[clang] [clang] add array out-of-bounds access constraints using llvm.assume (PR #159046)

[Sebastian Pop via cfe-commits]

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@@ -4559,6 +4559,134 @@ void CodeGenFunction::EmitCountedByBoundsChecking(
   }
 }
 
+/// Emit array bounds constraints using llvm.assume for optimization hints.
+///
+/// C Standard (ISO/IEC 9899:2011 - C11)
+/// Section J.2 (Undefined behavior): An array subscript is out of range, even
+/// if an object is apparently accessible with the given subscript (as in the
+/// lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).
+///
+/// Section 6.5.6 (Additive operators): If both the pointer operand and the
+/// result point to elements of the same array object, or one past the last
+/// element of the array object, the evaluation shall not produce an overflow;
+/// otherwise, the behavior is undefined.
+///
+/// C++ Standard (ISO/IEC 14882 - 2017)
+/// Section 8.7 (Additive operators):
+/// 4 When an expression that has integral type is added to or subtracted from a
+///   pointer, the result has the type of the pointer operand. If the expression
+///   P points to element x[i] of an array object x with n elements,^86 the
+///   expressions P + J and J + P (where J has the value j) point to the
+///   (possibly-hypothetical) element x[i + j] if 0 ≤ i + j ≤ n; otherwise, the
+///   behavior is undefined. Likewise, the expression P - J points to the
+///   (possibly-hypothetical) element x[i − j] if 0 ≤ i − j ≤ n; otherwise, the
+///   behavior is undefined.
+/// ^86 A pointer past the last element of an array x of n elements is
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sebpop wrote:

That is a foot note as it appears in the standard pdf doc, number 86.

https://github.com/llvm/llvm-project/pull/159046