RISC-V: Use symbolic instructions on inline assembly

While many intrinsics use `.insn` to generate raw machine code from
numbers, all ratified instructions can be symbolic
using `.option` directives.

By saving the assembler environment with `.option push` then modifying
the architecture with `.option arch`, we can temporarily enable certain
extensions (as we use `.option pop` immediately after the target
instruction, surrounding environment is completely intact in this
commit; *almost* completely intact in general).

Note:

`hinval.vvma` and `hinval.gvma` instructions are a part of the Svinval
extension (the H extension is not directly required on LLVM) but denoted
as `+h,+svinval` to show semantic context (they are useful only when the
H extension is present and according to the documentation, they are
provided only if the hypervisor extension is enabled).

Author: a4lg

crates/core_arch/src/riscv64/mod.rs

@@ -21,7 +21,10 @@ pub use zk::*;
 pub unsafe fn hlv_wu(src: *const u32) -> u32 {
     let value: u32;
     asm!(
-        ".insn i 0x73, 0x4, {}, {}, 0x681",
+        ".option push",
+        ".option arch, +h",
+        "hlv.wu {}, 0({})",
+        ".option pop",
         lateout(reg) value,
         in(reg) src,
         options(readonly, nostack, preserves_flags)
@@ -44,7 +47,10 @@ pub unsafe fn hlv_wu(src: *const u32) -> u32 {
 pub unsafe fn hlv_d(src: *const i64) -> i64 {
     let value: i64;
     asm!(
-        ".insn i 0x73, 0x4, {}, {}, 0x6C0",
+        ".option push",
+        ".option arch, +h",
+        "hlv.d {}, 0({})",
+        ".option pop",
         lateout(reg) value,
         in(reg) src,
         options(readonly, nostack, preserves_flags)
@@ -64,7 +70,10 @@ pub unsafe fn hlv_d(src: *const i64) -> i64 {
 #[unstable(feature = "riscv_ext_intrinsics", issue = "114544")]
 pub unsafe fn hsv_d(dst: *mut i64, src: i64) {
     asm!(
-        ".insn r 0x73, 0x4, 0x37, x0, {}, {}",
+        ".option push",
+        ".option arch, +h",
+        "hsv.d {}, 0({})",
+        ".option pop",
         in(reg) dst,
         in(reg) src,
         options(nostack, preserves_flags)