Add -Z small-data-threshold

This flag allows specifying the threshold size above which LLVM should
consider placing small objects in a .sdata or .sbss section.

Author: paulmenage

compiler/rustc_codegen_llvm/src/context.rs

@@ -351,6 +351,17 @@ pub unsafe fn create_module<'ll>(
         );
     }
 
+    if let Some(threshold) = sess.opts.unstable_opts.small_data_threshold {
+        if sess.target.arch.starts_with("riscv") {
+            llvm::LLVMRustAddModuleFlag(
+                llmod,
+                llvm::LLVMModFlagBehavior::Error,
+                "SmallDataLimit\0".as_ptr().cast(),
+                threshold as u32,
+            )
+        }
+    }
+
     // Insert `llvm.ident` metadata.
     //
     // On the wasm targets it will get hooked up to the "producer" sections

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -118,6 +118,21 @@ unsafe fn configure_llvm(sess: &Session) {
         for arg in sess_args {
             add(&(*arg), true);
         }
+
+        if let Some(threshold) = sess.opts.unstable_opts.small_data_threshold {
+            match sess.target.arch.as_ref() {
+                "hexagon" => {
+                    add(&format!("--hexagon-small-data-threshold={threshold}"), false);
+                    add("--hexagon-statics-in-small-datax", false)
+                }
+                "m68k" => add(&format!("--m68k-ssection-threshold={threshold}"), false),
+                arch @ _ => {
+                    if arch.starts_with("mips") {
+                        add(&format!("--mips-ssection-threshold={threshold}"), false);
+                    }
+                }
+            }
+        }
     }
 
     if sess.opts.unstable_opts.llvm_time_trace {

compiler/rustc_session/src/options.rs

@@ -1805,6 +1805,8 @@ written to standard error output)"),
     simulate_remapped_rust_src_base: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],
         "simulate the effect of remap-debuginfo = true at bootstrapping by remapping path \
         to rust's source base directory. only meant for testing purposes"),
+    small_data_threshold: Option<usize> = (None, parse_opt_number, [TRACKED],
+        "Set the threshold for objects to be stored in a \"small data\" section"),
     span_debug: bool = (false, parse_bool, [UNTRACKED],
         "forward proc_macro::Span's `Debug` impl to `Span`"),
     /// o/w tests have closure@path

tests/assembly/small_data_threshold.rs

@@ -0,0 +1,45 @@
+// Test for -Z small_data_threshold=...
+// revisions: RISCV MIPS HEXAGON M68K
+// assembly-output: emit-asm
+// compile-flags: -Z small_data_threshold=4
+// [RISCV] compile-flags:--target=riscv32im-unknown-none-elf
+// [MIPS] compile-flags:--target=mips-unknown-linux-musl
+// [HEXAGON] compile-flags:--target=hexagon-unknown-linux-musl
+// [M68K] compile-flags:--target=m68k-unknown-linux-gnu
+
+#![feature(no_core, lang_items)]
+#![no_std]
+#![no_core]
+#![crate_type = "lib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[lang = "drop_in_place"]
+fn drop_in_place<T>(_: *mut T) {}
+
+#[used]
+#[no_mangle]
+/// X is below the threshold, should be in sdata
+static mut X: u16 = 123;
+
+#[used]
+#[no_mangle]
+/// Y is at the threshold, should be in sdata
+static mut Y: u32 = 123;
+
+#[used]
+#[no_mangle]
+/// Z is over the threshold, should be in its own section
+static mut Z: u64 = 123;
+
+// Currently, only RISCV successfully puts any objects in the small data
+// section.
+
+// RISCV: .section .sdata,
+// RISCV-NOT: .section
+// RISCV: X:
+// RISCV: Y:
+// CHECK: .section .data.Z,
+// CHECK-NOT: .section
+// CHECK: Z: