[llvm-dev] [RFC] Adding range metadata to array subscripts. (original) (raw)

James Courtier-Dutton via llvm-dev llvm-dev at lists.llvm.org
Sun Mar 28 01:49:51 PDT 2021

Hi,

char* test_fill(int size) { char *test1 = malloc(size) for (n = 0; n <= size; n++) { test1[n] = 'A'; } }

Would it be worth making the "range" information a little richer and be able to use algebraic expressions as well as numeric ranges. Note: the above example code has an off by one overflow, and it would be helpful if one could catch that at compile time. In this case, it could catch that n must be less than size, and not less than or equal to size. Thus putting the range value on the test1 pointer as being from address of test1 to test1 + (size - 1)

This can only be achieved if algebraic expressions are used for ranges, and not just constant values. Actual use cases can get much more complicated with for example, non-contiguous ranges. e.g. 0,1,4,5 ok, but 2,3,6,7 not ok.

Another useful thing to catch at compile time, would be a warning that a pointer is being dereferenced, and we were not able to apply a range expression to it. I.e. warn about unbounded dereferences.

I think it would be useful to at least consider how we would capture this more complex range information/metadata in LLVM IR.

Kind Regards

James

>>>> On 3/24/21 9:06 AM, Clement Courbet wrote: >>>>> On Wed, Mar 24, 2021 at 2:20 PM Johannes Doerfert <_ _>>>>> johannesdoerfert at gmail.com> wrote: >>>>> >>>>>> I really like encoding more (range) information in the IR, >>>>>> more thoughts inlined. >>>>>> >>>>>> On 3/24/21 4:14 AM, Clement Courbet via llvm-dev wrote: >>>>>>> struct Histogram { >>>>>>> >>>>>>> int values[256]; >>>>>>> >>>>>>> int total; >>>>>>> >>>>>>> }; >>>>>>> >>>>>>> Histogram DoIt(const int* image, int size) { >>>>>>> >>>>>>> Histogram histogram; >>>>>>> >>>>>>> for (int i = 0; i < size; ++i) {_ _>>>>>>> >>>>>>> ++histogram.values[image[i]]; // (A) >>>>>>> >>>>>>> ++histogram.total; // (B) >>>>>>> >>>>>>> } >>>>>>> >>>>>>> return histogram; >>>>>>> >>>>>>> }

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