[CaptureTracking] Make MaxUsesToExplore cheaper (NFC)

The change in D78624 had a noticeable negative compile-time impact.
It seems that going through a function call for the MaxUsesToExplore
default is fairly expensive, at least if LLVM is not built with LTO.

This patch makes MaxUsesToExpore default to 0 and assigns the actual
default in the implementation instead. This recovers most of the
regression.

Differential Revision: https://reviews.llvm.org/D78734

Author: nikic

llvm/include/llvm/Analysis/CaptureTracking.h

@@ -33,12 +33,12 @@ namespace llvm {
   /// counts as capturing it or not.  The boolean StoreCaptures specified
   /// whether storing the value (or part of it) into memory anywhere
   /// automatically counts as capturing it or not.
-  /// MaxUsesToExplore specifies how many uses should the analysis explore for
-  /// one value before giving up due too "too many uses".
+  /// MaxUsesToExplore specifies how many uses the analysis should explore for
+  /// one value before giving up due too "too many uses". If MaxUsesToExplore
+  /// is zero, a default value is assumed.
   bool PointerMayBeCaptured(const Value *V, bool ReturnCaptures,
                             bool StoreCaptures,
-                            unsigned MaxUsesToExplore =
-                                getDefaultMaxUsesToExploreForCaptureTracking());
+                            unsigned MaxUsesToExplore = 0);
 
   /// PointerMayBeCapturedBefore - Return true if this pointer value may be
   /// captured by the enclosing function (which is required to exist). If a
@@ -50,13 +50,13 @@ namespace llvm {
   /// (or part of it) into memory anywhere automatically counts as capturing it
   /// or not. Captures by the provided instruction are considered if the
   /// final parameter is true.
-  /// MaxUsesToExplore specifies how many uses should the analysis explore for
-  /// one value before giving up due too "too many uses".
+  /// MaxUsesToExplore specifies how many uses the analysis should explore for
+  /// one value before giving up due too "too many uses". If MaxUsesToExplore
+  /// is zero, a default value is assumed.
   bool PointerMayBeCapturedBefore(
       const Value *V, bool ReturnCaptures, bool StoreCaptures,
       const Instruction *I, const DominatorTree *DT, bool IncludeI = false,
-      unsigned MaxUsesToExplore =
-          getDefaultMaxUsesToExploreForCaptureTracking());
+      unsigned MaxUsesToExplore = 0);
 
   /// This callback is used in conjunction with PointerMayBeCaptured. In
   /// addition to the interface here, you'll need to provide your own getters
@@ -89,11 +89,11 @@ namespace llvm {
   /// PointerMayBeCaptured - Visit the value and the values derived from it and
   /// find values which appear to be capturing the pointer value. This feeds
   /// results into and is controlled by the CaptureTracker object.
-  /// MaxUsesToExplore specifies how many uses should the analysis explore for
-  /// one value before giving up due too "too many uses".
+  /// MaxUsesToExplore specifies how many uses the analysis should explore for
+  /// one value before giving up due too "too many uses". If MaxUsesToExplore
+  /// is zero, a default value is assumed.
   void PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker,
-                            unsigned MaxUsesToExplore =
-                                getDefaultMaxUsesToExploreForCaptureTracking());
+                            unsigned MaxUsesToExplore = 0);
 } // end namespace llvm
 
 #endif

llvm/lib/Analysis/CaptureTracking.cpp

@@ -230,6 +230,9 @@ bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
 void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker,
                                 unsigned MaxUsesToExplore) {
   assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
+  if (MaxUsesToExplore == 0)
+    MaxUsesToExplore = DefaultMaxUsesToExplore;
+
   SmallVector<const Use *, 20> Worklist;
   Worklist.reserve(getDefaultMaxUsesToExploreForCaptureTracking());
   SmallSet<const Use *, 20> Visited;