Thanks!
> I'd actually do this down at the gimple level. You'll have an SSA graph you
> can use to identify the masking and verify its producing a single bit result.
> You'll also have canonicalized comparisons, so there'll be fewer things to
> test. Depending on exactly where you put the optimization, you may also see
> more constants on the RHS due to propagation.
Ok, I can move it to the gimple level. However I don't know much about
GCC internals so I'd like to know what file/function I should look at.
> A completely different approach would be to have VRP identify objects which
> have values that are only powers of 2. Once such a value is in the lattice,
> you can identify and warn/optimize when they're compared against values which
> aren't powers of 2.
I don't want to limit this to numbers that are only powers of 2.
I want to catch this also:
if ((relocation & 0xfffc0003) != 0xffff0000)
Actual code: https://lkml.org/lkml/2011/11/17/344
I determine if a condition ((x & 0xc) != 0xf) is always true by
checking if this is true:
((0xc & 0xf) != 0xf)
I wonder if I can check if a tree is a macro or template parameter
somehow. I don't know if there are any false positives with
macros/templates but it might be a good idea to skip warnings if the
expression is a expanded macro or if any value is a template
parameter.
Best regards,
Daniel Marjamäki
2014-04-16 23:17 GMT+02:00 Jeff Law <[email protected]>:
> On 04/16/14 09:27, Daniel Marjamäki wrote:
>>
>> Hello!
>>
>> I am new to GCC.
>>
>> I want to add a warning to GCC when bit comparison is always true/false.
>>
>> Example:
>>
>> if ((x&4)==0) {} // <- no warning
>>
>> if ((x&4)==4) {} // <- no warning
>>
>> if ((x&4)==5) {} // <- warn!
>>
>> When this warning is triggered, the most common cause is that somebody
>> made a mistake when using bitmasks.
>>
>> I attach a proof-of-concept patch. I would like comments.
>>
>> The patch needs some cleanup before it's applied.. I would like it to
>> handle at least != also and not just ==. And I would like it to be
>> less strict about where integer constants are located.
>>
>> I wonder where I should put this code. Is gcc/c/c-typeck.c a good file
>> to put this in? Should I put it in somewhere else?
>>
>> What warning flags should be used to enable this? Is some
>> -Wcondition-bitop a good idea? Can this be added by -Wall?
>>
>> I wrote this check for Cppcheck years ago. In my experience this
>> warning has a good signal/noise ratio.
>
> I'd actually do this down at the gimple level. You'll have an SSA graph you
> can use to identify the masking and verify its producing a single bit
> result. You'll also have canonicalized comparisons, so there'll be fewer
> things to test. Depending on exactly where you put the optimization, you
> may also see more constants on the RHS due to propagation.
>
>
> A completely different approach would be to have VRP identify objects which
> have values that are only powers of 2. Once such a value is in the lattice,
> you can identify and warn/optimize when they're compared against values
> which aren't powers of 2.
>
> Jeff
>>
>>
>> Best regards,
>> Daniel Marjamäki
>>
>
