invoke.texi (-fsemantic-interposition): Document.

	* doc/invoke.texi (-fsemantic-interposition): Document.
	* common.opt (fsemantic-interposition): New flag.
	* varasm.c (decl_replaceable_p): Use it.
	* gcc.dg/tree-ssa/interposition.c: New testcase.

From-SVN: r212049

gcc/ChangeLog

+2014-06-26  Jan Hubicka  <hubicka@ucw.cz>
+
+	* doc/invoke.texi (-fsemantic-interposition): Document.
+	* common.opt (fsemantic-interposition): New flag.
+	* varasm.c (decl_replaceable_p): Use it.
+
 2014-06-26  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
 
 	PR target/61542

gcc/common.opt

@@ -1855,6 +1855,10 @@ 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

gcc/doc/invoke.texi

@@ -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 -fssa-phiopt -fstack-protector @gol
 -fstack-protector-all -fstack-protector-strong -fstrict-aliasing @gol
@@ -7738,6 +7739,22 @@ This option has no effect unless one of @option{-fselective-scheduling} or
 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,

gcc/testsuite/ChangeLog

+2014-06-26  Jan Hubicka  <hubicka@ucw.cz>
+
+	* gcc.dg/tree-ssa/interposition.c: New testcase.
+
 2014-06-26  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/56633

gcc/testsuite/gcc.dg/tree-ssa/interposition.c

+/* { dg-do compile } */
+/* { dg-options "-O1 -fno-semantic-interposition -fdump-tree-optimized -fPIC" } */
+int t(void)
+{
+  return 1;
+}
+int q(void)
+{
+  return t();
+}
+/* { dg-final { scan-tree-dump-times "return 1" 2 "optimized"} } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */

gcc/varasm.c

@@ -6830,6 +6830,9 @@ 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);
 }