Compile with unaligned-scalar-mem feature (#29)

This optimizes memory reads and writes so that they don't have to be
aligned, as this is not required for our CPU.

This saves abot 3 KB of instruction space in the runtime binary.

The VeeR CPU also supports bit manipulation instructions, but we don't
have complete support for those in the emulator, so I left that as a
TODO. That saves a few hundred bytes more (for now).

.cargo/config.toml

@@ -4,4 +4,8 @@
 xtask = "run --package xtask --"
 
 [target.riscv32imc-unknown-none-elf]
-rustflags = ["-C", "panic=abort"]
+rustflags = [
+    "-C panic=abort",
+    "-C target-feature=+relax,+unaligned-scalar-mem",
+    "-C force-frame-pointers=no"
+]

xtask/src/apps_build.rs

@@ -1,6 +1,6 @@
 // Licensed under the Apache-2.0 license
 
-use crate::runtime_build::{objcopy, target_binary, OBJCOPY_FLAGS};
+use crate::runtime_build::{objcopy, target_binary, OBJCOPY_FLAGS, RUSTFLAGS_COMMON};
 use crate::tbf::TbfHeader;
 use crate::{DynError, PROJECT_ROOT, TARGET};
 use std::process::Command;
@@ -114,12 +114,14 @@ INCLUDE runtime/apps/app_layout.ld",
         ),
     )?;
 
+    let ld_flag = format!("-C link-arg=-T{}", layout_ld.display());
+    let mut rustc_flags = Vec::from(RUSTFLAGS_COMMON);
+    rustc_flags.push(ld_flag.as_str());
+    let rustc_flags = rustc_flags.join(" ");
+
     let status = Command::new("cargo")
         .current_dir(&*PROJECT_ROOT)
-        .env(
-            "RUSTFLAGS",
-            format!("-C link-arg=-T{}", layout_ld.display()),
-        )
+        .env("RUSTFLAGS", rustc_flags)
         .env("LIBTOCK_LINKER_FLASH", format!("0x{:x}", offset))
         .env("LIBTOCK_LINKER_FLASH_LENGTH", "128K")
         .env("LIBTOCK_LINKER_RAM", "0x50000000")

xtask/src/rom.rs

@@ -1,12 +1,17 @@
 // Licensed under the Apache-2.0 license
 
+use crate::runtime_build::RUSTFLAGS_COMMON;
 use crate::{runtime_build::objcopy, DynError, PROJECT_ROOT, TARGET};
 use std::process::Command;
 
 pub fn rom_build() -> Result<(), DynError> {
+    let mut rustc_flags = Vec::from(RUSTFLAGS_COMMON);
+    rustc_flags.push("-C link-arg=-Trom/layout.ld");
+    let rustc_flags = rustc_flags.join(" ");
+
     let status = Command::new("cargo")
         .current_dir(&*PROJECT_ROOT)
-        .env("RUSTFLAGS", "-C link-arg=-Trom/layout.ld")
+        .env("RUSTFLAGS", rustc_flags)
         .args(["b", "-p", "rom", "--release", "--target", TARGET])
         .status()?;
     if !status.success() {

xtask/src/runtime_build.rs

@@ -19,6 +19,13 @@ const RAM_START: usize = 0x5000_0000;
 const RAM_SIZE: usize = 128 * 1024;
 const BSS_SIZE: usize = 5000; // this is approximate. Increase it is there are "sram" errors when linking
 
+// TODO: Add +b/+zb* features when the emulator supports the extra bit manipulation instructions
+// This saves several hundred bytes of instruction space, and the VeeR core supports them.
+pub const RUSTFLAGS_COMMON: [&str; 2] = [
+    "-C target-feature=+relax,+unaligned-scalar-mem",
+    "-C force-frame-pointers=no",
+];
+
 pub fn target_binary(name: &str) -> PathBuf {
     PROJECT_ROOT
         .join("target")
@@ -133,21 +140,18 @@ INCLUDE runtime/kernel_layout.ld
     // - `-C symbol-mangling-version=v0`: Opt-in to Rust v0 symbol mangling scheme.
     //   See https://github.com/rust-lang/rust/issues/60705 and
     //   https://github.com/tock/tock/issues/3529.
-    let rustc_flags = [
-        format!("-C link-arg=-T{}", ld_file_path.display()).as_str(),
+    let ld_arg = format!("-C link-arg=-T{}", ld_file_path.display());
+    let mut rustc_flags = Vec::from(RUSTFLAGS_COMMON);
+    rustc_flags.extend_from_slice(&[
+        ld_arg.as_str(),
         "-C linker=rust-lld",
         "-C linker-flavor=ld.lld",
         "-C relocation-model=static",
         "-C link-arg=-nmagic", // don't page align sections, link against static libs
         "-C link-arg=-icf=all", // identical code folding
         "-C symbol-mangling-version=v0",
-        // RISC-V-specific flags.
-        "-C force-frame-pointers=no",
-        //   Ensure relocations generated is eligible for linker relaxation.
-        //   This provide huge space savings.
-        "-C target-feature=+relax",
-    ]
-    .join(" ");
+    ]);
+    let rustc_flags = rustc_flags.join(" ");
 
     // RUSTC_FLAGS_TOCK by default extends RUSTC_FLAGS with options that are global
     // to all Tock boards.