Re: [FFmpeg-devel] [PATCH] configure: add LTO optarg

["Martin Storsjö" <martin@martin.st>]

On Thu, 23 Mar 2023, J. Dekker wrote:

> This allows users to call with './configure --enable-lto' (or more
> explicitly './configure --enable-lto=full') for the old functionality or
> specify './configure --enable-lto=thin' to use ThinLTO.
>
> Signed-off-by: J. Dekker <jdek@itanimul.li>
> ---
> configure | 23 ++++++++++++++---------
> 1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/configure b/configure
> index 8980cec7ee..b65001d39d 100755
> --- a/configure
> +++ b/configure
> @@ -412,7 +412,7 @@ Toolchain options:
>   --build-suffix=SUFFIX    library name suffix []
>   --enable-pic             build position-independent code
>   --enable-thumb           compile for Thumb instruction set
> -  --enable-lto             use link-time optimization
> +  --enable-lto[=arg]       use link-time optimization
>   --env="ENV=override"     override the environment variables
>
> Advanced options (experts only):
> @@ -2524,7 +2524,6 @@ CMDLINE_SELECT="
>     debug
>     extra_warnings
>     logging
> -    lto
>     optimizations
>     rpath
>     stripping
> @@ -4170,6 +4169,12 @@ for opt do
>         --enable-sdl)
>             enable sdl2
>         ;;
> +        --enable-lto)
> +            lto=full
> +        ;;
> +        --enable-lto=*)
> +            lto="$optval"
> +        ;;
>         --enable-*=*|--disable-*=*)
>             eval $(echo "${opt%%=*}" | sed 's/--/action=/;s/-/ thing=/')
>             is_in "${thing}s" $COMPONENT_LIST || die_unknown "$opt"
> @@ -4639,7 +4644,7 @@ icl_flags(){
>             # on Windows, does enable remarks so disable them here.
>             -Wall)                echo $flag -Qdiag-disable:remark ;;
>             -std=c99)             echo -Qstd=c99 ;;
> -            -flto)                echo -ipo ;;
> +            -flto*)               echo -ipo ;;
>         esac
>     done
> }
> @@ -4647,7 +4652,7 @@ icl_flags(){
> icc_flags(){
>     for flag; do
>         case $flag in
> -            -flto)                echo -ipo ;;
> +            -flto*)               echo -ipo ;;
>             *)                    echo $flag ;;
>         esac
>     done
> @@ -7182,17 +7187,17 @@ fi
>
> check_optflags(){
>     check_cflags "$@"
> -    enabled lto && check_ldflags "$@"
> +    [ -z "$lto" ] || check_ldflags "$@"

With [ -n "$lto" ] you could retain the old logic with && instead of ||, 
although I don't think it matters much.

Also, this extra check when LTO is enabled, mirrors kinda what we've seen 
that we could need in meson, if LTO would be enabled surprisingly there.

> }
>
> check_optflags $optflags
> check_optflags -fno-math-errno
> check_optflags -fno-signed-zeros
>
> -if enabled lto; then
> +if [ ! -z "$lto" ]; then

This can be [ -n "$lto" ] to avoid the negation.

>     test "$cc_type" != "$ld_type" && die "LTO requires same compiler and linker"
> -    check_cflags  -flto
> -    check_ldflags -flto $cpuflags
> +    check_cflags  -flto=$lto
> +    check_ldflags -flto=$lto $cpuflags

Does GCC support -flto=full too, and is that the same thing as just -flto? 
Or should we stick to just passing -flto to the compiler without any 
argument if the user configured with plain --enable-lto?

>     disable inline_asm_direct_symbol_refs
> fi
>
> @@ -7223,7 +7228,7 @@ if enabled icc; then
>     # icc 11.0 and 11.1 work with ebp_available, but don't pass the test
>     enable ebp_available
>     # The test above does not test linking
> -    enabled lto && disable symver_asm_label
> +    [ -z "$lto" ] || disable symver_asm_label

[ -n "$lto" ] would retain the previous code structure, making the patch 
even clearer.

// Martin

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