Re: [Minios-devel] [UNIKRAFT PATCH v2 2/2] build: Provide LTO as build option

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

Hello Simon,

Please find the comments below:

On 05/11/2018 04:24 PM, Simon Kuenzer wrote:
> Introduces a build option that enables link time optmizations (LTO).
> When enabled, GCC is doing code optimization also while linking.
> Although the overall building time is increased, more efficient
> Unikraft images can be created.
>
> Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
> ---
>   Config.uk   | 8 ++++++++
>   Makefile.uk | 6 ++++++
>   2 files changed, 14 insertions(+)
>
> diff --git a/Config.uk b/Config.uk
> index eea961e..21cec9b 100644
> --- a/Config.uk
> +++ b/Config.uk
> @@ -78,6 +78,14 @@ config OPTIMIZE_DEADELIM
>   comment "Hint: Enable dropping of unused code to further reduce target image 
> size"
>         depends on OPTIMIZE_SIZE && !OPTIMIZE_DEADELIM
>   
> +config OPTIMIZE_LTO
> +       bool "Link time optimizations (LTO)"
> +       default n
> +       help
> +               Enables GCC to do code optimizations during linking. This option
> +               will increase overall building time but creates more efficient
> +               Unikraft binaries.
> +
gcc support stable LTO option since version 4.7. We may have to add 
checks to verify if the LTO option can be enabled with the gcc version.
>   config DEBUG_SYMBOLS
>         bool "Debugging information"
>         default n
> diff --git a/Makefile.uk b/Makefile.uk
> index c540ada..1964563 100644
> --- a/Makefile.uk
> +++ b/Makefile.uk
> @@ -34,6 +34,12 @@ CFLAGS-$(OPTIMIZE_DEADELIM)        += -fdata-sections 
> -ffunction-sections
>   CXXFLAGS-$(OPTIMIZE_DEADELIM)      += -fdata-sections -ffunction-sections
>   LDFLAGS-$(OPTIMIZE_DEADELIM)       += -Wl,--gc-sections
>   
> +# LTO requires the compiler flags to be handed over also for linking
> +CFLAGS-$(OPTIMIZE_LTO)             += -flto
> +CXXFLAGS-$(OPTIMIZE_LTO)           += -flto
> +LIBLDFLAGS-$(OPTIMIZE_LTO)         += $(CFLAGS) $(CFLAGS-y)
> +LDFLAGS-$(OPTIMIZE_LTO)            += $(CFLAGS) $(CFLAGS-y)
> +
>   ifneq ($(DEBUG_SYMBOLS),y)
>   CFLAGS                             += -g0
>   CXXFLAGS                           += -g0
The rest of the patch is fine.


Thanks & Regards
Sharan Santhanam

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