Add -Z small-data-threshold

This flag allows specifying the threshold size above which LLVM should
not consider placing small objects in a .sdata or .sbss section.

Support is indicated in the target options via the
small-data-threshold-support target option, which can indicate either an
LLVM argument or an LLVM module flag.  To avoid duplicate specifications
in a large number of targets, the default value for support is
DefaultForArch, which is translated to a concrete value according to the
target's architecture.

2 files changed, 32 insertions, 2 deletions

diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 01aae24ab56..73c2c15717f 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -27,7 +27,7 @@ use rustc_span::source_map::Spanned;
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::abi::call::FnAbi;
 use rustc_target::abi::{HasDataLayout, TargetDataLayout, VariantIdx};
-use rustc_target::spec::{HasTargetSpec, RelocModel, Target, TlsModel};
+use rustc_target::spec::{HasTargetSpec, RelocModel, SmallDataThresholdSupport, Target, TlsModel};
 use smallvec::SmallVec;
 
 use crate::back::write::to_llvm_code_model;
@@ -387,6 +387,24 @@ pub(crate) unsafe fn create_module<'ll>(
         }
     }
 
+    match (sess.opts.unstable_opts.small_data_threshold, sess.target.small_data_threshold_support())
+    {
+        // Set up the small-data optimization limit for architectures that use
+        // an LLVM module flag to control this.
+        (Some(threshold), SmallDataThresholdSupport::LlvmModuleFlag(flag)) => {
+            let flag = SmallCStr::new(flag.as_ref());
+            unsafe {
+                llvm::LLVMRustAddModuleFlagU32(
+                    llmod,
+                    llvm::LLVMModFlagBehavior::Error,
+                    flag.as_c_str().as_ptr(),
+                    threshold as u32,
+                )
+            }
+        }
+        _ => (),
+    };
+
     // Insert `llvm.ident` metadata.
     //
     // On the wasm targets it will get hooked up to the "producer" sections
diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index d55220ba5c3..ea0abb1fa53 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -14,7 +14,7 @@ use rustc_middle::bug;
 use rustc_session::config::{PrintKind, PrintRequest};
 use rustc_session::Session;
 use rustc_span::symbol::Symbol;
-use rustc_target::spec::{MergeFunctions, PanicStrategy};
+use rustc_target::spec::{MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
 use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
 
 use crate::back::write::create_informational_target_machine;
@@ -125,6 +125,18 @@ unsafe fn configure_llvm(sess: &Session) {
         for arg in sess_args {
             add(&(*arg), true);
         }
+
+        match (
+            sess.opts.unstable_opts.small_data_threshold,
+            sess.target.small_data_threshold_support(),
+        ) {
+            // Set up the small-data optimization limit for architectures that use
+            // an LLVM argument to control this.
+            (Some(threshold), SmallDataThresholdSupport::LlvmArg(arg)) => {
+                add(&format!("--{arg}={threshold}"), false)
+            }
+            _ => (),
+        };
     }
 
     if sess.opts.unstable_opts.llvm_time_trace {