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So to avoid slowing down > >> all string functions, bti c must be removed completely, not just from > >> A64FX memcpy. Using a real NOP is fine in all cases as long as > >> HAVE_AARCH64_BTI is not defined. > > > > I'm probably confused, but: If BTI is active, many more glibc functions > > will have BTI markers. What makes the string functions special? > > Exactly. And at that point trying to remove it from memcpy is just pointless. > > The case we are discussing is where BTI is not turned on in GLIBC but we still > emit a BTI at the start of assembler functions for simplicity. By using a NOP > instead, A64FX will not execute BTI anywhere in GLIBC. the asm ENTRY was written with the assumption that bti c behaves like a nop when bti is disabled, so we don't have to make the asm conditional based on cflags. if that's not the case i agree with the patch, however we will have to review some other code (e.g. libgcc outline atomics asm) where we made the same assumption.