MinGW x86_64 ABI problem

[John Fine]

The Windows version of the x86_64 ABI specifies that registers 
xmm6-xmm15 be callee saved.

I'm building a .dll with MinGW used by an .exe from a different compiler.

I debugged a failure to the point of determining that the code produced 
by MinGW clobbers xmm9 in violation of the ABI.

I tried a bunch of Google searches, expecting this problem would be 
discussed somewhere, but couldn't find it.

I'm using a downloaded binary copy of MinGW for x86_64.  I expect I will 
need to download source code and figure out how to build it, in order to 
deal with this problem.

But I don't know where to look in the gcc source code included in MinGW 
for the registers that should be callee saved.

1) Can you tell me where callee saved registers are specified in the gcc 
source code.

2) I tried adding "-f-call-saved-xmm9" to the g++ command to see if that 
would fix the problem for one register.  That compile aborted with
internal compiler error: in copyprop_hardreg_forward_1, at regrename.c:1590
Is that switch so incorrect?  I understand it would generate .obj code 
incompatible with anything generated without that switch.  But why 
should it crash gcc?