ralph_2 wrote:
but then the evaluated values have to be stored back into their respective variables
Nope. Why do you think that?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
unsigned long i; //4-bytes, int: 4-bytes
short s; //2-bytes
signed char c; //1-bytei = (s<<15) + (c <<9);
How will type conversion take place in this case? Is it that: 1. s will remain short(2 bytes) thus shifting the LSB into the MSB and remaining bits turn 0; c is promoted to short then left-shift 9 performed. The final R-value is promoted to unsigned long and saved into i. or 2. s and c both get converted to unsigned long before performing the shift operation (thus we do not lose any of the left-shifted bits) or something else happens. I tried it in VS6.0 and the result seem to indicate that in both s and c I did not lose any bit upon left-shifting as if s and c acted as if they were a 4-byte data type (long or int)
please have look on the msdn [^]
Величие не Бога может быть недооценена.
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Ralph_2 wrote:
but then the evaluated values have to be stored back into their respective variables
Nope. Why do you think that?
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Do you mean to say that since these are R-values they themselves do not get modified. Well, yes, my mistake. R-values will not be altered. My statement is incorrect. So now that we are talking at register-level then where and how does C's rules of type conversion come into picture?
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Do you mean to say that since these are R-values they themselves do not get modified. Well, yes, my mistake. R-values will not be altered. My statement is incorrect. So now that we are talking at register-level then where and how does C's rules of type conversion come into picture?
Without talking about registers, types promotion occurs because of (in the original expression) the assignment was done to an integer. [edit] actually, running the following code
unsigned int u = 0xFFFFFFFF;
unsigned long long ul = (u << 8);make me think the above expression cannot be true :^) [/edit] :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]modified on Thursday, October 22, 2009 4:08 AM
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Without talking about registers, types promotion occurs because of (in the original expression) the assignment was done to an integer. [edit] actually, running the following code
unsigned int u = 0xFFFFFFFF;
unsigned long long ul = (u << 8);make me think the above expression cannot be true :^) [/edit] :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]modified on Thursday, October 22, 2009 4:08 AM
In the book C-The Complete Reference by Herbert Schildt theree is an example
char ch;
int i;
float f
double d;
result=(ch/i)+(f*d)-(f+i)Here it does not talk about the data type of the variable "result". In any case, the conversion here is as follows: First ch is converted to int since i is int, then ch/i is performed. The result of ch/i is then converted to double as f*d is double. The final value is a double. Now if "result" is type float then the final value will be truncated to float and stored in "result", isnt it?
CPallini wrote:
types promotion occurs because of (in the original expressio) the assignment was done to an integer.
As we see in the above example the variable to which the final value is assigned, its type matters only when we have solved the expression on the right side. Please correct me if wrong anywhere. The mentioned book also says: "First, all char and short int values are automatically elevated to int" This seems to be the reason why I don't lose the shifted out bits and also because I store the value back in an unsigned long :)
modified on Thursday, October 22, 2009 4:51 AM
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In the book C-The Complete Reference by Herbert Schildt theree is an example
char ch;
int i;
float f
double d;
result=(ch/i)+(f*d)-(f+i)Here it does not talk about the data type of the variable "result". In any case, the conversion here is as follows: First ch is converted to int since i is int, then ch/i is performed. The result of ch/i is then converted to double as f*d is double. The final value is a double. Now if "result" is type float then the final value will be truncated to float and stored in "result", isnt it?
CPallini wrote:
types promotion occurs because of (in the original expressio) the assignment was done to an integer.
As we see in the above example the variable to which the final value is assigned, its type matters only when we have solved the expression on the right side. Please correct me if wrong anywhere. The mentioned book also says: "First, all char and short int values are automatically elevated to int" This seems to be the reason why I don't lose the shifted out bits and also because I store the value back in an unsigned long :)
modified on Thursday, October 22, 2009 4:51 AM
Ralph_2 wrote:
Now if "result" is type float then the final value will be truncated to float and stored in "result", isnt it?
Yes (and the compiler warns about).
Ralph_2 wrote:
As we see in the above example the variable to which the final value is assigned, its type matters only when we have solved the expression on the right side.
Yes. As I added in my previous post, you're right and looks like registers matter... :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
In the book C-The Complete Reference by Herbert Schildt theree is an example
char ch;
int i;
float f
double d;
result=(ch/i)+(f*d)-(f+i)Here it does not talk about the data type of the variable "result". In any case, the conversion here is as follows: First ch is converted to int since i is int, then ch/i is performed. The result of ch/i is then converted to double as f*d is double. The final value is a double. Now if "result" is type float then the final value will be truncated to float and stored in "result", isnt it?
CPallini wrote:
types promotion occurs because of (in the original expressio) the assignment was done to an integer.
As we see in the above example the variable to which the final value is assigned, its type matters only when we have solved the expression on the right side. Please correct me if wrong anywhere. The mentioned book also says: "First, all char and short int values are automatically elevated to int" This seems to be the reason why I don't lose the shifted out bits and also because I store the value back in an unsigned long :)
modified on Thursday, October 22, 2009 4:51 AM
Ralph_2 wrote:
"First, all char and short int values are automatically elevated to int"
Yes, and it is also explained here [^]. I didn't know
inthad such a special role. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles] -
Ralph_2 wrote:
"First, all char and short int values are automatically elevated to int"
Yes, and it is also explained here [^]. I didn't know
inthad such a special role. :)If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]Thanks for your assistance. So the answer to my question is "Integral promotion" :) Now, if only
intwere 2-bytes then I lose the left-shifted bits and will have a different result forunsigned long = (short<<15) + (char<<9).....Ahem, i hope this is right now ;P -
I understand that the shifts take place in registers but then the evaluated values have to be stored back into their respective variables, which are of type short and char, which cannot hold 4-byte values.
This is the assembly output for the statement
i = (s << 15) + (c << 9);movsx eax, WORD PTR _s$[ebp]
shl eax, 15 ; 0000000fH
movsx ecx, BYTE PTR _c$[ebp]
shl ecx, 9
add eax, ecx
mov DWORD PTR _i$[ebp], eaxSo as you can see nothing is stored back into any variable. The two shifts happen in the
eaxandecxregisters. It is then added together and the stored into the variable i, which is also 4 bytes in length.«_Superman_» I love work. It gives me something to do between weekends.
Microsoft MVP (Visual C++)
