Add flags customizing behaviour of MIR inlining

* `-Zinline-mir-threshold` to change the default threshold.
* `-Zinline-mir-hint-threshold` to change the threshold used by
  functions with inline hint.

Author: tmiasko

compiler/rustc_interface/src/tests.rs

@@ -554,6 +554,8 @@ fn test_debugging_options_tracking_hash() {
     tracked!(function_sections, Some(false));
     tracked!(human_readable_cgu_names, true);
     tracked!(inline_in_all_cgus, Some(true));
+    tracked!(inline_mir_threshold, 123);
+    tracked!(inline_mir_hint_threshold, 123);
     tracked!(insert_sideeffect, true);
     tracked!(instrument_coverage, true);
     tracked!(instrument_mcount, true);

compiler/rustc_mir/src/transform/inline.rs

@@ -16,9 +16,6 @@ use crate::transform::MirPass;
 use std::iter;
 use std::ops::{Range, RangeFrom};
 
-const DEFAULT_THRESHOLD: usize = 50;
-const HINT_THRESHOLD: usize = 100;
-
 const INSTR_COST: usize = 5;
 const CALL_PENALTY: usize = 25;
 const LANDINGPAD_PENALTY: usize = 50;
@@ -248,7 +245,11 @@ impl Inliner<'tcx> {
             }
         }
 
-        let mut threshold = if hinted { HINT_THRESHOLD } else { DEFAULT_THRESHOLD };
+        let mut threshold = if hinted {
+            self.tcx.sess.opts.debugging_opts.inline_mir_hint_threshold
+        } else {
+            self.tcx.sess.opts.debugging_opts.inline_mir_threshold
+        };
 
         // Significantly lower the threshold for inlining cold functions
         if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {

compiler/rustc_session/src/options.rs

@@ -929,6 +929,10 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         (default: no)"),
     incremental_verify_ich: bool = (false, parse_bool, [UNTRACKED],
         "verify incr. comp. hashes of green query instances (default: no)"),
+    inline_mir_threshold: usize = (50, parse_uint, [TRACKED],
+        "a default MIR inlining threshold (default: 50)"),
+    inline_mir_hint_threshold: usize = (100, parse_uint, [TRACKED],
+        "inlining threshold for functions with inline hint (default: 100)"),
     inline_in_all_cgus: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "control whether `#[inline]` functions are in all CGUs"),
     input_stats: bool = (false, parse_bool, [UNTRACKED],

src/test/mir-opt/inline/inline-options.rs

@@ -0,0 +1,19 @@
+// Checks that inlining threshold can be controlled with
+// inline-mir-threshold and inline-hint-threshold options.
+//
+// compile-flags: -Zinline-mir-threshold=90
+// compile-flags: -Zinline-mir-hint-threshold=50
+
+// EMIT_MIR inline_options.main.Inline.after.mir
+fn main() {
+    not_inlined();
+    inlined::<u32>();
+}
+
+// Cost is approximately 3 * 25 + 5 = 80.
+#[inline]
+pub fn not_inlined() { g(); g(); g(); }
+pub fn inlined<T>() { g(); g(); g(); }
+
+#[inline(never)]
+fn g() {}

src/test/mir-opt/inline/inline_options.main.Inline.after.mir

@@ -0,0 +1,56 @@
+// MIR for `main` after Inline
+
+fn main() -> () {
+    let mut _0: ();                      // return place in scope 0 at $DIR/inline-options.rs:8:11: 8:11
+    let _1: ();                          // in scope 0 at $DIR/inline-options.rs:9:5: 9:18
+    let _2: ();                          // in scope 0 at $DIR/inline-options.rs:10:5: 10:21
+    scope 1 (inlined inlined::<u32>) {   // at $DIR/inline-options.rs:10:5: 10:21
+        let _3: ();                      // in scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        let _4: ();                      // in scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        let _5: ();                      // in scope 1 at $DIR/inline-options.rs:10:5: 10:21
+    }
+
+    bb0: {
+        StorageLive(_1);                 // scope 0 at $DIR/inline-options.rs:9:5: 9:18
+        _1 = not_inlined() -> bb1;       // scope 0 at $DIR/inline-options.rs:9:5: 9:18
+                                         // mir::Constant
+                                         // + span: $DIR/inline-options.rs:9:5: 9:16
+                                         // + literal: Const { ty: fn() {not_inlined}, val: Value(Scalar(<ZST>)) }
+    }
+
+    bb1: {
+        StorageDead(_1);                 // scope 0 at $DIR/inline-options.rs:9:18: 9:19
+        StorageLive(_2);                 // scope 0 at $DIR/inline-options.rs:10:5: 10:21
+        StorageLive(_3);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        _3 = g() -> bb2;                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+                                         // mir::Constant
+                                         // + span: $DIR/inline-options.rs:10:5: 10:21
+                                         // + literal: Const { ty: fn() {g}, val: Value(Scalar(<ZST>)) }
+    }
+
+    bb2: {
+        StorageDead(_3);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        StorageLive(_4);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        _4 = g() -> bb3;                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+                                         // mir::Constant
+                                         // + span: $DIR/inline-options.rs:10:5: 10:21
+                                         // + literal: Const { ty: fn() {g}, val: Value(Scalar(<ZST>)) }
+    }
+
+    bb3: {
+        StorageDead(_4);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        StorageLive(_5);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        _5 = g() -> bb4;                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+                                         // mir::Constant
+                                         // + span: $DIR/inline-options.rs:10:5: 10:21
+                                         // + literal: Const { ty: fn() {g}, val: Value(Scalar(<ZST>)) }
+    }
+
+    bb4: {
+        StorageDead(_5);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        _2 = const ();                   // scope 1 at $DIR/inline-options.rs:10:5: 10:21
+        StorageDead(_2);                 // scope 0 at $DIR/inline-options.rs:10:21: 10:22
+        _0 = const ();                   // scope 0 at $DIR/inline-options.rs:8:11: 11:2
+        return;                          // scope 0 at $DIR/inline-options.rs:11:2: 11:2
+    }
+}