Why Does the Function `f` Push RAX onto the Stack Before Other Operations?

Function Call Stack Alignment

In the assembly code provided, it might be perplexing to see RAX being pushed onto the stack as the first operation within the function f. Understanding the reasoning behind this action requires an examination of the 64-bit ABI.

The 64-bit ABI mandates that the stack be 16-byte aligned prior to a call instruction. However, the call instruction pushes an 8-byte return address onto the stack, disrupting this alignment. Consequently, the compiler is obligated to implement measures to realign the stack to a multiple of 16 before proceeding to the next call.

In this context, pushing a don't-care value, such as RAX, serves an efficient purpose. It provides a means to align the stack without incurring the overhead of executing an additional sub rsp, 8 instruction, which may be less efficient on CPUs equipped with a stack engine.

The comparison to a tailcall without a std::function wrapper illustrates this principle. In the trivial function g, the compiler can simply execute a jmp instruction without any prior stack alignment actions. However, in the case of f, the additional push of RAX is necessary to maintain the 16-byte alignment, ensuring compatibility with the ABI requirements.

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