I think Keil C51 version 8 has a bug in the code generation. The following source shows the bug:
typedef unsigned char BYTE; typedef unsigned short WORD; #define FIXED_VAULE 64 #define MAKEWORD(h, l) (((WORD )(h) << 8) | (BYTE )(l)) void main(void) { BYTE x = 2; BYTE y = 2; WORD z; // The code generation for this line is incorrect z = MAKEWORD(FIXED_VAULE - x - 1, FIXED_VAULE - y - 1); // The Keil v8 asm listing shows the generated code for this // line is different to the previous line's. z = MAKEWORD(FIXED_VAULE - 1 - x, FIXED_VAULE - 1 - y); // However, if MAKEWORD is defined as: // #define MAKEWORD(h, l) (((WORD )(h) << 8) | (WORD )(l)) // the ASM code for 2 lines are the same. }
Hi,
I don't have access to the compiler at the moment to see what I get.
Could you say what the results of the code are? i.e., what is z after step#1 and step#2.
Is there a difference when the optimisation level is changed?
Maybe you should make z volatile - The compiler might see a chance to optimize out the first calculation to some extent.
Remember, the compiler and optimizer are free to shuffle around code as they see fit - So long as the result is the same.
If the results are correct but the underlying code produced by the compiler is different, then there is (probably) no problem.
The incorrectly generated code is:
;---- Variable 'x' assigned to Register 'R5' ---- ;---- Variable 'y' assigned to Register 'R4' ---- MOV R4,#02H CLR C MOV A,#03FH SUBB A,#02H MOV R7,A MOV A,R7 MOV R6,A MOV A,R4 CPL A ; Incorrect INC A ; Incorrect CLR C ; Incorrect SUBB A,#03FH ; Incorrect MOV R3,A MOV A,R6 MOV z,A MOV A,R3 MOV z+01H,A
z should be 0x3d3d. However, the result of the incorrectly generated code is 0x3dbf
Are you seeing this with version 8.15 (i.e., the latest release)?
I tried the code with v8.05 and v8.15.
The code is OK with v7.50a
I see Keil have just come out with another update 8.16a
Should be worth trying with that.