Open Radar

 
Summary:

XCode Version 4.3.2 (4E2002) with Apple LLVM 3.1 creates corrupted results from C function returning 64-bit integer when compiled with -mthumb (which is the implicit default when setting up a new project in XCode)


Steps to Reproduce:

Compile and run the attached problem demo app as RELEASE for iOS and run it on an ARMv7 device.
See below under notes for the function that fails.


Expected Results:

All three calculations should render the same result (1333152000000000)


Actual Results:

With the following conditions met:
- compiled for ARMv7
- with THUMB enabled (default setting in XCode generated app template or CFlags -mthumb)
- RELEASE build (full optimisation)
- actual function call (inline prevented)

...the function result is severely wrong (1333151995705032)


Regression:

Problem has appeared by upgrading project (which was fine in fall 2011) according to XCode suggestions, which included switching to Apple LLVM 3.1 compiler.
Insofar it is a regression, as this type of miscalculation was caused only by taking the recommended upgrade path to current developer tools and settings.


Notes:

The problem is heavily dependent on the actual calculation done inside the function, and apparently also dependent on the value passed in.

The function is:

signed long long failingWithTHUMB_whenNotInlined(signed long long aInput) 
{
  // shows the problem called as a real function (NOT inlined)
  return (aInput-210866803200000)*1000000ll/1000ll;  
}


- Called with aInput 212199955200000, it should return 1333152000000000 (and does so in non-THUMB, and also when inlined)

- But actually actually called as function (in the sample, I enforced this with  __attribute__ ((noinline)), in the real program the problem appeared caller and callee were in separate .c files), the function returns the completely wrong result 1333151995705032