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disable global visibility override hack (vis.h) by default

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neither current compilers nor linkers treat protected visibility the
way I expected, as having fixed source-level semantics rather than
being dependent on target-specific ABI details, and change seems
unlikely. while the use here does not actually depend on the specific
semantics, at least some versions of some linkers, especially lld,
refuse to allow linking to a libc.so where the symbols have protected
visibility. this cannot be detected at configure-time because linking
libc.so itself works fine, and because even if we could test linking
an application against libc.so successfully, we could not justifiably
assume that the same linker used to link libc.so would also be used
later to link applications.

disable the vis.h hack by default at the configure level, but add an
explicit "auto" option to request the old configure-time detection
rather than just removing it. this leaves it easy to evaluate whether
it actually resulted in significant size or performance benefits while
ensuring that out-of-the-box builds are not unlinkable for some users.

fortunately, preliminary evaluation suggests that at least x86_64,
arm, and aarch64 don't suffer at all from the change, and impact on
other archs is low. if low is not low enough, it should not be hard to
analyze where the significant PLT call ABI costs are present and
mitigate them without the hack.
This commit is contained in:
Rich Felker 2017-08-11 00:17:00 -04:00
parent 947d330f68
commit dc2f368e56
1 changed files with 3 additions and 2 deletions
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5
configure vendored
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@ -31,7 +31,7 @@ Optional features:
--enable-optimize=... optimize listed components for speed over size [auto]
--enable-debug build with debugging information [disabled]
--enable-warnings build with recommended warnings flags [disabled]
--enable-visibility use global visibility options to optimize PIC [auto]
--enable-visibility use global visibility options to optimize PIC [no]
--enable-wrapper=... build given musl toolchain wrapper [auto]
--disable-shared inhibit building shared library [enabled]
--disable-static inhibit building static library [enabled]
@ -135,7 +135,7 @@ target=
optimize=auto
debug=no
warnings=no
visibility=auto
visibility=no
shared=auto
static=yes
wrapper=auto
@ -163,6 +163,7 @@ case "$arg" in
--disable-debug|--enable-debug=no) debug=no ;;
--enable-warnings|--enable-warnings=yes) warnings=yes ;;
--disable-warnings|--enable-warnings=no) warnings=no ;;
--enable-visibility=auto) visibility=auto ;;
--enable-visibility|--enable-visibility=yes) visibility=yes ;;
--disable-visibility|--enable-visibility=no) visibility=no ;;
--enable-wrapper|--enable-wrapper=yes) wrapper=detect ;;