feat: use vectoring to jump straight to interruptN (#11)

* feat: use vectoring to jump straight to interruptN

Previously, the vector table was set up in a pseudo-direct mode where
all interrupts were redirected to a single `_start_trap` handler
regardless of number.

This change introduces 31 new `_start_trapN` targets that encode which
`interruptN` function to jump to, at a cost of 180 bytes per function
(~5kb total).

Not shown (because the linker scripts live in the HAL) are the big block
of `PROVIDES`:

```ld
PROVIDE(_start_trap1 = default_start_trap1)
PROVIDE(_start_trap2 = default_start_trap2)
...
PROVIDE(_start_trap31 = default_start_trap31)
```

As a side benefit, this does enable overriding on a per-irq basis and
could even allow someone to switch back to the current psuedo-direct
mode if they so chose (though, I think they wouldn't see anything get
GC'd yet).

Potential improvements include:

* Shrinking the saved register set to just being the callee-save parts
  of the C ABI, since only `default_start_trap` really has a use for the
  full frame
* Finer-grained control around which interrupts are vectored, and which
  are semi-direct: maybe we can get away with just three modes at the
  start (all direct, only 1-16 vectored, all vectored), but I'm curious
  if we could find  a way to precisely express in a downstream crate "I
  want precisely 1, 2, 4..." to be vectored. Maybe a proc macro or other
  generator script?

* fix(ci): use nightly clippy

Required if we want to use `#![feature(...)]`s, else clippy complains:

```
error[E0554]: `#![feature]` may not be used on the stable release channel
  --> src/lib.rs:18:1
   |
18 | #![feature(naked_functions)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0554`.
error: could not compile `esp-riscv-rt` due to previous error
Error: Process completed with exit code 101.
```

* Revert "feat: use vectoring to jump straight to interruptN"

This reverts commit 9b77193.

* feat: allow vectoring per-slot overrides

Previously, the vector table was set up in a pseudo-direct mode where
all interrupts were redirected to a single `_start_trap` handler
regardless of number.

This change introduces 31 new `_start_trapN` targets that, by default,
retain the pseudo-direct behavior. However, they're weak symbols, so
they allow for individually overriding entries in the _vector_table, as
well as overriding the entire _vector_table itself.

This is in contrast with the earlier attempt in 9b77193 that jumped
straight to the corresponding `extern "C" interruptN`; instead, this
defers that work to the HAL and other downstream crates.

* fix: restore per-slot handlers

Unless any of the `.weak` symbols are overridden, this produces the
exact same assembly as before. But it permits overriding handlers on a
slot-by-slot basis by overriding those `.weak` symbols individually.

Co-Authored-By: onsdagens <[email protected]>
Co-Authored-By: sethp <[email protected]>

---------

Co-authored-by: onsdagens <[email protected]>

Author: sethp

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@v1
         with:
-          toolchain: stable
+          toolchain: nightly
           components: clippy
       - uses: Swatinem/rust-cache@v2
 

src/lib.rs

@@ -426,15 +426,78 @@ _abs_start:
     .cfi_endproc
 
 /*
-    Trap entry point (_start_trap)
+    Trap entry points (_start_trap, _start_trapN for N in 1..=31)
 
-    Saves caller saved registers ra, t0..6, a0..7, calls _start_trap_rust,
-    restores caller saved registers and then returns.
+    The default implementation saves all registers to the stack and calls
+    _start_trap_rust, then restores all saved registers before `mret`
 */
 .section .trap, "ax"
-.global default_start_trap
-
-default_start_trap:
+.weak _start_trap
+.weak _start_trap1
+.weak _start_trap2
+.weak _start_trap3
+.weak _start_trap4
+.weak _start_trap5
+.weak _start_trap6
+.weak _start_trap7
+.weak _start_trap8
+.weak _start_trap9
+.weak _start_trap10
+.weak _start_trap11
+.weak _start_trap12
+.weak _start_trap13
+.weak _start_trap14
+.weak _start_trap15
+.weak _start_trap16
+.weak _start_trap17
+.weak _start_trap18
+.weak _start_trap19
+.weak _start_trap20
+.weak _start_trap21
+.weak _start_trap22
+.weak _start_trap23
+.weak _start_trap24
+.weak _start_trap25
+.weak _start_trap26
+.weak _start_trap27
+.weak _start_trap28
+.weak _start_trap29
+.weak _start_trap30
+.weak _start_trap31
+
+_start_trap1:
+_start_trap2:
+_start_trap3:
+_start_trap4:
+_start_trap5:
+_start_trap6:
+_start_trap7:
+_start_trap8:
+_start_trap9:
+_start_trap10:
+_start_trap11:
+_start_trap12:
+_start_trap13:
+_start_trap14:
+_start_trap15:
+_start_trap16:
+_start_trap17:
+_start_trap18:
+_start_trap19:
+_start_trap20:
+_start_trap21:
+_start_trap22:
+_start_trap23:
+_start_trap24:
+_start_trap25:
+_start_trap26:
+_start_trap27:
+_start_trap28:
+_start_trap29:
+_start_trap30:
+_start_trap31:
+
+_start_trap:
     addi sp, sp, -40*4
 
     sw ra, 0*4(sp)
@@ -534,7 +597,7 @@ abort:
 */
 
 .section .trap, "ax"
-.global _vector_table
+.weak _vector_table
 .type _vector_table, @function
 
 .option push
@@ -544,9 +607,37 @@ abort:
 
 _vector_table:
     j _start_trap
-    .rept 31
-    j _start_trap
-    .endr
+    j _start_trap1
+    j _start_trap2
+    j _start_trap3
+    j _start_trap4
+    j _start_trap5
+    j _start_trap6
+    j _start_trap7
+    j _start_trap8
+    j _start_trap9
+    j _start_trap10
+    j _start_trap11
+    j _start_trap12
+    j _start_trap13
+    j _start_trap14
+    j _start_trap15
+    j _start_trap16
+    j _start_trap17
+    j _start_trap18
+    j _start_trap19
+    j _start_trap20
+    j _start_trap21
+    j _start_trap22
+    j _start_trap23
+    j _start_trap24
+    j _start_trap25
+    j _start_trap26
+    j _start_trap27
+    j _start_trap28
+    j _start_trap29
+    j _start_trap30
+    j _start_trap31
 
 .option pop
 "#,