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Make libc symbols hidden in static PIE

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Hidden visibility can avoid indirections and RELATIVE relocs in
static PIE libc.

The check should use IS_IN_LIB instead of IS_IN(libc) since all
symbols are defined locally in static PIE and the optimization is
useful in all libraries not just libc. However the test system
links objects from libcrypt.a into dynamic linked test binaries
where hidden visibility does not work.  I think mixing static and
shared libc components in the same binary should not be supported
usage, but to be safe only use hidden in libc.a.

On some targets (i386) this optimization cannot be applied because
hidden visibility PIE ifunc functions don't work, so it is gated by
NO_HIDDEN_EXTERN_FUNC_IN_PIE.

From -static-pie linked 'int main(){}' this shaves off 71 relative
relocs on aarch64 and reduces code size by about 2k.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Szabolcs Nagy 2021-01-06 11:31:04 +00:00
parent 86d439b06f
commit 2682695e5c
1 changed files with 7 additions and 2 deletions
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9
include/libc-symbols.h
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@ -434,13 +434,18 @@ for linking")
strong_alias(real, name)
#endif
#if defined SHARED || defined LIBC_NONSHARED \
|| (BUILD_PIE_DEFAULT && IS_IN (libc))
#if defined SHARED || defined LIBC_NONSHARED
# define attribute_hidden __attribute__ ((visibility ("hidden")))
#else
# define attribute_hidden
#endif
/* Mark all symbols hidden in static PIE libc to avoid GOT indirections. */
#if BUILD_PIE_DEFAULT && !defined NO_HIDDEN_EXTERN_FUNC_IN_PIE \
&& IS_IN (libc) && !defined LIBC_NONSHARED
# pragma GCC visibility push(hidden)
#endif
#define attribute_tls_model_ie __attribute__ ((tls_model ("initial-exec")))
#define attribute_relro __attribute__ ((section (".data.rel.ro")))