Re: LTO vs GCC 8

[Freddie Chopin <freddie_chopin@op.pl>]

On Fri, 2018-05-11 at 13:06 +0200, David Brown wrote:
> For the Cortex-M devices (and probably many other RISC targets),
> -fdata-sections comes at a big cost - it effectively blocks
> -fsection-anchors and makes access to file-static data a lot bigger.
> People often use -fdata-sections and -ffunction-sections along with
> -Wl,--gc-sections with the aim of removing unused code and data (and
> thus saving space, useful on small devices) - I would expect LTO
> would
> manage that anyway.  The other purpose of these is to improve
> locality
> of reference - again LTO should do that for you.  But even without
> LTO,
> I find the cost of -fdata-sections high compared to -fsection-
> anchors.

Unfortunatelly having LTO doesn't make -ffunction-sections + -fdata-
sections + --gc-sections useless.

My test project compiled:
- without LTO and without these attributes - 150824 B ROM + 4240 B RAM
- with LTO and without these attributes - 133812 B ROM + 4208 B RAM
- without LTO and with these attributes - 124456 B ROM + 3484 B RAM
- with LTO and with these attributes - 120280 B ROM + 3680 B RAM

As you see these attributes give much more than LTO in terms of size.

As for the -fsection-anchors I guess this has no use for non-PIC code
for arm-none-eabi. Whether I use it or not, the sizes are identical.

Regards,
FCh