[llvm] 7d950f0 - [NFC][TLI] Replace std::lower_bound call in getLibFunc with DenseMap lookup

[Dhruv Chawla via llvm-commits]

Author: Dhruv Chawla
Date: 2023-08-15T22:37:18+05:30
New Revision: 7d950f040e3da66ec83f91b58d8c2220483d6335

URL: https://github.com/llvm/llvm-project/commit/7d950f040e3da66ec83f91b58d8c2220483d6335
DIFF: https://github.com/llvm/llvm-project/commit/7d950f040e3da66ec83f91b58d8c2220483d6335.diff

LOG: [NFC][TLI] Replace std::lower_bound call in getLibFunc with DenseMap lookup

While std::lower_bound takes logarithmic time (relative to the length of
the array) to execute, DenseMap gives better performance characteristics
as it traverses few (if any) elements when collisions do occur,
especially when the number of elements are known in advance.

This gives a speedup of 0.24%:
https://llvm-compile-time-tracker.com/compare.php?from=ac00cca3d9c6c3e9118ebbe47aa5b3ba1ee7404f&to=7f3d4c8ce8cee3a236a2328e46b2a8374672b46e&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D157951

Added: 
    

Modified: 
    llvm/lib/Analysis/TargetLibraryInfo.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
index bb9dbd0eb0e1b8..15ba6468a30708 100644
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Support/CommandLine.h"
@@ -940,16 +941,26 @@ static StringRef sanitizeFunctionName(StringRef funcName) {
   return GlobalValue::dropLLVMManglingEscape(funcName);
 }
 
+static DenseMap<StringRef, LibFunc>
+buildIndexMap(ArrayRef<StringLiteral> StandardNames) {
+  DenseMap<StringRef, LibFunc> Indices;
+  unsigned Idx = 0;
+  Indices.reserve(LibFunc::NumLibFuncs);
+  for (const auto &Func : StandardNames)
+    Indices[Func] = static_cast<LibFunc>(Idx++);
+  return Indices;
+}
+
 bool TargetLibraryInfoImpl::getLibFunc(StringRef funcName, LibFunc &F) const {
   funcName = sanitizeFunctionName(funcName);
   if (funcName.empty())
     return false;
 
-  const auto *Start = std::begin(StandardNames);
-  const auto *End = std::end(StandardNames);
-  const auto *I = std::lower_bound(Start, End, funcName);
-  if (I != End && *I == funcName) {
-    F = (LibFunc)(I - Start);
+  static const DenseMap<StringRef, LibFunc> Indices =
+      buildIndexMap(StandardNames);
+
+  if (auto Loc = Indices.find(funcName); Loc != Indices.end()) {
+    F = Loc->second;
     return true;
   }
   return false;