diff --git a/.cirrus.yml b/.cirrus.yml
index 7b3db44..818ab76 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -69,6 +69,7 @@ task:
     - env: {WIDEMUL:  int64,  RECOVERY: yes}
     - env: {WIDEMUL:  int64,                 ECDH: yes, SCHNORRSIG: yes}
     - env: {WIDEMUL: int128}
+    - env: {WIDEMUL: int128_struct}
     - env: {WIDEMUL: int128,  RECOVERY: yes,            SCHNORRSIG: yes}
     - env: {WIDEMUL: int128,                 ECDH: yes, SCHNORRSIG: yes}
     - env: {WIDEMUL: int128,  ASM: x86_64}
@@ -272,20 +273,22 @@ task:
     EXPERIMENTAL: yes
     SCHNORRSIG: yes
     CTIMETEST: no
+    # Use a MinGW-w64 host to tell ./configure we're building for Windows.
+    # This will detect some MinGW-w64 tools but then make will need only
+    # the MSVC tools CC, AR and NM as specified below.
+    HOST: x86_64-w64-mingw32
+    CC: /opt/msvc/bin/x64/cl
+    AR: /opt/msvc/bin/x64/lib
+    NM: /opt/msvc/bin/x64/dumpbin -symbols -headers
     # Set non-essential options that affect the CLI messages here.
     # (They depend on the user's taste, so we don't want to set them automatically in configure.ac.)
     CFLAGS: -nologo -diagnostics:caret
     LDFLAGS: -XCClinker -nologo -XCClinker -diagnostics:caret
-  # Use a MinGW-w64 host to tell ./configure we're building for Windows.
-  # This will detect some MinGW-w64 tools but then make will need only
-  # the MSVC tools CC, AR and NM as specified below.
   matrix:
     - name: "x86_64 (MSVC): Windows (Debian stable, Wine)"
+    - name: "x86_64 (MSVC): Windows (Debian stable, Wine, int128_struct)"
       env:
-        HOST: x86_64-w64-mingw32
-        CC: /opt/msvc/bin/x64/cl
-        AR: /opt/msvc/bin/x64/lib
-        NM: /opt/msvc/bin/x64/dumpbin -symbols -headers
+        WIDEMUL: int128_struct
     - name: "i686 (MSVC): Windows (Debian stable, Wine)"
       env:
         HOST: i686-w64-mingw32
diff --git a/src/tests.c b/src/tests.c
index 34681b4..a6a18ed 100644
--- a/src/tests.c
+++ b/src/tests.c
@@ -26,6 +26,7 @@
 #include "modinv32_impl.h"
 #ifdef SECP256K1_WIDEMUL_INT128
 #include "modinv64_impl.h"
+#include "int128_impl.h"
 #endif
 
 #define CONDITIONAL_TEST(cnt, nam) if (count < (cnt)) { printf("Skipping %s (iteration count too low)\n", nam); } else
@@ -430,6 +431,47 @@ void run_scratch_tests(void) {
     secp256k1_context_destroy(none);
 }
 
+
+#ifdef SECP256K1_WIDEMUL_INT128
+void run_int128_tests(void) {
+    {   /* secp256k1_u128_accum_mul */
+        secp256k1_uint128 res;
+
+        /* Check secp256k1_u128_accum_mul overflow */
+        secp256k1_u128_from_u64(&res, 0);
+        secp256k1_u128_accum_mul(&res, UINT64_MAX, UINT64_MAX);
+        secp256k1_u128_accum_mul(&res, UINT64_MAX, UINT64_MAX);
+        CHECK(secp256k1_u128_to_u64(&res) == 2);
+        CHECK(secp256k1_u128_hi_u64(&res) == 18446744073709551612U);
+    }
+    {   /* secp256k1_u128_accum_mul */
+        secp256k1_int128 res;
+
+        /* Compute INT128_MAX = 2^127 - 1 with secp256k1_i128_accum_mul */
+        secp256k1_i128_from_i64(&res, 0);
+        secp256k1_i128_accum_mul(&res, INT64_MAX, INT64_MAX);
+        secp256k1_i128_accum_mul(&res, INT64_MAX, INT64_MAX);
+        CHECK(secp256k1_i128_to_i64(&res) == 2);
+        secp256k1_i128_accum_mul(&res, 4, 9223372036854775807);
+        secp256k1_i128_accum_mul(&res, 1, 1);
+        CHECK((uint64_t)secp256k1_i128_to_i64(&res) == UINT64_MAX);
+        secp256k1_i128_rshift(&res, 64);
+        CHECK(secp256k1_i128_to_i64(&res) == INT64_MAX);
+
+        /* Compute INT128_MIN = - 2^127 with secp256k1_i128_accum_mul */
+        secp256k1_i128_from_i64(&res, 0);
+        secp256k1_i128_accum_mul(&res, INT64_MAX, INT64_MIN);
+        CHECK(secp256k1_i128_to_i64(&res) == INT64_MIN);
+        secp256k1_i128_accum_mul(&res, INT64_MAX, INT64_MIN);
+        CHECK(secp256k1_i128_to_i64(&res) == 0);
+        secp256k1_i128_accum_mul(&res, 2, INT64_MIN);
+        CHECK(secp256k1_i128_to_i64(&res) == 0);
+        secp256k1_i128_rshift(&res, 64);
+        CHECK(secp256k1_i128_to_i64(&res) == INT64_MIN);
+    }
+}
+#endif
+
 void run_ctz_tests(void) {
     static const uint32_t b32[] = {1, 0xffffffff, 0x5e56968f, 0xe0d63129};
     static const uint64_t b64[] = {1, 0xffffffffffffffff, 0xbcd02462139b3fc3, 0x98b5f80c769693ef};
@@ -7100,6 +7142,9 @@ int main(int argc, char **argv) {
     run_rand_bits();
     run_rand_int();
 
+#ifdef SECP256K1_WIDEMUL_INT128
+    run_int128_tests();
+#endif
     run_ctz_tests();
     run_modinv_tests();
     run_inverse_tests();