We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.
Hello,
We came across a mystery/bug yesterday. This was tested with C51 v9.53 (Simplicity Studio) and 9.56 (uVision).
unsigned char data c; unsigned int data d; // The values 0x7B and 0xFF7B are wrong c = (200 - 50) * (255 - 0) / (255 - 50); 0000738d: MOV 65H, #7BH d = (200 - 50) * (255 - 0) / (255 - 50); 00007390: MOV 66H, #0FFH 00007393: MOV 67H, #7BH //These are correct c = (200 - 50) * (255u - 0) / (255 - 50); 0000738d: MOV 65H, #0BAH d = (200 - 50) * (255u - 0) / (255 - 50); 00007390: MOV 66H, #00H 00007393: MOV 67H, #0BAH
The uVision docs say that numeric constants default to 16 bits. Is this not true? Or is this an issue with the "Constant Folding" optimizing step?
Any insights appreciated, Darren
In the above example, the constant 38250 was upgraded to unsigned int.
No. Or rather: if it had been, that would have constituted a compiler bug. For a 16-bit target, the constant 38250 is of type (signed) long int. Which is at least 32 bit wide, but could be wider.
My guess is unsigned int is 16-bits on your machine.
No need for a guess: it's C51, so yes, int is 16 bits wide.
In this case you specify to the compiler a type "higher" than integer (unsigned integer) so all are converted to this.
No. Only one of the constants in that expression will be converted to unsigned: the 0 in (255u - 0). The other 4 stay as signed ints. Then the results of their subtractions get converted to unsigned. I.e. what actually happens is equivalent to:
((unsigned)(200 - 50) * (255u - (unsigned)0)) / (unsigned)(255 - 50)
but the -1 will cause a "problem" as it does not fit within the range of an unsigned int.
No it won't, because that -1 is never promoted to unsigned.
For OP -
You were correct that 3000000 would be considered a long int (32-bit signed value). My guess on the SYSCLK would make no difference to this. Any comments based on SYSCLK "helping" to upconvert the 3000000 should be ignored. Hans-Bernhard Broekers specific example of when the conversions to unsigned int happen are correct.
//working - all 16-bit calculations, signed on 2 of the inner calculations, unsigned on the other inner calculation, unsigned on all 3 of the "outer" most calculations. ((unsigned)(200 - 50) * (255u - (unsigned)0)) / (unsigned)(255 - 50) //original - all 16-bit signed calculations (unsigned) ((200 - 50) * (255 - 0)) / (255 - 50))
Where a signed constant such as -1 will always be signed, calculations that use this constant can do conversions on it.
unsigned x = 255u * -1; // 65281 - but you already know this can happen. unsigned x = 255u * ((unsigned) -1);