[PATCH] D90320: [llvm][StringExtras] Use a lookup table for `hexDigitValue`

[River Riddle via Phabricator via llvm-commits]

rriddle updated this revision to Diff 301373.
rriddle added a comment.

Simplify LUT


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90320/new/

https://reviews.llvm.org/D90320

Files:
  llvm/include/llvm/ADT/StringExtras.h


Index: llvm/include/llvm/ADT/StringExtras.h
===================================================================
--- llvm/include/llvm/ADT/StringExtras.h
+++ llvm/include/llvm/ADT/StringExtras.h
@@ -66,10 +66,22 @@
 ///
 /// If \p C is not a valid hex digit, -1U is returned.
 inline unsigned hexDigitValue(char C) {
-  if (C >= '0' && C <= '9') return C-'0';
-  if (C >= 'a' && C <= 'f') return C-'a'+10U;
-  if (C >= 'A' && C <= 'F') return C-'A'+10U;
-  return -1U;
+  struct HexTable {
+    unsigned LUT[255] = {};
+    constexpr HexTable() {
+      // Default initialize everything to invalid.
+      for (int i = 0; i < 255; ++i)
+        LUT[i] = -1U;
+      // Initialize `0`-`9`.
+      for (int i = 0; i < 10; ++i)
+        LUT['0' + i] = i;
+      // Initialize `A`-`F` and `a`-`f`.
+      for (int i = 0; i < 6; ++i)
+        LUT['A' + i] = LUT['a' + i] = 10 + i;
+    }
+  };
+  constexpr HexTable HexTable;
+  return HexTable.LUT[static_cast<unsigned char>(C)];
 }
 
 /// Checks if character \p C is one of the 10 decimal digits.


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