bootstrap: Allow for setting the ThinLTO import limit used for compiler the compiler.

config.toml.example

@@ -406,6 +406,13 @@
 # Whether to verify generated LLVM IR
 #verify-llvm-ir = false
 
+# Compile the compiler with a non-default ThinLTO import limit. This import
+# limit controls the maximum size of functions imported by ThinLTO. Decreasing
+# will make code compile faster at the expense of lower runtime performance.
+# If `incremental` is set to true above, the import limit will default to 10
+# instead of LLVM's default of 100.
+#thin-lto-import-instr-limit = 100
+
 # Map all debuginfo paths for libstd and crates to `/rust/$sha/$crate/...`,
 # generally only set for releases
 #remap-debuginfo = false

src/bootstrap/builder.rs

@@ -1183,6 +1183,21 @@ impl<'a> Builder<'a> {
             rustflags.arg("-Cprefer-dynamic");
         }
 
+        // When building incrementally we default to a lower ThinLTO import limit
+        // (unless explicitly specified otherwise). This will produce a somewhat
+        // slower code but give way better compile times.
+        {
+            let limit = match self.config.rust_thin_lto_import_instr_limit {
+                Some(limit) => Some(limit),
+                None if self.config.incremental => Some(10),
+                _ => None,
+            };
+
+            if let Some(limit) = limit {
+                rustflags.arg(&format!("-Cllvm-args=-import-instr-limit={}", limit));
+            }
+        }
+
         Cargo { command: cargo, rustflags }
     }
 

src/bootstrap/config.rs

@@ -108,6 +108,7 @@ pub struct Config {
     pub rust_dist_src: bool,
     pub rust_codegen_backends: Vec<Interned<String>>,
     pub rust_verify_llvm_ir: bool,
+    pub rust_thin_lto_import_instr_limit: Option<u32>,
     pub rust_remap_debuginfo: bool,
 
     pub build: Interned<String>,
@@ -325,6 +326,7 @@ struct Rust {
     deny_warnings: Option<bool>,
     backtrace_on_ice: Option<bool>,
     verify_llvm_ir: Option<bool>,
+    thin_lto_import_instr_limit: Option<u32>,
     remap_debuginfo: Option<bool>,
     jemalloc: Option<bool>,
     test_compare_mode: Option<bool>,
@@ -569,6 +571,7 @@ impl Config {
             set(&mut config.deny_warnings, flags.deny_warnings.or(rust.deny_warnings));
             set(&mut config.backtrace_on_ice, rust.backtrace_on_ice);
             set(&mut config.rust_verify_llvm_ir, rust.verify_llvm_ir);
+            config.rust_thin_lto_import_instr_limit = rust.thin_lto_import_instr_limit;
             set(&mut config.rust_remap_debuginfo, rust.remap_debuginfo);
 
             if let Some(ref backends) = rust.codegen_backends {