When setting up space for local variables on the stack, the compiler generates code like this:

/* prologue: frame size=20 */
        push r28
        push r29
        in r28,__SP_L__
        in r29,__SP_H__
        sbiw r28,20
        in __tmp_reg__,__SREG__
        cli
        out __SP_H__,r29
        out __SREG__,__tmp_reg__
        out __SP_L__,r28
/* prologue end (size=10) */

It reads the current stack pointer value, decrements it by the required amount of bytes, then disables interrupts, writes back the high part of the stack pointer, writes back the saved SREG (which will eventually re-enable interrupts if they have been enabled before), and finally writes the low part of the stack pointer.

At the first glance, there's a race between restoring SREG, and writing SPL. However, after enabling interrupts (either explicitly by setting the I flag, or by restoring it as part of the entire SREG), the AVR hardware executes (at least) the next instruction still with interrupts disabled, so the write to SPL is guaranteed to be executed with interrupts disabled still. Thus, the emitted sequence ensures interrupts will be disabled only for the minimum time required to guarantee the integrity of this operation.

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