Load string indices with inline asm to save space.

Author: Dirbaio

CHANGELOG.md

@@ -52,6 +52,7 @@ We have several packages which live in this repository. Changes are tracked sepa
 
 ### [defmt-next]
 
+- [#879]: Load string indices with inline asm to save space.
 * [#974]: Ensure typechecking is still performed on disabled log statement.
 * [#960]: Fix `Format` not accepting multiple helper attribute instances
 * [#937]: add support for `#[defmt(transparent)]` on `Format` derive

macros/build.rs

@@ -1,3 +1,27 @@
 fn main() {
     println!("cargo:rerun-if-env-changed=DEFMT_LOG");
+
+    // Check rustc version
+    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string());
+    let output = std::process::Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to execute rustc");
+    let version_str = String::from_utf8_lossy(&output.stdout);
+    // rustc 1.91.0-beta.1 (1bffa2300 2025-09-15)
+    let is_ge_1_91 = version_str
+        .split_whitespace()
+        .nth(1)
+        .and_then(|v| {
+            let mut parts = v.split('.');
+            let major = parts.next()?.parse::<u32>().ok()?;
+            let minor = parts.next()?.parse::<u32>().ok()?;
+            Some((major, minor))
+        })
+        .map(|(major, minor)| major > 1 || (major == 1 && minor >= 91))
+        .unwrap_or(false);
+    if is_ge_1_91 {
+        println!("cargo:rustc-cfg=rustc_ge_1_91");
+    }
+    println!("cargo:rustc-check-cfg=cfg(rustc_ge_1_91)");
 }

macros/src/construct.rs

@@ -46,10 +46,41 @@ pub(crate) fn interned_string(
         quote!({ #defmt_path::export::fetch_add_string_index() })
     } else {
         let var_item = static_variable(&var_name, string, tag, prefix);
-        quote!({
-            #var_item
+
+        // defmt string indices are 16 bits, which can be loaded with a single `movw`.
+        // However, the compiler doesn't know that, so it generates `movw+movt` or `ldr rX, [pc, #offs]`
+        // because it sees we're loading an address of a symbol, which could be any 32bit value.
+        // This wastes space, so we load the value with asm manually to avoid this.
+        let val_arm_optimized = quote!(
+            let res: u16;
+            unsafe { ::core::arch::asm!(
+                "movw {res}, #:lower16:{y}",
+                res = lateout(reg) res,
+                y = sym #var_name,
+                options(pure, nomem, nostack, preserves_flags)
+            )};
+            resss
+        );
+        let val_standard = quote!(
             &#var_name as *const u8 as u16
-        })
+        );
+
+        // using symbols with quotes in `asm!(sym)` only works in Rust 1.91+
+        if cfg!(rustc_ge_1_91) {
+            quote!({
+                #var_item
+
+                #[cfg(target_arch = "arm")]
+                { #val_arm_optimized }
+                #[cfg(not(target_arch = "arm"))]
+                { #val_standard }
+            })
+        } else {
+            quote!({
+                #var_item
+                #val_standard
+            })
+        }
     };
 
     quote!({