[LLVMbugs] [Bug 6971] New: Code generation for non-constant __builtin_offsetof

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Wed Apr 28 18:39:59 PDT 2010


           Summary: Code generation for non-constant __builtin_offsetof
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: dgregor at apple.com
                CC: llvmbugs at cs.uiuc.edu

__builtin_offsetof is represented by an OffsetOfExpr in C++, and we would like
to use that representation in C as well to eliminate UnaryOperator::OffsetOf
(which is a hack). However, we need code generation support for non-constant
offsetof expressions, such as the one in this example:

  struct sockaddr_un {
   unsigned char sun_len;
   char sun_path[104];

  int test(int len) {
    return __builtin_offsetof(struct sockaddr_un, sun_path[len+1]);

The UnaryOperator::OffsetOf implementation handles this by creating an lvalue,
then applying various array-index and member-reference operations to that value
and subtracting the original lvalue's address from the resulting lvalue's
address. We'll likely need to do the same for OffsetOfExpr. Once implemented,
we can kill off UnaryOperator::OffsetOf.

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