[FFmpeg-devel] Re: [DISCUSSION] Adding ARM64EC support to FFmpeg

["Martin Storsjö via ffmpeg-devel" <ffmpeg-devel@ffmpeg.org>]

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Hi,

On Wed, 22 Oct 2025, Harish Raja Selvan wrote:

> 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.

Thanks! This version is nicer than the previous ones, and almost looks 
good to me - but there are a couple of smaller details I'd still like to 
fix up.

See the attached patch for my final fixed up version of this; is that one 
ok for you? If it is, I would prefer going with that. I'm commenting 
inline below about the issues I found:

> 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"*)

Checking for a literal / here is overly restrictive, as the option can be 
passed as either /machine or -machine. We probably don't need to 
explicitly match either of the flags literally, it may be enough to look 
for "machine:arm64ec".

(Note that I think link.exe may accept these flags in uppercase too, and 
then we wouldn't match it. But it's probably ok to not catch that here...)

> +        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')

There's two checks for $exit_thunk here - the later one matches a wider 
pattern than the former one. So the former one can be skipped entirely.

The added sed expression 's/^[[:space:]]*[0-9A-Fa-f]* //' makes the later 
"cut" command redundant, so I suggest removing that for clarity. (In the 
updated patch I refactored this to require 1 or more occurrances of the 
hex digits, and match 1 or more spaces after that as well, to make sure it 
eats up all spaces.

For mingw mode, we use a separate invocation of $NM above. I've extended 
that with an expression to remove the leading '#' as well, and remove the 
entry/exit thunks. With that in place, this works fine for arm64ec mingw 
as well.

>  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)"

This is the mingw version (where we pass NM=...) - here we don't need to 
check LDFLAGS, so we don't need to edit this line at all.

With those changes, in the attached patch, this looks good to me. I also 
extended the commit message a bit to mention the other subtle details in 
this change.

// Martin

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From e1a23db7d5a87cbc737386c4bf64c2957721473d Mon Sep 17 00:00:00 2001
From: Harishmcw <harish.rajaselvan@multicorewareinc.com>
Date: Fri, 17 Oct 2025 18:03:44 +0530
Subject: [PATCH] Fix .def file generation for ARM64EC builds on Windows
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

When building DLLs on ARM64EC, the default use of `dumpbin
-linkermember:1` fails because ARM64EC static libraries use a
different linker member format. Use `-linkermember:32` for ARM64EC
to correctly extract symbols.

Additionally, MSVC inserts $exit_thunk and $entry_thunk symbols
for ARM64EC to handle x64 ↔ ARM64 transitions. These are internal
thunks and must not be exported. Filter them out when generating
the .def file to avoid unresolved symbols or invalid exports.

Trim out the leading '#' on ARM64EC function symbols. This is only
relevant on ARM64EC, but it is benign to do that filtering on
all architectures (such symbols aren't expected on other
architectures).

Simplify the sed command by removing the symbol address with a
sed expression instead of a later "cut" command.

This ensures correct symbol extraction and stable DLL generation
on ARM64EC targets, while keeping behavior unchanged for other
Windows architectures.

Signed-off-by: Harishmcw <harish.rajaselvan@multicorewareinc.com>
---
 compat/windows/makedef | 25 ++++++++++++++++++++-----
 configure              |  2 +-
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/compat/windows/makedef b/compat/windows/makedef
index add8222d13..af42f08fd5 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 [ $? != 0 ]; then
     echo "Could not create temporary library." >&2
@@ -106,12 +112,21 @@ if [ -n "$NM" ]; then
               grep -v : |
               grep -v ^$ |
               cut -d' ' -f3 |
-              sed -e "s/^${prefix}//")
+              sed -e "s/^${prefix}//" -e "s/^#//" |
+              grep -v '\$entry_thunk' |
+              grep -v '\$exit_thunk')
 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/,$d' -e 's/^[[:space:]]*[0-9A-Fa-f]\{1,\}[[:space:]]\{1,\}//' -e "s/^${prefix}//" -e 's/^#//' |
               tail -n +2 |
-              cut -d' ' -f3)
+              grep -v '\$entry_thunk' |
+              grep -v '\$exit_thunk')
 fi
 
 rm ${libname}
diff --git a/configure b/configure
index 39e69d217d..db18aa2c28 100755
--- a/configure
+++ b/configure
@@ -6079,7 +6079,7 @@ 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)'
-- 
2.43.0


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