Hi,
as disucssed some time ago, our assumption that every symbol of shared library
can
be interposed at runtime is expensive and prevents a lot of useful
optimizations,
including inlining or IPA propagation.
While this is useful feature, it is rather incommon to use it for bigger C++
projects, like firefox and at least clang seems to ignore the ELF interposition
rules and always inline/propagate. This patch adds flag to control the
behaviour.
Symbols explicitly delcared WEAK are still considered as interposable.
Bootstrapped/regtested x86_64-linux, will commit it tomorrow if there are
no complains. (Feedback is welcome!)
Honza
* doc/invoke.texi (-fsemantic-interposition): New flag.
* common.opt (fsemantic-interposition): Use it.
* varasm.c (decl_replaceable_p): Use it.
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi (revision 210653)
+++ doc/invoke.texi (working copy)
@@ -411,6 +411,7 @@ Objective-C and Objective-C++ Dialects}.
-fschedule-insns -fschedule-insns2 -fsection-anchors @gol
-fselective-scheduling -fselective-scheduling2 @gol
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops @gol
+-fsemantic-interposition @gol
-fshrink-wrap -fsignaling-nans -fsingle-precision-constant @gol
-fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector @gol
-fstack-protector-all -fstack-protector-strong -fstrict-aliasing @gol
@@ -7709,6 +7710,22 @@ This option has no effect unless one of
When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect unless @option{-fsel-sched-pipelining} is turned on.
+@item -fsemantic-interposition
+@opindex fsemantic-interposition
+Some object formats, like ELF, allow interposing of symbols by dynamic linker.
+This means that for symbols exported from the DSO compiler can not perform
+inter-procedural propagation, inlining and other optimizations in anticipation
+that the function or variable in question may change. While this feature is
+useful, for example, to rewrite memory allocation functions by a debugging
+implementation, it is expensive in the terms of code quality.
+With @option{-fno-semantic-inteposition} compiler assumest that if
interposition
+happens for functions the overwritting function will have
+precisely same semantics (and side effects). Similarly if interposition happens
+for variables, the constructor of the variable will be the same. The flag
+has no effect for functions explicitly declared inline, where
+interposition changing semantic is never allowed and for symbols explicitly
+declared weak.
+
@item -fshrink-wrap
@opindex fshrink-wrap
Emit function prologues only before parts of the function that need it,
Index: common.opt
===================================================================
--- common.opt (revision 210653)
+++ common.opt (working copy)
@@ -1854,6 +1854,10 @@ fsel-sched-reschedule-pipelined
Common Report Var(flag_sel_sched_reschedule_pipelined) Init(0) Optimization
Reschedule pipelined regions without pipelining
+fsemantic-interposition
+Common Report Var(flag_semantic_interposition) Init(1)
+Allow interposing function (or variables) by ones with different semantics (or
initializer) respectively by dynamic linker
+
; sched_stalled_insns means that insns can be moved prematurely from the queue
; of stalled insns into the ready list.
fsched-stalled-insns
Index: varasm.c
===================================================================
--- varasm.c (revision 210654)
+++ varasm.c (working copy)
@@ -6870,6 +6870,9 @@ decl_replaceable_p (tree decl)
gcc_assert (DECL_P (decl));
if (!TREE_PUBLIC (decl) || DECL_COMDAT (decl))
return false;
+ if (!flag_semantic_interposition
+ && !DECL_WEAK (decl))
+ return false;
return !decl_binds_to_current_def_p (decl);
}
