Some investigation on timing and comparing the SO answers of http://stackoverflow.com/a/25541942/466611 and http://stackoverflow.com/a/25541929/466611

gistfile1.md

Testing code:

#include <stdio.h>
#include <string.h>
#include <assert.h>

/*
 * Implements http://stackoverflow.com/a/25541929/466611
 */
static inline unsigned int test1(int n)
{
    return (1u << n) - 1;
}

/*
 * Implements http://stackoverflow.com/a/25541942/466611
 */
static inline unsigned int test2(int n)
{
    return 0xffffffff >> (32 - n);
}

int main(int argc, char **argv)
{
    assert((argc == 2) || (argc == 3));
    unsigned int repeat = 1 << 24;
    if (argc > 2)
        repeat = atol(argv[2]);
    if (!strcmp("test1", argv[1]))
    {
        volatile unsigned int tmp;
        unsigned int i;
        for (i = 0; i < repeat; i++)
            tmp = test1(i & 31);
    }
    else if (!strcmp("test2", argv[1]))
    {
        volatile unsigned int tmp;
        unsigned int i;
        for (i = 0; i < repeat; i++)
            tmp = test2(i & 31);
    }
    else
        assert(0);
    return 0;
}

Compiling parameters and sample results:

[pengyu@GLaDOS tmp]$ gcc a.c -O2
[pengyu@GLaDOS tmp]$ time ./a.out test1

real	0m0.031s
user	0m0.027s
sys	0m0.003s
[pengyu@GLaDOS tmp]$ time ./a.out test2

real	0m0.040s
user	0m0.037s
sys	0m0.000s

Testing platform info:

[pengyu@GLaDOS tmp]$ cat /proc/cpuinfo | grep "model name" | head -n 1
model name	: Intel(R) Core(TM) i7-3740QM CPU @ 2.70GHz
[pengyu@GLaDOS tmp]$ uname -a
Linux GLaDOS 3.16.1-1-ARCH #1 SMP PREEMPT Thu Aug 14 07:40:19 CEST 2014 x86_64 GNU/Linux
[pengyu@GLaDOS tmp]$ gcc --version | head -n 1
gcc (GCC) 4.9.1
[pengyu@GLaDOS tmp]$ ld --version | head -n 1
GNU ld (GNU Binutils) 2.24

Core function assembly code:

Generated using: gcc a.c -O2 -S -fverbose-asm

Core function code for test1:

 92 .L6:                                                                           
 93     movl    %eax, %edx  # tmp126, D.2768                                       
 94     sall    %cl, %edx   # i, D.2768
 95     addl    $1, %ecx    #, i                                                   
 96     subl    $1, %edx    #, D.2768                                              
 97     cmpl    %ecx, %ebx  # i, repeat
 98     movl    %edx, 24(%rsp)  # D.2768, tmp                                      
 99     ja  .L6 #,

Core function code for test2:

124 .L8:
125     movl    %edx, %eax  # i, D.2767
126     movl    %edi, %ecx  # tmp127, D.2767
127     addl    $1, %edx    #, i
128     andl    $31, %eax   #, D.2767
129     subl    %eax, %ecx  # D.2767, D.2767
130     movl    %esi, %eax  # tmp128, D.2768
131     shrl    %cl, %eax   # D.2767, D.2768
132     cmpl    %ebx, %edx  # repeat, i
133     movl    %eax, 28(%rsp)  # D.2768, tmp
134     jne .L8 #,