[LLVMdev] Request to review patch for bug #14792

[Yun-Wei Lee]

http://llvm.org/bugs/show_bug.cgi?id=14792

Problem:
  In the i386 ABI Page 3-10, it said that the stack is aligned. However,
the two example code show that does not handle the alignment correctly when
using variadic function. For example, if the size of the first argument is
17, the overflow_arg_area in va_list will be set to "address of first
argument + 16" instead of "address of first argument + 24" after calling
va_start.
  In addition, #6636 showed the same problem because in AMD64, arguments is
passed by register at first, then pass by memory when run out of register
(AMD64 ABI 3.5.7 rule 10).

Why this problem happened?
  When calling va_start to set va_list, overflow_arg_area is not set
correctly. To set the overflow_arg_area correctly, we need to get the
FrameIndex correctly. Now, here comes the problem, llvm doesn't handle it
correctly. It accounts for StackSize to compute the FrameIndex, and if the
StackSize is not aligned, it will compute the wrong FrameIndex. As a result
overflow_arg_area will not be set correctly.

My Solution:
1. Record the Align if it is located in Memory.
2. If it is variadic function and needs to set FrameIndex, adjust the
stacksize.
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