emulator: Remove vestiges of special handling of x(0) #7387
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In the BEAM emulator, X register 0 (`x(0)`) used to be specially handled by the macro `r(0)` so that it could be placed in a CPU register. That made sense for platforms with many registers such as Sparc and PowerPC. For the register-starved x86_64 platform it is not possible to allocate a CPU register for `x(0)`. Therefore, in 1f996cc (part of Erlang/OTP 19), the special handling of `x(0)` was removed. Removing that special case simplified the code in many places. Get rid of the vestiges of the special handling of `x(0)`. While at it, also re-introduce the `REG_xregs` attribute for the variable holding the pointer to the X registers.
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In the BEAM emulator, X register 0 (
x(0)) used to be specially handled by the macror(0)so that it could be placed in a CPU register.That made sense for platforms with many registers such as Sparc and PowerPC. For the register-starved x86_64 platform it is not possible to allocate a CPU register for
x(0). Therefore, in 1f996cc (part of Erlang/OTP 19), the special handling ofx(0)was removed. Removing that special case simplified the code in many places.Get rid of the vestiges of the special handling of
x(0). While at it, also re-introduce theREG_xregsattribute for the variable holding the pointer to the X registers.