Speedup some unicode rendering

Use a fast path for column width computation for ascii characters. Especially
relevant for llvm-objdump.

before:

    % time ./bin/llvm-objdump -D  -j .text /lib/libc.so.6 >/dev/null
    ./bin/llvm-objdump -D -j .text /lib/libc.so.6 > /dev/null  0.75s user 0.01s system 99% cpu 0.757 total

after:

    % time ./bin/llvm-objdump -D  -j .text /lib/libc.so.6 >/dev/null
    ./bin/llvm-objdump -D -j .text /lib/libc.so.6 > /dev/null  0.37s user 0.01s system 99% cpu 0.378 total

Differential Revision: https://reviews.llvm.org/D92180

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Support/Unicode.cpp b/llvm/lib/Support/Unicode.cpp
index 4d19506..bb6e755 100644
--- a/llvm/lib/Support/Unicode.cpp
+++ b/llvm/lib/Support/Unicode.cpp
@@ -339,11 +339,22 @@ static inline int charWidth(int UCS)
   return 1;
 }
 
+static bool isprintableascii(char c) { return c > 31 && c < 127; }
+
 int columnWidthUTF8(StringRef Text) {
   unsigned ColumnWidth = 0;
   unsigned Length;
   for (size_t i = 0, e = Text.size(); i < e; i += Length) {
     Length = getNumBytesForUTF8(Text[i]);
+
+    // fast path for ASCII characters
+    if (Length == 1) {
+      if (!isprintableascii(Text[i]))
+        return ErrorNonPrintableCharacter;
+      ColumnWidth += 1;
+      continue;
+    }
+
     if (Length <= 0 || i + Length > Text.size())
       return ErrorInvalidUTF8;
     UTF32 buf[1];