К clang много вопросов, не догадывается до достаточно простых вещей. В книге, кстати, рассматривается также и "переписывать какую-то функциональность самостоятельно", но что-то не впечатляет. Можно предположить, что только 1 вариант неплохо работает (но он ощутимо медленнее "улучшенных" вариантов):

        time -p ./bin/readfile_string.gcc "$TESTFILE"
        time -p ./bin/readfile_string.gcc "$TESTFILE"
        time -p ./bin/readfile_string.gcc "$TESTFILE"
---
real 12.70
user 1.54
sys 1.93
---
real 6.53
user 1.59
sys 1.76
---
real 5.95
user 1.63
sys 1.59
        time -p ./bin/readfile_string.clang "$TESTFILE"
        time -p ./bin/readfile_string.clang "$TESTFILE"
        time -p ./bin/readfile_string.clang "$TESTFILE"
---
real 11.67
user 0.92
sys 1.96
---
real 6.69
user 0.87
sys 1.68
---
real 5.73
user 0.88
sys 1.68
        time -p ./bin/readfile_resize.gcc "$TESTFILE"
        time -p ./bin/readfile_resize.gcc "$TESTFILE"
        time -p ./bin/readfile_resize.gcc "$TESTFILE"
---
real 10.72
user 0.12
sys 1.33
---
real 1.14
user 0.14
sys 0.99
---
real 1.14
user 0.15
sys 0.98
        time -p ./bin/readfile_resize.clang "$TESTFILE"
        time -p ./bin/readfile_resize.clang "$TESTFILE"
        time -p ./bin/readfile_resize.clang "$TESTFILE"
---
real 10.79
user 0.42
sys 1.44
---
real 1.35
user 0.47
sys 0.87
---
real 1.32
user 0.43
sys 0.87
        time -p ./bin/readfile_assign.gcc "$TESTFILE"
        time -p ./bin/readfile_assign.gcc "$TESTFILE"
        time -p ./bin/readfile_assign.gcc "$TESTFILE"
---
real 11.03
user 0.41
sys 1.86
---
real 3.86
user 0.44
sys 1.61
---
real 3.84
user 0.40
sys 1.56
        time -p ./bin/readfile_assign.clang "$TESTFILE"
        time -p ./bin/readfile_assign.clang "$TESTFILE"
        time -p ./bin/readfile_assign.clang "$TESTFILE"
---
real 11.04
user 0.76
sys 2.03
---
real 3.99
user 0.71
sys 1.55
---
real 5.06
user 0.71
sys 1.70
        time -p ./bin/readfile_whileread.gcc "$TESTFILE"
        time -p ./bin/readfile_whileread.gcc "$TESTFILE"        
        time -p ./bin/readfile_whileread.gcc "$TESTFILE"
---
real 31.52
user 29.94
sys 0.77
---
real 30.23
user 29.93
sys 0.25
---
real 30.20
user 29.90
sys 0.26
        time -p ./bin/readfile_whileread.clang "$TESTFILE"
        time -p ./bin/readfile_whileread.clang "$TESTFILE"
        time -p ./bin/readfile_whileread.clang "$TESTFILE"
---
real 101.05
user 93.37
sys 4.81
---
real 90.21
user 83.18
sys 4.93
---
real 89.42
user 83.58
sys 4.34