[llvm-bugs] [Bug 37319] New: wrong code at -O2 for trivial_abi class that inspects its own "this" pointer

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37319

            Bug ID: 37319
           Summary: wrong code at -O2 for trivial_abi class that inspects
                    its own "this" pointer
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: llvm-bugs at lists.llvm.org

cat > test.cc <<EOF
#include <assert.h>
#include <stdint.h>

#define TRIVIAL_ABI __attribute__((trivial_abi))

template<class T>
class TRIVIAL_ABI trivial_offset_ptr {
    intptr_t value_;
public:
    trivial_offset_ptr(T *p) : value_((const char*)p - (const char*)this) {}
    trivial_offset_ptr(const trivial_offset_ptr& rhs) : value_((const
char*)rhs.get() - (const char*)this) {}
    T *get() const { return (T *)((const char *)this + value_); }
    trivial_offset_ptr& operator=(const trivial_offset_ptr& rhs) {
        value_ = ((const char*)rhs.get() - (const char*)this);
        return *this;
    }
    trivial_offset_ptr& operator+=(int diff) {
        value_ += (diff * sizeof (T));
        return *this;
    }
};

trivial_offset_ptr<int>
incr(trivial_offset_ptr<int> p) {
    p += 1;
    return p;
}

int main() {
    int a[10];
    trivial_offset_ptr<int> op = &a[4];
    trivial_offset_ptr<int> top = &a[4];
    top = incr(top);
    assert(top.get() == &a[5]);
}
EOF

clang++ -std=c++17 -O1 test.cc -o a.out
./a.out  # works great!

clang++ -std=c++17 -O2 test.cc -o a.out
./a.out

Assertion failed: (top.get() == &a[5]), function main, file test.cc, line 34.
Abort trap: 6

====

This is super sneaky UB-ful code in theory, I'm sure, but IMHO Clang is still
doing something not-conservative-enough here.  Especially, notice that if you
remove the unused local variable `op` from `main`, the assertion will pass. Put
the unused variable back, and the assertion will fail.

Documentation for [[trivial_abi]] is here. It does not mention any caveats
about "don't implement offset_ptr with this attribute." I would grudgingly
accept that outcome if it were proposed (but I'd rather the code Just Work).
https://clang.llvm.org/docs/AttributeReference.html#trivial-abi-clang-trivial-abi

P.S. I am aware that [[trivial_abi]] is super new, and I love it, and I don't
mean to be harsh at all in this first corner-case bug filed against it. :)

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