[LLVMbugs] [Bug 603] NEW: [llvm-gcc] CFE does not lay out structures correctly when funny darwin rules apply

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=603

           Summary: [llvm-gcc] CFE does not lay out structures correctly
                    when funny darwin rules apply
           Product: tools
           Version: 1.0
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: llvm-gcc
        AssignedTo: sabre at nondot.org
        ReportedBy: sabre at nondot.org
                CC: natebegeman at mac.com


llvm-gcc is currently miscompiling the following code:

----
typedef union {
  int A; long long L;
} X;
typedef struct {
  X   x;
  int B;
} Y;
Y* foo(Y *l) { return l+1; }
---

In this case, the X union should have 8-byte alignment due to funny darwin rules (even though long 
long's are normally only 4-byte aligned).  When X is the first element of a struct, that increases its 
alignment requirements to 8-bytes, which increases the size of Y from 12 to 16 bytes.

llvm-gcc currently compiles this to:

---
        %struct.Y = type { %union.X, int }
        %union.X = type { long }

%struct.Y* %_Z3fooP1Y(%struct.Y* %l) {
        %tmp.2 = getelementptr %struct.Y* %l, int 1             ; <%struct.Y*> [#uses=1]
        ret %struct.Y* %tmp.2
}
---

... which is incorrect: Y is only 12 bytes in size.  This causes us to emit the following PPC code:

__Z3fooP1Y:
        addi r3, r3, 12
        blr

... instead of the correct code:

__Z3fooP1Y:
        addi r3, r3, 16
        blr

This problem was reduced from Bug 449.

-Chris



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