Improve equivalence checking of pointer arithmetic expressions

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#596

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The Checked C specification calls for expanding pointer arithmetic to bytewise arithmetic before checking equivalence of expressions. We're not doing that, which is causing various pointer expressions to not be considered equivalent when they are.

For example, given an expression e1 that is a pointer to T, (Array_ptr<char>) e1 + (sizeof(T) * i) is equivalent to e + i.

This kind of arithmetic arises when a programmer declared bounds for a variable using count, but a programmer does explicit size calculation as part of allocation. For example:

_Array_ptr<int> x : count(i) = malloc(sizeof(int) * i)

[secure-sw-dev-bot]

Comment from @dtarditi:

With the changes to address work item #594, the compiler will emit additional warning messages when compiling the PtrDist and Olden benchmarks that have been converted to Checked C. The additional messages are due to this issue when malloc or calloc are used.

Here are the benchmarks that had additional warnings:

• Olden

  • EM3D: make_graph.c (use of calloc)
  • MST: hash.c, makegraphic. (use of calloc, also requires bundled blocks).

• PtrDist

  • Anagram: anagram.c (use of calloc)
  • YACR2
    • channel.c: use of malloc
    • hcg.c: use of malloc
    • maze.c: use of free. Note: there are unrelated warningss due to lack of relational arithmetic. Need to check for possible off by 1 issue
    • vcg.c: use of malloc and free

[secure-sw-dev-bot]

Comment from @mgrang:

@dtarditi Can we close this bug now that this has been implemented by @ppashakhanloo ?