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lib: sbi: Avoid GOT indirection for global symbol references

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OpenSBI is compiled with -fPIE, which generally implies dynamic linking.
This causes the compiler to generate GOT references for global symbols
in order to support runtime symbol interposition. However, OpenSBI does
not actually perform dynamic linking, so the GOT indirection just adds
unnecessary overhead.

The GOT references can be avoided by declaring global symbols with
hidden visibility, thus making them local to this dynamic object and
non-interposable. GCC/Clang's -fvisibility parameter is insufficient for
this purpose when referencing objects from other translation units;
either __attribute__((visibility(...)) or the pragma is required. Use
the pragma since it is easier to apply to every symbol. Additionally
clean up the one GOT reference from inline assembly.

With this change, a firmware linked with LLD does not contain either a
GOT or a PLT, and a firmware linked with BFD ld contains only a GOT with
a single (unreferenced, legacy) _GLOBAL_OFFSET_TABLE_ entry.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
This commit is contained in:
Samuel Holland
2025-02-20 09:53:01 -08:00
committed by Anup Patel
parent 98c0a3860a
commit 86c01a73ff
3 changed files with 18 additions and 1 deletions
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1
Makefile
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@@ -350,6 +350,7 @@ ifeq ($(BUILD_INFO),y)
GENFLAGS += -DOPENSBI_BUILD_TIME_STAMP="\"$(OPENSBI_BUILD_TIME_STAMP)\""
GENFLAGS += -DOPENSBI_BUILD_COMPILER_VERSION="\"$(OPENSBI_BUILD_COMPILER_VERSION)\""
endif
GENFLAGS += -include $(include_dir)/sbi/sbi_visibility.h
ifdef PLATFORM
GENFLAGS += -include $(KCONFIG_AUTOHEADER)
endif