[LLVMbugs] [Bug 12384] New: Clang miscompiles trivial init-family methods if the return type is unexpected

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12384

             Bug #: 12384
           Summary: Clang miscompiles trivial init-family methods if the
                    return type is unexpected
           Product: clang
           Version: unspecified
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: csdavec at swan.ac.uk
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


This simple method:


@interface Fail @end

@protocol X @end
@implementation Fail
- (id<X>) initWithX
{
    return (id)self;
}
@end

When compiled with clang, the first thing you'll spot is a very strange error:


banlu.m:8:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^

Control obviously doesn't reach the end of the function - the line before this
is a return statement!

However, checking the generated IR, it appears that it does:


define internal i8* @"\01-[Fail initWithX]"(%0* %self, i8* %_cmd) nounwind
uwtable {
entry:
  %retval = alloca i8*, align 8
  %self.addr = alloca %0*, align 8
  %_cmd.addr = alloca i8*, align 8
  store %0* %self, %0** %self.addr, align 8
  store i8* %_cmd, i8** %_cmd.addr, align 8
  %0 = load i8** %retval
  ret i8* %0
}

Note that retval is never written to.  This becomes clearer with optimisation
enabled:


define internal noalias i8* @"\01-[Fail initWithX]"(%0* nocapture %self, i8*
nocapture %_cmd) nounwind uwtable readnone {
entry:
  ret i8* undef
}


This is obviously wrong.  

This is independent of the Objective-C runtime being targeted - it appears that
the return statement never makes it into the AST at all...

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