On Thu, May 20, 2021 at 01:24:10PM +0200, Marek Behún wrote:
On ARM, the gd pointer is stored in registers r9 / x18. For this the -ffixed-r9 / -ffixed-x18 flag is used when compiling, but using global register variables causes errors when building with LTO, and these errors are very difficult to overcome.
Richard Biener says [1]: Note that global register vars shouldn't be used with LTO and if they are restricted to just a few compilation units the recommended fix is to build those CUs without -flto.
We cannot do this for U-Boot since all CUs use -ffixed-reg flag.
It seems that with LTO we could in fact store the gd pointer differently and gain performance or size benefit by allowing the compiler to use r9 / x18. But this would need more work.
So for now, when building with LTO, go the clang way, and instead of declaring gd a global register variable, we make it a function call via macro.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68384
Signed-off-by: Marek Behún marek.behun@nic.cz Reviewed-by: Simon Glass sjg@chromium.org
Applied to u-boot/master, thanks!