While msabi and sysv do agree what to do with floating point args for 4 floating point args (pass in xmm0-4), they dont agree what to do with mixing
msabi will choose the register corresponding with argument position. so if you have (int foo, float bar), bar will use xmm1
sysv instead will choose the first register available in the group. so with the previous example, you instead have bar using xmm0
the simple solution is to simply prohibit mixed args for now. maybe someday we could support mixing, but that's probably overkill (best use a struct at that point)
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