[PATCH] D90265: [llvm][StringExtras] Add a fail-able version of `fromHex` and optimize hex digit lookup

[River Riddle via Phabricator via llvm-commits]

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This revision adds a fail-able/checked version of `fromHex` that fails when the input string contains a non-hex character. This removes the need for users to have a separate check for if the string contains all hex digits. This becomes very costly for large hex strings given that checking if a string contains only hex digits is effectively the same as just converting it in the first place.

This revision also refactors `hexDigitValue` to use a lookup table instead of if statements, which also leads to a large speedup of hex conversion.

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 (2 seconds -> 300 miliseconds).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D90265

Files:
  llvm/include/llvm/ADT/StringExtras.h
  llvm/unittests/ADT/StringExtrasTest.cpp

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