ARM: make gd a function call for LTO and set via set_gd()

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>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/arm/mach-imx/imx8m/soc.c b/arch/arm/mach-imx/imx8m/soc.c
index 36abb2e57f..0c44022a6d 100644
--- a/arch/arm/mach-imx/imx8m/soc.c
+++ b/arch/arm/mach-imx/imx8m/soc.c
@@ -537,7 +537,7 @@ enum boot_device get_boot_device(void)
 
 	ret = g_rom_api->query_boot_infor(QUERY_BT_DEV, &boot,
 					  ((uintptr_t)&boot) ^ QUERY_BT_DEV);
-	gd = pgd;
+	set_gd(pgd);
 
 	if (ret != ROM_API_OKAY) {
 		puts("ROMAPI: failure at query_boot_info\n");