include __builtin_popcount replacement function

Some systems/compilers lack __builtin_popcount(), so replace it as
necessary. Reported by Dennis Clarke; ok dtucker@

Author: djmdjm

configure.ac

@@ -2084,6 +2084,19 @@ AC_CHECK_FUNCS([ \
 	warn \
 ])
 
+AC_MSG_CHECKING([whether compiler supports __builtin_popcount])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+		#include <stdlib.h>
+	]],
+	[[ int x = 123, y;
+	   y = __builtin_popcount(123);
+	   exit(y == 6 ? 0 : -1); ]])],
+	[ AC_MSG_RESULT([yes]) ], [
+		AC_MSG_RESULT([no])
+		AC_DEFINE([MISSING_BUILTIN_POPCOUNT], [1], [Define if your compiler lacks __builtin_popcount])
+	]
+)
+
 AC_CHECK_DECLS([bzero, memmem])
 
 dnl Wide character support.

libcrux_mlkem768_sha3.h

@@ -177,10 +177,14 @@ static inline uint32_t core_num__u32_8__from_le_bytes(uint8_t buf[4]) {
 }
 
 static inline uint32_t core_num__u8_6__count_ones(uint8_t x0) {
-#ifdef _MSC_VER
+#if defined(_MSC_VER)
   return __popcnt(x0);
-#else
+#elif !defined(MISSING_BUILTIN_POPCOUNT)
   return __builtin_popcount(x0);
+#else
+  const uint8_t v[16] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };
+  return v[x0 & 0xf] + v[(x0 >> 4) & 0xf];
+
 #endif
 }
 

mlkem768.sh

@@ -49,6 +49,11 @@ echo '#define KRML_HOST_EPRINTF(...)'
 echo '#define KRML_HOST_EXIT(x) fatal_f("internal error")'
 echo
 
+__builtin_popcount_replacement='
+  const uint8_t v[16] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };
+  return v[x0 & 0xf] + v[(x0 >> 4) & 0xf];
+'
+
 for i in $FILES; do
 	echo "/* from $i */"
 	# Changes to all files:
@@ -62,7 +67,10 @@ for i in $FILES; do
 		# Replace endian functions with versions that work.
 		perl -0777 -pe 's/(static inline void core_num__u64_9__to_le_bytes.*\n)([^}]*\n)/\1  v = htole64(v);\n\2/' |
 		perl -0777 -pe 's/(static inline uint64_t core_num__u64_9__from_le_bytes.*?)return v;/\1return le64toh(v);/s' |
-		perl -0777 -pe 's/(static inline uint32_t core_num__u32_8__from_le_bytes.*?)return v;/\1return le32toh(v);/s'
+		perl -0777 -pe 's/(static inline uint32_t core_num__u32_8__from_le_bytes.*?)return v;/\1return le32toh(v);/s' |
+		# Compat for popcount.
+		perl -0777 -pe 's/\#ifdef (_MSC_VER)(.*?return __popcnt\(x0\);)/\#if defined(\1)\2/s' |
+		perl -0777 -pe "s/\\#else(\\n\\s+return __builtin_popcount\\(x0\\);)/\\#elif !defined(MISSING_BUILTIN_POPCOUNT)\\1\\n#else$__builtin_popcount_replacement/s"
 		;;
 	# Default: pass through.
 	*)