https://github.com/boostorg/boost/commit/5c8898af4
https://github.com/boostorg/predef/commit/fc99bc2

Index: boost_1_71_0/boost/predef/architecture.h
===================================================================
--- boost_1_71_0.orig/boost/predef/architecture.h
+++ boost_1_71_0/boost/predef/architecture.h
@@ -21,6 +21,7 @@ http://www.boost.org/LICENSE_1_0.txt)
 #include <boost/predef/architecture/ppc.h>
 #include <boost/predef/architecture/ptx.h>
 #include <boost/predef/architecture/pyramid.h>
+#include <boost/predef/architecture/riscv.h>
 #include <boost/predef/architecture/rs6k.h>
 #include <boost/predef/architecture/sparc.h>
 #include <boost/predef/architecture/superh.h>
Index: boost_1_71_0/boost/predef/architecture/riscv.h
===================================================================
--- /dev/null
+++ boost_1_71_0/boost/predef/architecture/riscv.h
@@ -0,0 +1,41 @@
+/*
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE_1_0.txt or copy at
+http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#ifndef BOOST_PREDEF_ARCHITECTURE_RISCV_H
+#define BOOST_PREDEF_ARCHITECTURE_RISCV_H
+
+#include <boost/predef/version_number.h>
+#include <boost/predef/make.h>
+
+/*`
+[heading `BOOST_ARCH_RISCV`]
+
+[@http://en.wikipedia.org/wiki/RISC-V] architecture.
+
+[table
+    [[__predef_symbol__] [__predef_version__]]
+
+    [[`__riscv`] [__predef_detection__]]
+    ]
+ */
+
+#define BOOST_ARCH_RISCV BOOST_VERSION_NUMBER_NOT_AVAILABLE
+
+#if defined(__riscv)
+#   undef BOOST_ARCH_RISCV
+#   define BOOST_ARCH_RISCV BOOST_VERSION_NUMBER_AVAILABLE
+#endif
+
+#if BOOST_ARCH_RISCV
+#   define BOOST_ARCH_RISCV_AVAILABLE
+#endif
+
+#define BOOST_ARCH_RISCV_NAME "RISC-V"
+
+#endif
+
+#include <boost/predef/detail/test.h>
+BOOST_PREDEF_DECLARE_TEST(BOOST_ARCH_RISCV,BOOST_ARCH_RISCV_NAME)
Index: boost_1_71_0/boostcpp.jam
===================================================================
--- boost_1_71_0.orig/boostcpp.jam
+++ boost_1_71_0/boostcpp.jam
@@ -607,7 +607,7 @@ rule address-model ( )
     return <conditional>@boostcpp.deduce-address-model ;
 }
 
-local deducable-architectures = arm mips1 power sparc x86 combined ;
+local deducable-architectures = arm mips1 power riscv sparc x86 combined ;
 feature.feature deduced-architecture : $(deducable-architectures) : propagated optional composite hidden ;
 for a in $(deducable-architectures)
 {
@@ -618,11 +618,12 @@ rule deduce-architecture ( properties *
 {
     local result ;
     local filtered = [ toolset-properties $(properties) ] ;
-    local names = arm mips1 power sparc x86 combined ;
+    local names = arm mips1 power riscv sparc x86 combined ;
     local idx = [ configure.find-builds "default architecture" : $(filtered)
         : /boost/architecture//arm
         : /boost/architecture//mips1
         : /boost/architecture//power
+        : /boost/architecture//riscv
         : /boost/architecture//sparc
         : /boost/architecture//x86
         : /boost/architecture//combined ] ;