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diff --git a/m4/asm-underscore.m4 b/m4/asm-underscore.m4
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+# asm-underscore.m4 serial 5
+dnl Copyright (C) 2010-2020 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl From Bruno Haible. Based on as-underscore.m4 in GNU clisp.
+
+# gl_ASM_SYMBOL_PREFIX
+# Tests for the prefix of C symbols at the assembly language level and the
+# linker level. This prefix is either an underscore or empty. Defines the
+# C macro USER_LABEL_PREFIX to this prefix, and sets ASM_SYMBOL_PREFIX to
+# a stringified variant of this prefix.
+
+AC_DEFUN([gl_ASM_SYMBOL_PREFIX],
+[
+  AC_REQUIRE([AC_PROG_EGREP])
+  dnl We don't use GCC's __USER_LABEL_PREFIX__ here, because
+  dnl 1. It works only for GCC.
+  dnl 2. It is incorrectly defined on some platforms, in some GCC versions.
+  AC_REQUIRE([gl_C_ASM])
+  AC_CACHE_CHECK(
+    [whether C symbols are prefixed with underscore at the linker level],
+    [gl_cv_prog_as_underscore],
+    [cat > conftest.c <<EOF
+#ifdef __cplusplus
+extern "C" int foo (void);
+#endif
+int foo(void) { return 0; }
+EOF
+     # Look for the assembly language name in the .s file.
+     AC_TRY_COMMAND(${CC-cc} $CFLAGS $CPPFLAGS $gl_c_asm_opt conftest.c) >/dev/null 2>&1
+     if LC_ALL=C $EGREP '(^|[[^a-zA-Z0-9_]])_foo([[^a-zA-Z0-9_]]|$)' conftest.$gl_asmext >/dev/null; then
+       gl_cv_prog_as_underscore=yes
+     else
+       gl_cv_prog_as_underscore=no
+     fi
+     rm -f conftest*
+    ])
+  if test $gl_cv_prog_as_underscore = yes; then
+    USER_LABEL_PREFIX=_
+  else
+    USER_LABEL_PREFIX=
+  fi
+  AC_DEFINE_UNQUOTED([USER_LABEL_PREFIX], [$USER_LABEL_PREFIX],
+    [Define to the prefix of C symbols at the assembler and linker level,
+     either an underscore or empty.])
+  ASM_SYMBOL_PREFIX='"'${USER_LABEL_PREFIX}'"'
+  AC_SUBST([ASM_SYMBOL_PREFIX])
+])
+
+# gl_C_ASM
+# Determines how to produce an assembly language file from C source code.
+# Sets the variables:
+#   gl_asmext - the extension of assembly language output,
+#   gl_c_asm_opt - the C compiler option that produces assembly language output.
+
+AC_DEFUN([gl_C_ASM],
+[
+  AC_EGREP_CPP([MicrosoftCompiler],
+    [
+#ifdef _MSC_VER
+MicrosoftCompiler
+#endif
+    ],
+    [dnl Microsoft's 'cl' and 'clang-cl' produce an .asm file, whereas 'clang'
+     dnl produces a .s file. Need to distinguish 'clang' and 'clang-cl'.
+     rm -f conftest*
+     echo 'int dummy;' > conftest.c
+     AC_TRY_COMMAND(${CC-cc} $CFLAGS $CPPFLAGS -c conftest.c) >/dev/null 2>&1
+     if test -f conftest.o; then
+       gl_asmext='s'
+       gl_c_asm_opt='-S'
+     else
+       gl_asmext='asm'
+       gl_c_asm_opt='-c -Fa'
+     fi
+     rm -f conftest*
+    ],
+    [gl_asmext='s'
+     gl_c_asm_opt='-S'
+    ])
+])