Hello,

This patch adds support for ARM64EC builds on Windows by updating .def file generation logic for shared libraries and ensuring the correct machine flag is passed to lib.exe and dumpbin.exe.

In-Lined patch:

diff --git a/compat/windows/makedef b/compat/windows/makedef
index add8222d13..261e7d463f 100755
--- a/compat/windows/makedef
+++ b/compat/windows/makedef
@@ -48,7 +48,13 @@ trap 'rm -f -- $libname' EXIT
 if [ -n "$AR" ]; then
     $AR rcs ${libname} $@ >/dev/null
 else
-    lib.exe -out:${libname} $@ >/dev/null
+    machine_flag=""
+    case "$LDFLAGS" in
+    *"/machine:arm64ec"*)
+        machine_flag="-machine:arm64ec"
+        ;;
+    esac
+    lib.exe ${machine_flag} -out:${libname} $@ >/dev/null
 fi
 if [ $? != 0 ]; then
     echo "Could not create temporary library." >&2
@@ -108,10 +114,19 @@ if [ -n "$NM" ]; then
               cut -d' ' -f3 |
               sed -e "s/^${prefix}//")
 else
-    dump=$(dumpbin.exe -linkermember:1 ${libname} |
-              sed -e '/public symbols/,$!d' -e '/^ \{1,\}Summary/,$d' -e "s/ \{1,\}${prefix}/ /" -e 's/ \{1,\}/ /g' |
+    member=1
+    case "$LDFLAGS" in
+    *"/machine:arm64ec"*)
+        member=32
+        ;;
+    esac
+    dump=$(dumpbin.exe -linkermember:${member} ${libname} |
+              sed -e '/public symbols/,$!d' -e '/^ \{1,\}Summary/,$d' -e 's/^[[:space:]]*[0-9A-Fa-f]* //' -e "s/^${prefix}//" -e 's/^#//' |
               tail -n +2 |
-              cut -d' ' -f3)
+              cut -d' ' -f3 |
+              grep -v '\$exit_thunk$' |
+              grep -v '\$entry_thunk' |
+              grep -v '\$exit_thunk')
 fi

 rm ${libname}
diff --git a/configure b/configure
index 7828381b5d..fbf5ab38a4 100755
--- a/configure
+++ b/configure
@@ -6040,7 +6040,7 @@ case $target_os in
         SLIB_INSTALL_LINKS=
         SLIB_INSTALL_EXTRA_SHLIB='$(SLIBNAME:$(SLIBSUF)=.lib)'
         SLIB_INSTALL_EXTRA_LIB='lib$(SLIBNAME:$(SLIBSUF)=.dll.a) $(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.def)'
-        SLIB_CREATE_DEF_CMD='EXTERN_PREFIX="$(EXTERN_PREFIX)" AR="$(AR_CMD)" NM="$(NM_CMD)" $(SRC_PATH)/compat/windows/makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
+        SLIB_CREATE_DEF_CMD='LDFLAGS="$(LDFLAGS)" EXTERN_PREFIX="$(EXTERN_PREFIX)" AR="$(AR_CMD)" NM="$(NM_CMD)" $(SRC_PATH)/compat/windows/makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
         SHFLAGS='-shared -Wl,--out-implib,$(SUBDIR)lib$(SLIBNAME:$(SLIBSUF)=.dll.a) -Wl,--disable-auto-image-base $$(@:$(SLIBSUF)=.def)'
         enabled x86_64 && objformat="win64" || objformat="win32"
         dlltool="${cross_prefix}dlltool"
@@ -6078,7 +6078,7 @@ case $target_os in
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
-        SLIB_CREATE_DEF_CMD='EXTERN_PREFIX="$(EXTERN_PREFIX)" $(SRC_PATH)/compat/windows/makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
+        SLIB_CREATE_DEF_CMD='LDFLAGS="$(LDFLAGS)" EXTERN_PREFIX="$(EXTERN_PREFIX)" $(SRC_PATH)/compat/windows/makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
         SLIB_INSTALL_NAME='$(SLIBNAME_WITH_MAJOR)'
         SLIB_INSTALL_LINKS=
         SLIB_INSTALL_EXTRA_SHLIB='$(SLIBNAME:$(SLIBSUF)=.lib)'
--
2.50.1.windows.1

For reference, here is the configuration used to build FFmpeg with MSVC for ARM64EC:

AR_CMD="lib.exe -machine:arm64ec"
ARCH="arm64"

"${SRC_DIR}/configure" \
    --toolchain=msvc \
    --target-os=win64 \
    --cc=cl.exe \
    --cxx=cl.exe \
    --extra-cxxflags="-arm64EC" \
    --extra-cflags="-arm64EC" \
    --extra-ldflags="/machine:arm64ec" \
    --as="armasm64.exe -machine ARM64EC" \
    --ld=link.exe \
    --ar="${AR_CMD}" \
    --arch="${ARCH}" \
    --enable-shared

Thanks,
Harish Raja Selvan.