[clang] [llvm] [RISCV][MC] Add experimental `Zvvmm` MC support (PR #193956)

[Pengcheng Wang via cfe-commits]

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@@ -0,0 +1,11 @@
+# RUN: llvm-mc -triple=riscv64 -show-encoding --mattr=+experimental-zvvmm %s \
+# RUN:        | FileCheck %s --check-prefixes=CHECK-ENCODING,CHECK-INST
+
+# SEW=32, LMUL=1, ta, ma, lambda=4, altfmt_A=1, altfmt_B=1.
+li t1, 0x3c000000000000d0
+vsetvl t0, a0, t1
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wangpc-pp wrote:

> We may have an "extension" that supports encoding IME fields in the vtype-immediate, which will be transformed to li + vsetvl. I think this is better addressed in a separate patch, since it may require coordination with other compiler implementations like gcc?

That means a pseudo assembly instruction, which should also be defined in the spec if we need it I think. The pseudo still requires an extra register, so it may not be convenient.

https://github.com/llvm/llvm-project/pull/193956