This is a combination of several upstream patches which weren't accepted.
They were proposed by Fedora for spidermonkey 52 and were ultimately
accepted years later after some changes for a later version. It was
adapted slightly from both sets of patches to apply cleanly to mozjs-60.

https://bugzilla.mozilla.org/show_bug.cgi?id=1318905
https://bug1318905.bmoattachments.org/attachment.cgi?id=8812602
https://bug1318905.bmoattachments.org/attachment.cgi?id=8812603
https://bug1318905.bmoattachments.org/attachment.cgi?id=8812604
https://phabricator.services.mozilla.com/D78623
https://phabricator.services.mozilla.com/D78624
https://phabricator.services.mozilla.com/D78625


diff --git a/build/moz.configure/init.configure b/build/moz.configure/init.configure
index 83b8d705..59131525 100644
--- a/build/moz.configure/init.configure
+++ b/build/moz.configure/init.configure
@@ -676,6 +676,9 @@ def split_triplet(triplet, allow_unknown=False):
     elif cpu == 'sh4':
         canonical_cpu = 'sh4'
         endianness = 'little'
+    elif cpu.startswith('riscv64'):
+        canonical_cpu = 'riscv64'
+        endianness = 'little'
     elif allow_unknown:
         canonical_cpu = cpu
         endianness = 'unknown'
diff --git a/js/src/jit/AtomicOperations.h b/js/src/jit/AtomicOperations.h
index a8970b0d..6b947a3f 100644
--- a/js/src/jit/AtomicOperations.h
+++ b/js/src/jit/AtomicOperations.h
@@ -375,7 +375,7 @@ AtomicOperations::isLockfreeJS(int32_t size)
 # endif
 #elif defined(__ppc__) || defined(__PPC__)
 # include "jit/none/AtomicOperations-feeling-lucky.h"
-#elif defined(__sparc__)
+#elif defined(__sparc__) || defined(__riscv)
 # include "jit/none/AtomicOperations-feeling-lucky.h"
 #elif defined(__ppc64__) || defined(__PPC64__) || defined(__ppc64le__) || defined(__PPC64LE__)
 # include "jit/none/AtomicOperations-feeling-lucky.h"
diff --git a/js/src/jit/none/AtomicOperations-feeling-lucky.h b/js/src/jit/none/AtomicOperations-feeling-lucky.h
index da572284..6ce40c89 100644
--- a/js/src/jit/none/AtomicOperations-feeling-lucky.h
+++ b/js/src/jit/none/AtomicOperations-feeling-lucky.h
@@ -49,6 +49,12 @@
 #  define GNUC_COMPATIBLE
 #endif
 
+#if defined(__riscv) && __riscv_xlen == 64
+#  define HAS_64BIT_ATOMICS
+#  define HAS_64BIT_LOCKFREE
+#  define GNUC_COMPATIBLE
+#endif
+
 #ifdef __sparc__
 #  define GNUC_COMPATIBLE
 #  ifdef  __LP64__
diff --git a/js/src/jit/none/MacroAssembler-none.h b/js/src/jit/none/MacroAssembler-none.h
index 80143dc8..b430fedb 100644
--- a/js/src/jit/none/MacroAssembler-none.h
+++ b/js/src/jit/none/MacroAssembler-none.h
@@ -402,6 +402,10 @@ class MacroAssemblerNone : public Assembler
 #endif
 };
 
+    struct AutoPrepareForPatching {
+        explicit AutoPrepareForPatching(MacroAssemblerNone&) {}
+    };
+
 typedef MacroAssemblerNone MacroAssemblerSpecific;
 
 class ABIArgGenerator
diff --git a/mfbt/tests/TestPoisonArea.cpp b/mfbt/tests/TestPoisonArea.cpp
index 06c24ed0..c3fed0df 100644
--- a/mfbt/tests/TestPoisonArea.cpp
+++ b/mfbt/tests/TestPoisonArea.cpp
@@ -160,6 +160,9 @@
 #elif defined __aarch64__
 #define RETURN_INSTR 0xd65f03c0 /* ret */
 
+#elif defined __riscv
+#define RETURN_INSTR 0x80828082 /* ret; ret */
+
 #elif defined __ia64
 struct ia64_instr { uint32_t mI[4]; };
 static const ia64_instr _return_instr =
diff --git a/python/mozbuild/mozbuild/configure/constants.py b/python/mozbuild/mozbuild/configure/constants.py
index 33ae5a45..11a01d3b 100644
--- a/python/mozbuild/mozbuild/configure/constants.py
+++ b/python/mozbuild/mozbuild/configure/constants.py
@@ -50,6 +50,7 @@ CPU_bitness = {
     'mips64': 64,
     'ppc': 32,
     'ppc64': 64,
+    'riscv64': 64,
     's390': 32,
     's390x': 64,
     'sh4': 32,
@@ -82,6 +84,7 @@ CPU_preprocessor_checks = OrderedDict((
     ('s390', '__s390__'),
     ('ppc64', '__powerpc64__'),
     ('ppc', '__powerpc__'),
+    ('riscv64', '__riscv && __riscv_xlen == 64'),
     ('Alpha', '__alpha__'),
     ('hppa', '__hppa__'),
     ('sparc64', '__sparc__ && __arch64__'),
diff --git a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
index cb7ff709..9da41adf 100755
--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
@@ -1165,6 +1165,10 @@ class LinuxCrossCompileToolchainTest(BaseToolchainTest):
         'sh4-unknown-linux-gnu': little_endian + {
             '__sh__': 1,
         },
+        'riscv64-unknown-linux-gnu': little_endian + {
+            '__riscv': 1,
+            '__riscv_xlen': 64,
+        },
     }
 
     PLATFORMS['powerpc64le-unknown-linux-gnu'] = \