[llvm] f6a6f27 - [llvm][StringExtras] Use a lookup table for `hexDigitValue`

[River Riddle via llvm-commits]

Author: River Riddle
Date: 2020-10-28T16:58:06-07:00
New Revision: f6a6f27edb3991154393976e9e8f7b88542d406c

URL: https://github.com/llvm/llvm-project/commit/f6a6f27edb3991154393976e9e8f7b88542d406c
DIFF: https://github.com/llvm/llvm-project/commit/f6a6f27edb3991154393976e9e8f7b88542d406c.diff

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

This method is at the core of the conversion from hex to binary, and using a lookup table great improves the compile time of hex conversions.

Context: In MLIR we use hex strings to represent very large constants in the textual format of the IR. These changes lead to a large decrease in compile time when parsing these constants (>1 second -> 350 miliseconds).

Differential Revision: https://reviews.llvm.org/D90320

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/include/llvm/ADT/StringExtras.h b/llvm/include/llvm/ADT/StringExtras.h
index 6084a564b95d..77288f8514dc 100644
--- a/llvm/include/llvm/ADT/StringExtras.h
+++ b/llvm/include/llvm/ADT/StringExtras.h
@@ -66,10 +66,22 @@ inline ArrayRef<uint8_t> arrayRefFromStringRef(StringRef Input) {
 ///
 /// 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 Table;
+  return Table.LUT[static_cast<unsigned char>(C)];
 }
 
 /// Checks if character \p C is one of the 10 decimal digits.