Import small cold functions

The Rust code is often written under an assumption that for generic
methods inline attribute is mostly unnecessary, since for optimized
builds using ThinLTO, a method will be generated in at least one CGU and
available for import.

For example, deref implementations for Box, Vec, MutexGuard, and
MutexGuard are not currently marked as inline, neither is identity
implementation of From trait.

In PGO builds, when functions are determined to be cold, the default
multiplier of zero will stop the import, even for completely trivial
functions.

Increase slightly the default multiplier from 0 to 0.1 to import them
regardless.

Author: tmiasko

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -98,6 +98,9 @@ unsafe fn configure_llvm(sess: &Session) {
         // during inlining. Unfortunately these may block other optimizations.
         add("-preserve-alignment-assumptions-during-inlining=false", false);
 
+        // Use non-zero `import-instr-limit` multiplier for cold callsites.
+        add("-import-cold-multiplier=0.1", false);
+
         for arg in sess_args {
             add(&(*arg), true);
         }