[llvm] [llvm] Improve implementation of StringRef::find_last_of and cie (PR #71865)

[via llvm-commits]

https://github.com/serge-sans-paille created https://github.com/llvm/llvm-project/pull/71865

Using a simple LUT implies less operations than a bitset, at the expense of slightly more stack usage.

Mandatory performance report:

https://llvm-compile-time-tracker.com/compare.php?from=3f9d385e5844f2f1f144305037cfc904789c6187&to=86f685e9552acaf8d5db630161e91752348422a1&stat=instructions:u

>From 852a30c4fd4e4251d9234518e850d14d5abe586a Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Thu, 9 Nov 2023 20:41:40 +0100
Subject: [PATCH] [llvm] Improve implementation of StringRef::find_last_of and
 cie

Using a simple LUT implies less operations than a bitset, at the expense
of slightly more stack usage.

Mandatory performance report:

https://llvm-compile-time-tracker.com/compare.php?from=3f9d385e5844f2f1f144305037cfc904789c6187&to=86f685e9552acaf8d5db630161e91752348422a1&stat=instructions:u
---
 llvm/lib/Support/StringRef.cpp | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/llvm/lib/Support/StringRef.cpp b/llvm/lib/Support/StringRef.cpp
index feee47ca693b251..be6e340ff0d5901 100644
--- a/llvm/lib/Support/StringRef.cpp
+++ b/llvm/lib/Support/StringRef.cpp
@@ -13,7 +13,6 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/edit_distance.h"
 #include "llvm/Support/Error.h"
-#include <bitset>
 
 using namespace llvm;
 
@@ -236,12 +235,12 @@ size_t StringRef::rfind_insensitive(StringRef Str) const {
 /// Note: O(size() + Chars.size())
 StringRef::size_type StringRef::find_first_of(StringRef Chars,
                                               size_t From) const {
-  std::bitset<1 << CHAR_BIT> CharBits;
+  bool CharBits[1 << CHAR_BIT] = {false};
   for (char C : Chars)
-    CharBits.set((unsigned char)C);
+    CharBits[(unsigned)C] = true;
 
   for (size_type i = std::min(From, Length), e = Length; i != e; ++i)
-    if (CharBits.test((unsigned char)Data[i]))
+    if (CharBits[(unsigned char)Data[i]])
       return i;
   return npos;
 }
@@ -258,12 +257,12 @@ StringRef::size_type StringRef::find_first_not_of(char C, size_t From) const {
 /// Note: O(size() + Chars.size())
 StringRef::size_type StringRef::find_first_not_of(StringRef Chars,
                                                   size_t From) const {
-  std::bitset<1 << CHAR_BIT> CharBits;
+  bool CharBits[1 << CHAR_BIT] = {false};
   for (char C : Chars)
-    CharBits.set((unsigned char)C);
+    CharBits[(unsigned)C] = true;
 
   for (size_type i = std::min(From, Length), e = Length; i != e; ++i)
-    if (!CharBits.test((unsigned char)Data[i]))
+    if (!CharBits[(unsigned char)Data[i]])
       return i;
   return npos;
 }
@@ -274,12 +273,12 @@ StringRef::size_type StringRef::find_first_not_of(StringRef Chars,
 /// Note: O(size() + Chars.size())
 StringRef::size_type StringRef::find_last_of(StringRef Chars,
                                              size_t From) const {
-  std::bitset<1 << CHAR_BIT> CharBits;
+  bool CharBits[1 << CHAR_BIT] = {false};
   for (char C : Chars)
-    CharBits.set((unsigned char)C);
+    CharBits[(unsigned)C] = true;
 
   for (size_type i = std::min(From, Length) - 1, e = -1; i != e; --i)
-    if (CharBits.test((unsigned char)Data[i]))
+    if (CharBits[(unsigned char)Data[i]])
       return i;
   return npos;
 }
@@ -299,12 +298,12 @@ StringRef::size_type StringRef::find_last_not_of(char C, size_t From) const {
 /// Note: O(size() + Chars.size())
 StringRef::size_type StringRef::find_last_not_of(StringRef Chars,
                                                  size_t From) const {
-  std::bitset<1 << CHAR_BIT> CharBits;
+  bool CharBits[1 << CHAR_BIT] = {false};
   for (char C : Chars)
-    CharBits.set((unsigned char)C);
+    CharBits[(unsigned)C] = true;
 
   for (size_type i = std::min(From, Length) - 1, e = -1; i != e; --i)
-    if (!CharBits.test((unsigned char)Data[i]))
+    if (!CharBits[(unsigned char)Data[i]])
       return i;
   return npos;
 }