Hi everybody,
I noticed a strange behavior when executing certain shift operations and I
wonder whether this is because of a compiler bug or just my ignorance in
low-level C.
For example, when compiling (with no optimization) and then executing the
following lines of code the result I obtain is 16 instead of 64:
uint32_t u32_var;
int main(void)
{
int tmp;
...
u32_var = 0xa;
tmp = 16 << (u32_var & 0x07);
printf("tmp: %d\n", tmp);
...
}
However, if I change the type of tmp form "int" to "unsigned int" the
result is correct (i.e., 64).
This is assembly code generated when tmp is defined as int (i.e., when the
result is wrong).
5c44: b2 40 0a 00 mov #10, &0x1c02 ;#0x000a
5c48: 02 1c
5c4a: 82 43 04 1c mov #0, &0x1c04 ;r3 As==00
5c4e: 1e 42 02 1c mov &0x1c02,r14
5c52: 1f 42 04 1c mov &0x1c04,r15
5c56: 0f 4e mov r14, r15
5c58: 3f f0 07 00 and #7, r15 ;#0x0007
5c5c: 3e 40 10 00 mov #16, r14 ;#0x0010
5c60: 4f 4f mov.b r15, r15
5c62: 7f f0 0f 00 and.b #15, r15 ;#0x000f
5c66: 84 4e fc ff mov r14, -4(r4) ;0xfffc(r4)
5c6a: 4f 93 tst.b r15
5c6c: 03 24 jz $+8 ;abs 0x5c74
5c6e: 7f 53 add.b #-1, r15 ;r3 As==11
5c70: cf 18 0e 5e .rpt r15
addx r14, r14
5c74: 14 12 fc ff push -4(r4) ;0xfffc(r4)
5c78: 30 12 9a 66 push #26266 ;#0x669a
5c7c: b0 12 ae 5c call #0x5cae
As you can see, at address 0x5c66, register 14 (containing 16) is stored in
memory before being shifted. The stored valued (i.e., 16) is then pushed in
the stack (i.e., passed as an argument to the printf() function).
I have a similar problem also when using -Os, but I can't replicate it with
simple code because of the optimization. However, if needed, I can post the
(large) source code that is giving me this problem.
The version of msp430-gcc that I am using is:
msp430-gcc (GCC) 4.7.0 20120322 (mspgcc dev 20120911)
Thank you for any help you can provide.
Regards.
Daniele
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