fix asm! clobber, no inline-asm support, 4 insn per cycle

Author: japaric

asm/delay.s

@@ -0,0 +1,8 @@
+  .global __delay
+  .syntax unified
+  .thumb_func
+__delay:
+  nop
+  subs r0, #1
+  bne __delay
+  bx lr

build.rs

@@ -15,6 +15,7 @@ fn main() {
             .file("asm/control.s")
             .file("asm/cpsid.s")
             .file("asm/cpsie.s")
+            .file("asm/delay.s")
             .file("asm/dmb.s")
             .file("asm/dsb.s")
             .file("asm/faultmask.s")

src/asm.rs

@@ -24,26 +24,37 @@ pub fn bkpt() {
     }
 }
 
-/// Blocks the program for at least `n` instruction cycles
+/// Blocks the program for *at least* `n` instruction cycles
 ///
 /// This is implemented in assembly so its execution time is the same regardless of the optimization
 /// level.
 ///
 /// NOTE that the delay can take much longer if interrupts are serviced during its execution.
-#[inline(never)]
+#[inline]
 pub fn delay(_n: u32) {
     match () {
-        #[cfg(target_arch = "arm")]
+        #[cfg(all(cortex_m, feature = "inline-asm"))]
         () => unsafe {
             asm!("1:
+                  nop
                   subs $0, $$1
                   bne.n 1b"
+                 : "+r"(_n / 4 + 1)
                  :
-                 : "r"(_n / 3 + 1)
                  :
                  : "volatile");
         },
-        #[cfg(not(target_arch = "arm"))]
+
+        #[cfg(all(cortex_m, not(feature = "inline-asm")))]
+        () => unsafe {
+            extern "C" {
+                fn __delay(n: u32);
+            }
+
+            __delay(_n / 4 + 1);
+        },
+
+        #[cfg(not(cortex_m))]
         () => unimplemented!(),
     }
 }