Index: lib/libc/gdtoa/Makefile.inc
===================================================================
--- lib/libc/gdtoa/Makefile.inc
+++ lib/libc/gdtoa/Makefile.inc
@@ -18,4 +18,10 @@
 gdtoa_${src}: ${SRCTOP}/contrib/gdtoa/${src} .NOMETA
 	ln -sf ${.ALLSRC} ${.TARGET}
 CFLAGS.gdtoa_${src}+=-fno-strict-aliasing
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 100000
+# Avoid various floating-point optimizations that clang performs.
+CFLAGS.gdtoa_${src}+=	-ffp-exception-behavior=maytrap
+CFLAGS.gdtoa_${src}+=	-ffp-model=strict
+.endif
+
 .endfor
Index: lib/libc/gdtoa/_ldtoa.c
===================================================================
--- lib/libc/gdtoa/_ldtoa.c
+++ lib/libc/gdtoa/_ldtoa.c
@@ -35,8 +35,8 @@
 #include <math.h>
 #include <stdlib.h>
 #include "../stdio/floatio.h"
+#include "../stdio/gdtoa_wrapper.h"
 #include "fpmath.h"
-#include "gdtoaimp.h"
 
 /*
  * ldtoa() is a wrapper for gdtoa() that makes it smell like dtoa(),
Index: lib/libc/stdio/gdtoa_wrapper.h
===================================================================
--- /dev/null
+++ lib/libc/stdio/gdtoa_wrapper.h
@@ -0,0 +1,69 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright 2021 Alex Richardson <arichardson@FreeBSD.org>
+ *
+ * This software was developed by SRI International and the University of
+ * Cambridge Computer Laboratory under DARPA/AFRL contract (FA8750-10-C-0237)
+ * ("CTSRD"), as part of the DARPA CRASH research programme.
+ *
+ * This work was supported by Innovate UK project 105694, "Digital Security by
+ * Design (DSbD) Technology Platform Prototype".
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+/* Avoid references to fenv functions by requiring them to be always_inline. */
+#ifdef _FENV_H_
+#error "fenv.h was already included"
+#endif
+#define	__fenv_static static __always_inline
+#include <fenv.h>
+
+#include "gdtoaimp.h"
+/*
+ * For soft-float code, the fenv functions are defined out-of-line in libm,
+ * so we must avoid references to them inisde libc. We therefore only mask the
+ * FE_INEXACT flag for hard-float architectures.
+ */
+#ifndef SOFTFLOAT_FOR_GCC
+#undef dtoa
+#undef gdtoa
+
+/*
+ * Mask the inexact flag that can be set by dtoa due to (k = (int)ds;) to
+ * prevent printf() from changing the floating point exception flags.
+ */
+#define	dtoa(...) ({					\
+	/* TODO: _Pragma("STDC FENV_ACCESS ON"); */	\
+	int was_inexact = fetestexcept(FE_INEXACT);	\
+	char *result = __dtoa(__VA_ARGS__);		\
+	if (!was_inexact)				\
+		feclearexcept(FE_INEXACT);		\
+	result; })
+#define	gdtoa(...) ({					\
+	/* TODO: _Pragma("STDC FENV_ACCESS ON"); */	\
+	int was_inexact = fetestexcept(FE_INEXACT);	\
+	char *result = __gdtoa(__VA_ARGS__);		\
+	if (!was_inexact)				\
+		feclearexcept(FE_INEXACT);		\
+	result; })
+#endif
Index: lib/libc/stdio/printfcommon.h
===================================================================
--- lib/libc/stdio/printfcommon.h
+++ lib/libc/stdio/printfcommon.h
@@ -46,14 +46,10 @@
 
 
 #ifndef NO_FLOATING_POINT
-
-#define	dtoa		__dtoa
-#define	freedtoa	__freedtoa
-
 #include <float.h>
 #include <math.h>
 #include "floatio.h"
-#include "gdtoa.h"
+#include "gdtoa_wrapper.h"
 
 #define	DEFPREC		6
 
Index: lib/libc/stdio/xprintf_float.c
===================================================================
--- lib/libc/stdio/xprintf_float.c
+++ lib/libc/stdio/xprintf_float.c
@@ -41,13 +41,9 @@
 #include <assert.h>
 #include <locale.h>
 #include <limits.h>
-
-#define	dtoa		__dtoa
-#define	freedtoa	__freedtoa
-
 #include <float.h>
 #include <math.h>
-#include "gdtoa.h"
+#include "gdtoa_wrapper.h"
 #include "floatio.h"
 #include "printf.h"
 #include <un-namespace.h>