Merge #19403: build: improve __builtin_clz* detection

9952242 build: improve builtin_clz* detection (fanquake)

Pull request description:

  Fixes #19402.

  The way we currently test for `__builtin_clz*` support with `AC_CHECK_DECLS` does not work with Clang:
  ```bash
  configure:21492: clang++-10 -std=c++11 -c -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS conftest.cpp >&5
  conftest.cpp:100:10: error: builtin functions must be directly called
    (void) __builtin_clz;
           ^
  1 error generated.
  ```

  This also removes the `__builtin_clz()` check, as we don't actually use it anywhere, and it's trvial to re-add detection if we do start using it at some point. If this is controversial then I'll add a test for it as well.

ACKs for top commit:
  sipa:
    ACK 9952242
  laanwj:
    ACK 9952242

Tree-SHA512: 695abb1a694a01a25aaa483b4fffa7d598842f2ba4fe8630fbed9ce5450b915c33bf34bb16ad16a16b702dd7c91ebf49fe509a2498b9e28254fe0ec5177bbac0

Author: laanwj

build_msvc/bitcoin_config.h

@@ -137,18 +137,6 @@
    don't. */
 #define HAVE_DECL_STRNLEN 1
 
-/* Define to 1 if you have the declaration of `__builtin_clz', and to 0 if you
-   don't. */
-//#define HAVE_DECL___BUILTIN_CLZ 1
-
-/* Define to 1 if you have the declaration of `__builtin_clzl', and to 0 if
-   you don't. */
-//#define HAVE_DECL___BUILTIN_CLZL 1
-
-/* Define to 1 if you have the declaration of `__builtin_clzll', and to 0 if
-   you don't. */
-//#define HAVE_DECL___BUILTIN_CLZLL 1
-
 /* Define to 1 if you have the <dlfcn.h> header file. */
 /* #undef HAVE_DLFCN_H */
 

configure.ac

@@ -850,7 +850,21 @@ AC_CHECK_DECLS([bswap_16, bswap_32, bswap_64],,,
                  #include <byteswap.h>
                  #endif])
 
-AC_CHECK_DECLS([__builtin_clz, __builtin_clzl, __builtin_clzll])
+AC_MSG_CHECKING(for __builtin_clzl)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
+ (void) __builtin_clzl(0);
+  ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_BUILTIN_CLZL, 1, [Define this symbol if you have __builtin_clzl])],
+ [ AC_MSG_RESULT(no)]
+)
+
+AC_MSG_CHECKING(for __builtin_clzll)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
+  (void) __builtin_clzll(0);
+  ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_BUILTIN_CLZLL, 1, [Define this symbol if you have __builtin_clzll])],
+ [ AC_MSG_RESULT(no)]
+)
 
 dnl Check for malloc_info (for memory statistics information in getmemoryinfo)
 AC_MSG_CHECKING(for getmemoryinfo)

src/crypto/common.h

@@ -82,12 +82,12 @@ void static inline WriteBE64(unsigned char* ptr, uint64_t x)
 /** Return the smallest number n such that (x >> n) == 0 (or 64 if the highest bit in x is set. */
 uint64_t static inline CountBits(uint64_t x)
 {
-#if HAVE_DECL___BUILTIN_CLZL
+#if HAVE_BUILTIN_CLZL
     if (sizeof(unsigned long) >= sizeof(uint64_t)) {
         return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0;
     }
 #endif
-#if HAVE_DECL___BUILTIN_CLZLL
+#if HAVE_BUILTIN_CLZLL
     if (sizeof(unsigned long long) >= sizeof(uint64_t)) {
         return x ? 8 * sizeof(unsigned long long) - __builtin_clzll(x) : 0;
     }