Which %g registers can I count on not changing behind my back?

I've written some assembly code that needs lots of registers.  I ran
out of spare registers and started using the %g registers.  But in
tracking down bugs, I discovered that %g1 is being changed behind my
back by *something*.

The only thing the code calls is .mul, and looking at
/usr/src/lib/libc/arch/sparc/gen/mul.S, it looks to me as though it
runs entirely in the %o registers (which my code is prepared to have
clobbered by calls to .mul).  The only %g register it even refers to is
%g0, as far as I can see.  I even looked at the FUNC() entry prologue,
and it doesn't use %g1 even if compiling for profiling, which I'm not
doing anyway.

What am I missing?  What could be bashing %g1?  What other registers
can I expect to change mysteriously behind my back, and - perhaps most
important of all - where is all this stuff documented?

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