[PATCH] Warn on mismatching types to sizeof for memset and friends where length is of the form sizeof(Type) * factor.

[Nico Weber]

Did the warning find any legitimate bugs?

On Wed, Apr 8, 2015 at 5:09 PM, Oliver Chang <ochang at google.com> wrote:

> Here are some general results after some quick testing (some of which is
> stolen from thakis's testing on chromium). Unfortunately some of these
> warnings were difficult to triage and requires a significant amount of time
> (which I think makes a point for this warning).
>
> Discounting duplicates in the same file, both Firefox and Chromium give
> around 8-10 warnings.
>
> Some patterns observed:
>
> - Most seem to just be based on size assumptions between types, e.g. 2 *
> sizeof(uint16_t) == sizeof(uint32_t).
>
> Firefox had about 2 instances where 2 different classes were expected to
> have the same size (e.g. when 1 is a wrapper around another).
>
> - In Chromium there are some issues with structs and unions that solely
> contained members of a single type.
>
> e.g.
> struct M {
>
>   int A[4];
>
> };
>
> M a;
> memset(&a, 0, sizeof(int) * 4);
>
> thakis@ suggested decaying the struct type to the member type in this
> case for the comparison.
>
> - Firefox also had about 2 warnings related to multidimensional arrays,
> e.g.
>
> typedef int Foo[10];
> Foo m;
> memset(&m, 0, sizeof(Foo));
>
> Perhaps we can just compare the type 'Foo' in this case before trying to
> break it down further in this case.
>
> - There are some other warnings such as differences complex_t vs float[2],
> w_char vs short.
>
>
> http://reviews.llvm.org/D8881
>
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