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[analyzer] Do not assume that all pointers have the same bitwidth as void*

Authored by vabridgers on Jul 14 2021, 5:00 AM.

Description

[analyzer] Do not assume that all pointers have the same bitwidth as void*

This change addresses this assertion that occurs in a downstream
compiler with a custom target.

APInt.h:1151: bool llvm::APInt::operator==(const llvm::APInt &) const: Assertion `BitWidth == RHS.BitWidth && "Comparison requires equal bit widths"'

No covering test case is susbmitted with this change since this crash
cannot be reproduced using any upstream supported target. The test case
that exposes this issue is as simple as:

void test(int * p) {
  int * q = p-1;
  if (q) {}
  if (q) {} // crash
  (void)q;
}

The custom target that exposes this problem supports two address spaces,
16-bit chars, and a _Bool type that maps to 16-bits. There are no upstream
supported targets with similar attributes.

The assertion appears to be happening as a result of evaluating the
SymIntExpr (reg_$0<int * p>) != 0U in VisitSymIntExpr located in
SimpleSValBuilder.cpp. The LHS is evaluated to 32b and the RHS is
evaluated to 16b. This eventually leads to the assertion in APInt.h.

While this change addresses the crash and passes LITs, two follow-ups
are required:

  1. The remainder of getZeroWithPtrWidth() and getIntWithPtrWidth() should be cleaned up following this model to prevent future confusion.
  2. We're not sure why references are found along with the modified code path, that should not be the case. A more principled fix may be found after some further comprehension of why this is the case.

Acks: Thanks to @steakhal and @martong for the discussions leading to this
fix.

Reviewed By: NoQ

Differential Revision: https://reviews.llvm.org/D105974

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