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NaCl's ARM ABI uses 16 byte stack alignment, so set that in ARMSubtarget.cpp. Using 16 byte alignment exposes an issue in code generation in which a varargs function leaves a 4 byte gap between the values of r1-r3 saved to the stack and the following arguments that were passed on the stack. (Previously, this code only needed to support 4 byte and 8 byte alignment.) With this issue, llc generated: varargs_func: sub sp, sp, #16 push {lr} sub sp, sp, #12 add r0, sp, #16 // Should be 20 stm r0, {r1, r2, r3} ldr r0, .LCPI0_0 // Address of va_list add r1, sp, #16 str r1, [r0] bl external_func Fix the bug by checking for "Align > 4". Also simplify the code by using OffsetToAlignment(), and update comments. Differential Revision: http://llvm-reviews.chandlerc.com/D2677 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201497 91177308-0d34-0410-b5e6-96231b3b80d8