On Dec 19 2006 13:24, David Rientjes wrote:
>On Tue, 19 Dec 2006, Jan Engelhardt wrote:
>
>> > I don't know if anyone cares about them anymore, since I think gcc
>> > grew some smarts in the area recently, but there are a lot of lines of
>> > code matching "static int.*= *0;" and equivalents in the driver tree.
>>
>> I'd really like to see the C compiler being enhanced to detect
>> "stupid casts", i.e. those, which when removed, do not change (a) the outcome
>> (b) the compiler warnings/error output.
>
>If your desire is for the compiler warnings output to be unchanged, I'm
Oh well maybe I suck at writing exact definitions in one-phrase phrases.
Anyway, what was intended:
(1) Catch casts where they are usually not necessary, because the value
would be anyhow discarded
(void)strcpy(b, a);
(Does not apply to e.g. cmpxchg -- that's why I mentioned (b)!)
(2) Catch casts which do not change the type of an expression, e.g.
void *x = (void *)kmalloc(...)
(3) Catch casts which do not change the outcome, because from-to-void* is
warningless
struct foo *bar = (struct foo *)kmalloc(...)
Quite often seen in kernel code.
(4) extension and truncation, implicit conversions - unneessary casts
func_taking_u32( (u32)some_u16 );
func_taking_u16( (u16)some_u32 );
(5) Slightly harder one: Where the evaluation process changes, but the
outcome is the same
#define V_FL_BASE_HI(x) ((x) << 8)
#define V_FL_INDEX_LO(x) (x)
#define M_FL_INDEX_LO 0xFF
static void t3_write_reg(struct adapter *, u32, u32);
int t3_sge_init_flcntxt(struct adapter *adapter, unsigned int id,
int gts_enable, u64 base_addr, unsigned int size,
unsigned int bsize, unsigned int cong_thres, int gen,
unsigned int cidx)
{
t3_write_reg(adapter, A_SG_CONTEXT_DATA1,
V_FL_BASE_HI((u32) base_addr) |
V_FL_INDEX_LO(cidx & M_FL_INDEX_LO));
}
(Note that this is not exactly code found in the cxgb3 driver, but
tweaked for this example)
As far as I can see, even if base_addr was not truncated to u32, the end
result would be, when the u64 value is passed to t3_write_reg.
>not sure how you'd enhance the compiler from detecting these casts. All
>of the casts that have been removed in these cleanup patches do not change
>the assembly when using gcc; they simply reduce the amount of visual noise
>in the source code.
>
>This is also true in terms of global static variables being initialized to
>0 (or NULL). While it is indeed unnecessary by the standard, it simply
>moves the initialization from one segment of the assembly to the other,
>regardless of how many different functions it is referenced in. gcc does
>not emit movl $0, var for these cases.
>
>It _would_ be helpful to add a macro such as:
>
> #define SILENCE_GCC(x) = x
>
>to eliminate warnings such that:
>
> auto int a SILENCE_GCC(a);
> fill_a(&a);
> if (a)
> ...
>
>would not produce a "may be used uninitialized" warning.
__attribute__((used)) would be more appropriate, I think.
-`J'
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