X86 has several instructions which are documented as enabling interrupts exactly one instruction *after* the one which changes the SS segment register. Inserting a nop between these two instructions allows an interrupt to arrive before the execution of the following instruction which changes semantic behaviour.
The list of instructions is documented in "Table 24-3. Format of Interruptibility State" in Volume 3c of the Intel manual. They basically all come down to different ways to write to the SS register.
Differential Revision: https://reviews.llvm.org/D75359
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