[llvm-dev] Sparc64 stack realignment broken

[Michael Karcher via llvm-dev]

While trying to get rust running on Sparc64, I encountered an issue inside
llvm. For some reason I did not try to hunt down, rustc decides to do
strict (over-)alignment of some stack frames. At a certain point, it is
requesting an alignment of 64 bytes. This creates the following sparc
assembly code in the output from SparcFrameLowering.cpp:

andn %sp,63,%sp

This ensures (as intended) that the stack pointer has its low 6 bits
cleared and is perfectly aligned on 64 bytes. Alas, this does not take
Sparc64's stack pointer bias into account: The real register value is 2047
(0x7ff) lower than the effective stack pointer address. As the stack an
Sparc64 is always 8-byte aligned, the stack pointer register modulo 8 has
to be 1.

A crude fix to this is to not mask the lowest bit of the stack pointer
(which will keep it 0 on Sparc32 and 1 on Sparc64), which I have verified
to fix a Bus Error in rustc on Sparc64/Linux.

Kind regards,
  Michael Karcher