RISC-V: Improvements of inline assembly uses

This commit performs various improvements (better register allocation,
less register clobbering on the worst case and better readability) of
RISC-V inline assembly use cases.

Note that it does not change the `p` module (which defines the "P"
extension draft instructions but very likely to change).

1.  Use `lateout` as possible.
    Unlike `out(reg)` and `in(reg)`, `lateout(reg)` and `in(reg)` can
    share the same register, assuming that the register is written after
    all the reads are performed.
    It can improve the register allocation.
2.  Add `preserves_flags` option as possible.
    While RISC-V doesn't have regular condition codes, RISC-V inline
    assembly in the Rust language assumes that some registers
    (mainly vector state registers) may be overwritten by default.
    By adding `preserves_flags` to the intrinsics corresponding
    instructions without overwriting such registers, it can minimize
    register clobbering on the worst case and adds flexibility to
    reorder the instructions.
3.  Use trailing semicolon.
    As `asm!` declares an action and it doesn't return a value by
    itself, it would be better to have trailing semicolon to denote that
    an `asm!` call is effectively a statement.

Author: a4lg

crates/core_arch/src/riscv64/mod.rs

@@ -20,7 +20,12 @@ pub use zk::*;
 #[unstable(feature = "riscv_ext_intrinsics", issue = "114544")]
 pub unsafe fn hlv_wu(src: *const u32) -> u32 {
     let value: u32;
-    asm!(".insn i 0x73, 0x4, {}, {}, 0x681", out(reg) value, in(reg) src, options(readonly, nostack));
+    asm!(
+        ".insn i 0x73, 0x4, {}, {}, 0x681",
+        lateout(reg) value,
+        in(reg) src,
+        options(readonly, nostack, preserves_flags)
+    );
     value
 }
 
@@ -38,7 +43,12 @@ pub unsafe fn hlv_wu(src: *const u32) -> u32 {
 #[unstable(feature = "riscv_ext_intrinsics", issue = "114544")]
 pub unsafe fn hlv_d(src: *const i64) -> i64 {
     let value: i64;
-    asm!(".insn i 0x73, 0x4, {}, {}, 0x6C0", out(reg) value, in(reg) src, options(readonly, nostack));
+    asm!(
+        ".insn i 0x73, 0x4, {}, {}, 0x6C0",
+        lateout(reg) value,
+        in(reg) src,
+        options(readonly, nostack, preserves_flags)
+    );
     value
 }
 
@@ -53,5 +63,5 @@ pub unsafe fn hlv_d(src: *const i64) -> i64 {
 #[inline]
 #[unstable(feature = "riscv_ext_intrinsics", issue = "114544")]
 pub unsafe fn hsv_d(dst: *mut i64, src: i64) {
-    asm!(".insn r 0x73, 0x4, 0x37, x0, {}, {}", in(reg) dst, in(reg) src, options(nostack));
+    asm!(".insn r 0x73, 0x4, 0x37, x0, {}, {}", in(reg) dst, in(reg) src, options(nostack, preserves_flags));
 }