Why is Xcode/LLVM optimising out the variable this method returns when compiling for ARM64

Question

This is a known bug and is has been submitted to bugreporter.apple.com under bug id #16040090 NB: This issue only reproducible on a 64-bit (A7) iOS device. If run in the simulator or on a 32-bit device it works fine. Please note: This bug seems to be fixed in LLVM 5.1, this is the default compiler for Xcode 5.1 (currently in beta). I have a method `-pointValue` (see below), it works fine when compiled with the optimisation level set to `-O0`, but using `-Os` (the default for Release builds) the compiler optimises out the line that sets the variable `point`. `point` is used as the return value so any other method calling this gets random values. The issue can be fixed by declaring the `points` variable as `volatile`. However, why does the compiler think it can remove the line setting `points`? Is it something to do with how I have written the method? ``` - (CGPoint)pointValue { ULongLong encodedPoint = [self unsignedLongLongValue]; ULongLong *values = [self splitEncodedInteger:encodedP…

Accepted Answer

This issues seems to be a bug in the arm64 compiler; armv7(s) and x86 (Simulator) run the code without issue.

```
- (CGPoint)pointValue
{
    ULongLong encodedPoint = [self unsignedLongLongValue];
    ULongLong *values = [self splitEncodedInteger:encodedPoint withShift:SHIFT];

//volatile is required to prevent optimising out
    /*volatile*/ CGPoint point = CGPointMake(values[0], values[1]);

free(values); //clean up

return point;
}
```

What seems to be happening is the `free(values)` line is being reordered in the optimisation phase and run before `CGPointMake(...)` causing `pointer` to return with garbage values.

Thanks to Mike Ash, CRD and Rob Napier.

Why is Xcode/LLVM optimising out the variable this method returns when compiling for ARM64

Solution

Problem