Change in libosmocore[master]: hash/log2: Add generic implementations of fls() and fls64()

[laforge]

laforge has submitted this change. ( https://gerrit.osmocom.org/c/libosmocore/+/21562 )

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

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
---
M include/osmocom/core/log2.h
1 file changed, 59 insertions(+), 0 deletions(-)

Approvals:
  Jenkins Builder: Verified
  laforge: Looks good to me, approved



diff --git a/include/osmocom/core/log2.h b/include/osmocom/core/log2.h
index 06b20f8..dfe9d37 100644
--- a/include/osmocom/core/log2.h
+++ b/include/osmocom/core/log2.h
@@ -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

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Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: Ifa4898eb66c8d949618edd47961b7a0330ed35b5
Gerrit-Change-Number: 21562
Gerrit-PatchSet: 1
Gerrit-Owner: laforge <laforge at osmocom.org>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: laforge <laforge at osmocom.org>
Gerrit-MessageType: merged
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