Ignore hartid in single-hart mode.

Don't bother checking hart ID on startup in single-hart mode. Allows
use of cores with unusual mhartid values and saves a few instructions.

src/lib.rs

@@ -368,12 +368,11 @@ mod asm;
 
 use core::sync::atomic::{compiler_fence, Ordering};
 
-#[cfg(feature = "s-mode")]
+#[cfg(any(feature = "s-mode", feature = "single-hart"))]
 use riscv::register::{scause as xcause, stvec as xtvec, stvec::TrapMode as xTrapMode};
 
-#[cfg(not(feature = "s-mode"))]
+#[cfg(all(not(feature = "s-mode"), not(feature = "single-hart")))]
 use riscv::register::{mcause as xcause, mhartid, mtvec as xtvec, mtvec::TrapMode as xTrapMode};
-
 pub use riscv_rt_macros::{entry, pre_init};
 
 #[export_name = "error: riscv-rt appears more than once in the dependency graph"]
@@ -405,10 +404,12 @@ pub unsafe extern "C" fn start_rust(a0: usize, a1: usize, a2: usize) -> ! {
     }
 
     // sbi passes hartid as first parameter (a0)
-    #[cfg(feature = "s-mode")]
+    #[cfg(all(feature = "s-mode", not(feature = "single-hart")))]
     let hartid = a0;
-    #[cfg(not(feature = "s-mode"))]
+    #[cfg(all(not(feature = "s-mode"), not(feature = "single-hart")))]
     let hartid = mhartid::read();
+    #[cfg(feature = "single-hart")]
+    let hartid = 0;
 
     if _mp_hook(hartid) {
         __pre_init();
@@ -661,6 +662,7 @@ pub unsafe extern "Rust" fn default_pre_init() {}
 #[doc(hidden)]
 #[no_mangle]
 #[rustfmt::skip]
+#[cfg(not(feature = "single-hart"))]
 pub extern "Rust" fn default_mp_hook(hartid: usize) -> bool {
     match hartid {
         0 => true,
@@ -670,6 +672,14 @@ pub extern "Rust" fn default_mp_hook(hartid: usize) -> bool {
     }
 }
 
+#[doc(hidden)]
+#[no_mangle]
+#[rustfmt::skip]
+#[cfg(feature = "single-hart")]
+pub extern "Rust" fn default_mp_hook(_hartid: usize) -> bool {
+    true
+}
+
 /// Default implementation of `_setup_interrupts` that sets `mtvec`/`stvec` to a trap handler address.
 #[doc(hidden)]
 #[no_mangle]