diff --git a/libc/testlib/BUILD.mk b/libc/testlib/BUILD.mk
index ddd3ce16a..83fb3dd40 100644
--- a/libc/testlib/BUILD.mk
+++ b/libc/testlib/BUILD.mk
@@ -50,6 +50,7 @@ LIBC_TESTLIB_A_SRCS_C =						\
 	libc/testlib/clearxmmregisters.c			\
 	libc/testlib/contains.c					\
 	libc/testlib/endswith.c					\
+	libc/testlib/exactlyequallongdouble.c		 	\
 	libc/testlib/extract.c					\
 	libc/testlib/ezbenchcontrol.c				\
 	libc/testlib/ezbenchreport.c				\
diff --git a/libc/testlib/exactlyequallongdouble.c b/libc/testlib/exactlyequallongdouble.c
new file mode 100644
index 000000000..fa6f73e6c
--- /dev/null
+++ b/libc/testlib/exactlyequallongdouble.c
@@ -0,0 +1,35 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2024 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/math.h"
+#include "libc/testlib/testlib.h"
+
+bool testlib_exactlyequallongdouble(long double x, long double y) {
+  if (isnan(x) && isnan(y))
+    return true;
+  // Check that we don't have e.g. one input denormal and the other not
+  // (a denormal and a non-denormal can sometimes compare equal)
+  if (fpclassify(x) != fpclassify(y))
+    return false;
+  // Check that we don't have -0 and 0
+  if (signbit(x) != signbit(y))
+    return false;
+  if (x != y)
+    return false;
+  return true;
+}
diff --git a/libc/testlib/testlib.h b/libc/testlib/testlib.h
index 398c50004..9c8053c71 100644
--- a/libc/testlib/testlib.h
+++ b/libc/testlib/testlib.h
@@ -195,6 +195,13 @@ void TearDownOnce(void);
 #define ASSERT_LDBL_LT(VAL, GOT) \
   assertLongDoubleLessThan(VAL, GOT, #VAL " < " #GOT, true)
 
+#define ASSERT_FLOAT_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, true)
+#define ASSERT_DOUBLE_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, true)
+#define ASSERT_LDBL_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, true)
+
 /*───────────────────────────────────────────────────────────────────────────│─╗
 │ cosmopolitan § testing library » assert or log                           ─╬─│┼
 ╚────────────────────────────────────────────────────────────────────────────│*/
@@ -271,6 +278,13 @@ void TearDownOnce(void);
 #define EXPECT_LGBL_LT(VAL, GOT) \
   expectLongDoubleLessThan(VAL, GOT, #VAL " < " #GOT, false)
 
+#define EXPECT_FLOAT_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, false)
+#define EXPECT_DOUBLE_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, false)
+#define EXPECT_LDBL_EXACTLY_EQ(WANT, GOT) \
+  assertLongDoubleExactlyEquals(FILIFU WANT, GOT, #GOT, false)
+
 /*───────────────────────────────────────────────────────────────────────────│─╗
 │ cosmopolitan § testing library » implementation details                  ─╬─│┼
 ╚────────────────────────────────────────────────────────────────────────────│*/
@@ -404,6 +418,7 @@ void testlib_formatbinaryashex(const char *, const void *, size_t, char **,
 void testlib_formatbinaryasglyphs(const char16_t *, const void *, size_t,
                                   char **, char **);
 bool testlib_almostequallongdouble(long double, long double);
+bool testlib_exactlyequallongdouble(long double, long double);
 void testlib_incrementfailed(void);
 void testlib_clearxmmregisters(void);
 
@@ -696,5 +711,20 @@ forceinline void assertLongDoubleEquals(FILIFU_ARGS long double want,
   testlib_onfail2(isfatal);
 }
 
+forceinline void assertLongDoubleExactlyEquals(FILIFU_ARGS long double want,
+                                               long double got,
+                                               const char *gotcode,
+                                               bool isfatal) {
+  ++g_testlib_ran;
+  if (testlib_exactlyequallongdouble(want, got))
+    return;
+  if (g_testlib_shoulddebugbreak)
+    DebugBreak();
+  testlib_showerror(file, line, func, "assertLongDoubleExactlyEquals", "≠",
+                    gotcode, testlib_formatfloat(want),
+                    testlib_formatfloat(got));
+  testlib_onfail2(isfatal);
+}
+
 COSMOPOLITAN_C_END_
 #endif /* COSMOPOLITAN_LIBC_TESTLIB_H_ */
diff --git a/test/libc/stdio/fscanf_test.c b/test/libc/stdio/fscanf_test.c
index accfb4bdb..74005f3f4 100644
--- a/test/libc/stdio/fscanf_test.c
+++ b/test/libc/stdio/fscanf_test.c
@@ -27,7 +27,7 @@ TEST(fscanf, test_readAfterFloat) {
   EXPECT_EQ(4, fscanf(f, "%f%x%f%x", &f1, &i1, &f2, &i2));
   EXPECT_TRUE(isinf(f1));
   EXPECT_EQ(0xDEAD, i1);
-  EXPECT_EQ(-0.125e-2f, f2);
+  EXPECT_FLOAT_EXACTLY_EQ(-0.125e-2f, f2);
   EXPECT_EQ(0xBEEF, i2);
   fclose(f);
 }
diff --git a/test/libc/stdio/sscanf_test.c b/test/libc/stdio/sscanf_test.c
index 40e35f1c4..c134fa265 100644
--- a/test/libc/stdio/sscanf_test.c
+++ b/test/libc/stdio/sscanf_test.c
@@ -338,17 +338,17 @@ TEST(sscanf, flexdecimal_hex) {
 TEST(sscanf, floating_point_simple) {
   float x = 666.666f, y = x, z = y;
   EXPECT_EQ(3, sscanf("0.3715 .3715 3715", "%f %f %f", &x, &y, &z));
-  EXPECT_EQ(0.3715f, x);
-  EXPECT_EQ(0.3715f, y);
-  EXPECT_EQ(3715.0f, z);
+  EXPECT_FLOAT_EXACTLY_EQ(0.3715f, x);
+  EXPECT_FLOAT_EXACTLY_EQ(0.3715f, y);
+  EXPECT_FLOAT_EXACTLY_EQ(3715.0f, z);
 }
 
 TEST(sscanf, floating_point_simple_double_precision) {
   double x = 666.666, y = x, z = y;
   EXPECT_EQ(3, sscanf("0.3715 .3715 3715", "%lf %lf %lf", &x, &y, &z));
-  EXPECT_EQ(0.3715, x);
-  EXPECT_EQ(0.3715, y);
-  EXPECT_EQ(3715.0, z);
+  EXPECT_DOUBLE_EXACTLY_EQ(0.3715, x);
+  EXPECT_DOUBLE_EXACTLY_EQ(0.3715, y);
+  EXPECT_DOUBLE_EXACTLY_EQ(3715.0, z);
 }
 
 TEST(sscanf, floating_point_nan) {
@@ -426,12 +426,12 @@ TEST(sscanf, floating_point_documentation_examples) {
       2, sscanf("0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz -0.0000000123junk junk",
                 "%f %f %f %f %f", &f, &g, &h, &i, &j));
 
-  EXPECT_EQ(111.11f, a);
-  EXPECT_EQ(-2.22f, b);
+  EXPECT_FLOAT_EXACTLY_EQ(111.11f, a);
+  EXPECT_FLOAT_EXACTLY_EQ(-2.22f, b);
   EXPECT_TRUE(isnan(c));
   EXPECT_TRUE(isnan(d));
   EXPECT_TRUE(isinf(e));
-  EXPECT_EQ(0X1.BC70A3D70A3D7P+6f, f);
+  EXPECT_FLOAT_EXACTLY_EQ(0X1.BC70A3D70A3D7P+6f, f);
   EXPECT_TRUE(isinf(g));
 }
 
@@ -445,12 +445,12 @@ TEST(sscanf, floating_point_documentation_examples_double_precision) {
       2, sscanf("0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz -0.0000000123junk junk",
                 "%lf %lf %lf %lf %lf", &f, &g, &h, &i, &j));
 
-  EXPECT_EQ(111.11, a);
-  EXPECT_EQ(-2.22, b);
+  EXPECT_DOUBLE_EXACTLY_EQ(111.11, a);
+  EXPECT_DOUBLE_EXACTLY_EQ(-2.22, b);
   EXPECT_TRUE(isnan(c));
   EXPECT_TRUE(isnan(d));
   EXPECT_TRUE(isinf(e));
-  EXPECT_EQ(0X1.BC70A3D70A3D7P+6, f);
+  EXPECT_DOUBLE_EXACTLY_EQ(0X1.BC70A3D70A3D7P+6, f);
   EXPECT_TRUE(isinf(g));
 }
 
@@ -506,3 +506,9 @@ TEST(scanf, n) {
   ASSERT_EQ(1848, port);
   ASSERT_EQ(12, len);
 }
+
+TEST(sscanf, floating_point_hexadecimal) {
+  double a = 0;
+  ASSERT_EQ(1, sscanf("0x1.5014c3472bc2c0000000p-123", "%lf", &a));
+  ASSERT_DOUBLE_EXACTLY_EQ(0x1.5014c3472bc2c0000000p-123, a);
+}
diff --git a/third_party/gdtoa/gethex.c b/third_party/gdtoa/gethex.c
index 36f5cf751..22fd02d23 100644
--- a/third_party/gdtoa/gethex.c
+++ b/third_party/gdtoa/gethex.c
@@ -169,7 +169,9 @@ pcheck:
 			L = 0;
 			n = 0;
 		}
-		L |= (__gdtoa_hexdig[*s1] & 0x0f) << n;
+		// We can shift in a way that changes the sign bit or overflows,
+		// so we need to cast to unsigned to avoid undefined behavior
+		L |= (unsigned)(__gdtoa_hexdig[*s1] & 0x0f) << n;
 		n += 4;
 	}
 	*x++ = L;