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hash/log2: Add generic implementations of fls() and fls64() 

When importing the hashtable code in I8ef73a62fe9846ce45058eb21cf999dd3eed5741
I didn't import actual implementations of the fls() and fls64()
implementations, as at least gcc-10 was smart enough to detect
we only use it on constant types and hence the computation can happen
at build time via const_ilog2()

However, in our jenkins build verification' this doesn't appear to
happen, as we get below errors:

/build/deps/install/stow/libosmocore/include/osmocom/core/log2.h: In function ‘__ilog2_u32’:
/build/deps/install/stow/libosmocore/include/osmocom/core/log2.h:20:9: error: implicit declaration of function ‘fls’ [-Werror=implicit-function-declaration]
  return fls(n) - 1;
         ^~~
/build/deps/install/stow/libosmocore/include/osmocom/core/log2.h: In function ‘__ilog2_u64’:
/build/deps/install/stow/libosmocore/include/osmocom/core/log2.h:28:9: error: implicit declaration of function ‘fls64’ [-Werror=implicit-function-declaration]
  return fls64(n) - 1;
         ^~~~~

Let's provide some generic implementations for this case.  If needed
one could also introduce architecture-specific assembly implementations
like in the Linux kernel, but so far we managed to keep libosmocore free
of any assembly tweaks.

Change-Id: Ifa4898eb66c8d949618edd47961b7a0330ed35b5
This commit is contained in:
Harald Welte 2020-12-06 14:37:18 +01:00
parent c22e002890
commit 622cda3802
1 changed files with 59 additions and 0 deletions
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59
include/osmocom/core/log2.h
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@ -6,6 +6,65 @@
*/
#pragma once
#define __always_inline inline __attribute__((always_inline))
/* from linux/asm-generic/bitops/{fls,fls64}.h - could later be enhanced
* with architecture specific optimized versions */
/**
* fls - find last (most-significant) bit set
* @x: the word to search
*
* This is defined the same way as ffs.
* Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
*/
static __always_inline int fls(unsigned int x)
{
int r = 32;
if (!x)
return 0;
if (!(x & 0xffff0000u)) {
x <<= 16;
r -= 16;
}
if (!(x & 0xff000000u)) {
x <<= 8;
r -= 8;
}
if (!(x & 0xf0000000u)) {
x <<= 4;
r -= 4;
}
if (!(x & 0xc0000000u)) {
x <<= 2;
r -= 2;
}
if (!(x & 0x80000000u)) {
x <<= 1;
r -= 1;
}
return r;
}
/**
* fls64 - find last set bit in a 64-bit word
* @x: the word to search
*
* This is defined in a similar way as the libc and compiler builtin
* ffsll, but returns the position of the most significant set bit.
*
* fls64(value) returns 0 if value is 0 or the position of the last
* set bit if value is nonzero. The last (most significant) bit is
* at position 64.
*/
static __always_inline int fls64(__u64 x)
{
__u32 h = x >> 32;
if (h)
return fls(h) + 32;
return fls(x);
}
/*
* non-constant log of base 2 calculators