[llvm-commits] [PATCH] Miscompile Fix: Ensure that we do not constant fold Globals containing ObjC References in GlobalOpt.

[Michael Gottesman]

Please ignore this patch, I have a better alternate solution (I will post it in the same thread).

Michael

On Jan 11, 2013, at 11:55 AM, Michael Gottesman <mgottesman at apple.com> wrote:

> Currently we miscompile Objective-C++ in the case where we have a selector stored in a global constant structure. The miscompile occurs in the IR level pass GlobalOpt.
> 
> ----
> #include <Foundation/Foundation.h>
> 
> struct ButtonInitializer {
>  NSString *name;
>  SEL sel;
> };
> 
> const static ButtonInitializer buttonInitData[] = {
>  {
>    @"Print",
>    @selector(print:)
>  }
> };
> 
> @interface Test : NSObject {
>  NSMutableDictionary *dict;
> }
> - (id)init;
> @end
> 
> @implementation Test
> - (id)init {
>  self = [super init];
>  if (self) {
>    dict = [[NSMutableDictionary alloc] init];
>    for (int i = 0; i < 1; i++) {
>      [dict setValue:buttonInitData[i].sel forKey:buttonInitData[i].name];
>    }
>  }
>  return self;
> }
> @end
> ----
> 
> The specific error that occurs is that in GlobalOpt we evaluate the Objective C selector reference and put in the local method name string that the selector ref is pointing at in stead of the ref itself. This violates the semantics of objective-c++, which due to dynamic binding, explicitly says that the runtime reserves the right to change what method name string a specific Objective C selector reference is pointing at. If for whatever this occurs, a hard error (does not respond to selector) will occur causing the application to exit abnormally.
> 
> The following patch fixes this issue by making the following changes:
> 
> 1. A new instance method is added to GlobalValue called mayBeOverriddenAfterStartBeforeMain() that is named as such to suggest that said value may be altered at program start before main is called but after program start.
> 2. The static method mayBeOverridden is renamed mayBeOverriddenAtLinkTime() to contrast said method with mayBeOverriddenAfterStartBeforeMain() and to make the name of the function fit its function.
> 3. The instance method mayBeOverridden is changed to return false if and only if both mayBeOverriddenAtLinkTime() and mayBeOverriddenAfterStartBeforeMain() return false.
> 
> Additionally a test case is provided.
> 
> *NOTE* I am going to fix some comments in the file in a separate patch.
> 
> Please Review.
> 
> <0001-Fixed-a-bug-in-GlobalOpt-which-was-causing-evaluatio.patch>
> 
> Michael
> 
> 
> 
> 
> 
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