[LLVMbugs] [Bug 11516] New: Wrong alignment for variable of a reference type (e.g. char &foo)

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11516

             Bug #: 11516
           Summary: Wrong alignment for variable of a reference type (e.g.
                    char &foo)
           Product: clang
           Version: 3.0
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: pickensd at synopsys.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Look at the bitcode generated from the following function:

int eq(const char &c1, const char &c2) {
    return c1 < c2;
}

You'll see that the dereference of c1 and c2 are incorrectly aligned to 4
instead of 1. This causes misaligned loads  to be generated for some loop
transformations involving char pointers.

define i32 @_Z2eqRKcS0_(i8* %c1, i8* %c2) nounwind {
entry:
  %c1.addr = alloca i8*, align 4
  %c2.addr = alloca i8*, align 4
  store i8* %c1, i8** %c1.addr, align 4
  store i8* %c2, i8** %c2.addr, align 4
  %0 = load i8** %c1.addr
  %1 = load i8* %0, align 4   <-------------------WRONG!
  %conv = zext i8 %1 to i32
  %2 = load i8** %c2.addr
  %3 = load i8* %2, align 4   <-------------------WRONG!
  %conv1 = zext i8 %3 to i32
  %cmp = icmp slt i32 %conv, %conv1
  %conv2 = zext i1 %cmp to i32
  ret i32 %conv2
}

Here is a proposed fix in module clang/lib/CodeGen/CGExpr.cpp, function
CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) :


Change this:

    if (VD->getType()->isReferenceType())
      V = Builder.CreateLoad(V);


To this:

    if (VD->getType()->isReferenceType()) {
      V = Builder.CreateLoad(V);
      // Alignment is now that of referenced type!
      QualType refee = VD->getType()->getPointeeType();
      Alignment = getContext().getTypeAlignInChars(refee).getQuantity();
    }

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