ARM: Prevent the compiler from using NEON registers

For ARMv8-A, NEON is standard, so the compiler can use it even when no
special target flags are provided. For example, it can use stores from
NEON registers to zero-initialize large structures. GCC 11 decides to
do this inside the DRAM init code for the Allwinner H6.

However, GCC 11 has a bug where it generates misaligned NEON register
stores even with -mstrict-align. Since the MMU is not enabled this early
in SPL, the misaligned store causes an exception and breaks booting.

Work around this issue by restricting the compiler to using GPRs only,
not vector registers. This prevents any future surprises relating to
NEON use as well.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Andre Przywara <andre.przywara@arm.com>

diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index b684d8b416f77641fdcaa0a28c0e39e195fc8d9f..b107b1af27ac07571144532eda80fb86c9b223c1 100644 (file)
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -25,6 +25,7 @@ endif
 
 PLATFORM_RELFLAGS += -fno-common -ffixed-r9
 PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
+                    $(call cc-option,-mgeneral-regs-only) \
       $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 
 # LLVM support