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In D65234#1615649 <https://reviews.llvm.org/D65234#1615649>, @efriedma wrote:
> For the "=x" thing I was talking about, try the following testcase:
>
> void a() { struct S { unsigned x[4]; } z; asm volatile("%0":"=x"(z)); }
> void a2() { struct S { unsigned x[8]; } z; asm volatile("%0":"=x"(z)); }
>
>
> clang trunk gives "error: couldn't allocate output register for constraint
> 'x'" in the backend for both functions. gcc prints "%xmm0" for the first,
> and rejects the second; not exactly sure why it's rejecting the second,
> though. It would be nice if both worked, although I guess it's okay if we
> print a reasonable error message. Please add a testcase, at least.
Note that this is invalid assembly, as it emits a register name instead of an
assembly instruction. I don't think it should work at all.
If we replace "%0" with a "nop", Clang will reject the second test, but will
accept the first one.
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