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5 Commits

Author SHA1 Message Date
Fangrui Song d1fd72343c Refactor how -fno-semantic-interposition sets dso_local on default visibility external linkage definitions 
The idea is that the CC1 default for ELF should set dso_local on default
visibility external linkage definitions in the default -mrelocation-model pic
mode (-fpic/-fPIC) to match COFF/Mach-O and make output IR similar.

The refactoring is made available by 2820a2ca3a.

Currently only x86 supports local aliases. We move the decision to the driver.
There are three CC1 states:

* -fsemantic-interposition: make some linkages interposable and make default visibility external linkage definitions dso_preemptable.
* (default): selected if the target supports .Lfoo$local: make default visibility external linkage definitions dso_local
* -fhalf-no-semantic-interposition: if neither option is set or the target does not support .Lfoo$local: like -fno-semantic-interposition but local aliases are not used. So references can be interposed if not optimized out.

Add -fhalf-no-semantic-interposition to a few tests using the half-based semantic interposition behavior.
 2020-12-31 13:59:45 -08:00
Fangrui Song 9d55e4ee13 Make explicit -fno-semantic-interposition (in -fpic mode) infer dso_local 
-fno-semantic-interposition is currently the CC1 default. (The opposite
disables some interprocedural optimizations.) However, it does not infer
dso_local: on most targets accesses to ExternalLinkage functions/variables
defined in the current module still need PLT/GOT.

This patch makes explicit -fno-semantic-interposition infer dso_local,
so that PLT/GOT can be eliminated if targets implement local aliases
for AsmPrinter::getSymbolPreferLocal (currently only x86).

Currently we check whether the module flag "SemanticInterposition" is 0.
If yes, infer dso_local. In the future, we can infer dso_local unless
"SemanticInterposition" is 1: frontends other than clang will also
benefit from the optimization if they don't bother setting the flag.
(There will be risks if they do want ELF interposition: they need to set
"SemanticInterposition" to 1.)
 2020-05-25 20:48:18 -07:00
Fangrui Song 3ecba396e9 [Driver][test] Change %itanium_abi_triple to generic ELF 
x86_64-windows and darwin default to PIC. They don't use PIE.
 2020-02-02 21:46:48 -08:00
Fangrui Song 7eeb901528 [Driver] Fix fsemantic-interposition.c for Windows and Darwin 2020-02-02 20:55:24 -08:00
Fangrui Song aed488e3a4 [Driver] Move -fsemantic-interposition decision from cc1 to driver 
And add test/Driver/fsemantic-interposition.c
 2020-02-02 20:45:29 -08:00