GCC: and inlining failed in call to always_inline 'FOO': target specific option mismatch

AVX512 comes with the number of variants, and a compiler must know which AVX512 version it compiles.

GCC error inlining failed in call to always_inline 'FOO': target specific option mismatch occurs when a program containing some SIMD-intrinsics, and compiler has wrong or missing target options. The target option are introduced by "-m".

Lets look at the error from real world:

/usr/lib/gcc/x86_64-linux-gnu/5/include/avx512vlbwintrin.h:790:1: error: inlining failed in call to always_inline ‘_mm_movm_epi8’: target specific option mismatch

Now, when open avx512vlbwintrin.h, we see at the beginning of file:

...
#pragma GCC push_options
#pragma GCC target("avx512vl,avx512bw")
#define __DISABLE_AVX512VLBW__
...

Thus, in order to properly compile the program, gcc have to be feed with the two options listed at the target line: -mavx512vl and -mavx512bw.

Compiler warnings are your future errors

Months ago I was asked to upgrade GCC from version 4.7 to 4.9 and also cleanup configure scripts. Not very exciting, merely time consuming task. Oh, we were hit by a bug in libstdc++, but simple patch have fixed the problem. Few weeks later I was asked to change GCC switch from -std=c++11 to -std=c++14 -- the easiest task in the world. I had to modify single script, run configure, type make, then run tests... everything was OK. Quite boring so far. Read more ...

GCC - asm goto

Starting from GCC 4.5 the asm statement has new form: asm goto. Programmer can use any labels from C/C++ program, however output from this block is not allowed.

Using asm block in old form required more instructions and additional storage:

 uint32_t bit_set;
 asm (
  "bt %2, %%eax  \n"
  "setc %%al  \n"
  "movzx %%al, %%eax \n"
  : "=a" (bit_set)
  : "a" (number), "r" (bit)
  : "cc"
 );

 if (bit_set)
  goto has_bit;

Above code is compiled to:

 80483f6:       0f a3 d8                bt     %ebx,%eax
 80483f9:       0f 92 c0                setb   %al
 80483fc:       0f b6 c0                movzbl %al,%eax
 80483ff:       85 c0                   test   %eax,%eax
 8048401:       74 16                   je     8048419 

With asm goto the same task could be writting shorter and easier:

 asm goto (
  "bt %1, %0 \n"
  "jc %l[has_bit] \n"

  : /* no output */
  : "r" (number), "r" (bit)
  : "cc"
  : has_bit // name of label
 );

Complete demo is available. See also GCC documentation about Extended Asm.

Slow-paths in GNU libc strstr

I've observed that some patterns issued to strstr cause significant slowdown.

Sample program kill-strstr.c executes strstr(data, pattern), where data is a large string (16MB) filled with character ?; patterns are read from command line.

On my machine following times were recorded:

1. searching string 'johndoe'...
        time: 0.032
2. searching string '??????????????????a'...
        time: 0.050
3. searching string '??????????????????????????????a'...
        time: 0.049
4. searching string '???????????????????????????????a'...
        time: 0.274
5. searching string '??????????????????????????????a?'...
        time: 0.356
6. searching string '??????????????????????????????a??????????????????????????????'...
        time: 0.396

• Slowdown is visible in case 4 (5 times slower than pattern 3). Pattern has 32 characters, and contains '?', except last char.
• Even bigger slowdown occurs in case 5 (7 times slower). This pattern also contains 32 chars, but position of the single letter 'a' is last but one.
• Similar slowdown occurs in case 5 (nearly 8 times slower). In this pattern single letter 'a' is surrounded by 30 '?'.