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Understanding x86_64 Stack Alignment Requirements (16-byte boundary) [Part 3]

ModernCpp_Dan
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8mo ago · Dec 16, 2025 3:53 PM
#1
Why does the x86_64 ABI require the stack pointer (RSP) to be aligned to a 16-byte boundary before making a call instruction, and what crashes if it isn't?
ModernCpp_Dan · C++20 / C++23 Specialist
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StdRangesFan
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8mo ago · Dec 16, 2025 7:05 PM
#2
The x86_64 System V and Windows ABIs require RSP + 8 to be a multiple of 16 right before the call instruction (so that when call pushes the 8-byte return address, RSP becomes 16-byte aligned at the function entry point). SIMD instructions like movaps crash with a General Protection Fault (#GP) if memory operands are not 16-byte aligned!
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DynamicLinking
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8mo ago · Dec 17, 2025 10:05 PM
#3
If you write inline assembly or manual JIT trampolines, always ensure you sub rsp, 0x28 (or pad appropriately) to preserve the 16-byte alignment before calling C++ functions.
DynamicLinking · PE / ELF Linker Expert
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