* invoke.texi (-fstrict-aliasing): Document.

	* rtl.texi (MEM_ALIAS_SET): Document.
	* flags.h (flag_strict_aliasing): Declare.
	* toplev.c (flag_strict_aliasing): Define.
	(f_options): Add -strict-aliasing.
	(main): Set flag_strict_aliasing if -O2 or higher.
	* tree.h (tree_type): Add alias_set field.
	(TYPE_ALIAS_SET): New macro.
	(TYPE_ALIAS_SET_KNOWN_P): Likewise.
	(get_alias_set): Declare.
	* tree.c (lang_get_alias_set): Define.
	(make_node): Initialize TYPE_ALIAS_SET.
	(get_alias_set): New function.
	* print-tree.c (print_node): Dump the alias set for a type.
	* c-tree.h (c_get_alias_set): Declare.
	* c-common.c (c_get_alias_set): New function.
	* c-decl.c (init_decl_processing): Set lang_get_alias_set.
	* expr.c (protect_from_queue): Propogage alias sets.
	(expand_assignment): Calculate alias set for new MEMs.
	(expand_expr): Likewise.
	* function.c (put_var_into_stack): Likewise.
	(put_reg_into_stack): Likewise.
	(gen_mem_addressof): Likewise.
	(assign_parms): Likewise.
	* stmt.c (expand_decl): Likewise.
	* varasm.c (make_decl_rtl): Eliminate redundant clearing of
	DECL_RTL.  Calculate alias set for new MEMs.
	* rtl.def (REG): Add dummy operand.
	(MEM): Add extra operand to store the MEM_ALIAS_SET.
	* rtl.h (MEM_ALIAS_SET): New macro.
	(gen_rtx_MEM): Declare.
	* emit-rtl.c (gen_rtx_MEM): New function.
	* gengenrtl.c (sepcial_rtx): Make MEMs special.
	* alias.c (CHECK_ALIAS_SETS_FOR_CONSISTENCY): New macro.
	(DIFFERENT_ALIAS_SETS_P): Likewise.
	(canon_rtx): Propogate the alias set to the new MEM.
	(true_dependence): Check the alias sets.
	(anti_dependence): Likewise.
	(output_dependence): Likewise.
	* explow.c (stabilize): Progoate alias sets.
	* integrate.c (copy_rtx_and_substitute): Likewise.
	* final.c (alter_subreg): Make sure not to leave MEM_IN_STRUCT_P
	in an unpredictable state.  Propogate alias sets.
	* reload1.c (reload): Clear MEM_ALIAS_SET for new MEMs about which
	we have no alias information.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@20719 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 52 insertions, 1 deletions

diff --git a/gcc/invoke.texi b/gcc/invoke.texi
index 56bdfdbae47..3bc197f9b66 100644
--- a/gcc/invoke.texi
+++ b/gcc/invoke.texi
@@ -156,7 +156,7 @@ in the following sections.
 -frerun-cse-after-loop  -frerun-loop-opt -fschedule-insns
 -fschedule-insns2  -fstrength-reduce  -fthread-jumps
 -funroll-all-loops  -funroll-loops
--fmove-all-movables  -freduce-all-givs
+-fmove-all-movables  -freduce-all-givs -fstrict-aliasing
 -O  -O0  -O1  -O2  -O3 -Os
 @end smallexample
 
@@ -2419,6 +2419,57 @@ Some machines only support 2 operands per instruction.  On such
 machines, GNU CC might have to do extra copies.  The @samp{-fregmove}
 option overrides the default for the machine to do the copy before
 register allocation.
+
+@item -fstrict-aliasing
+Allows the compiler to assume the strictest aliasing rules applicable to
+the language being compiled.  For C (and C++), this activates
+optimizations based on the type of expressions.  In particular, an
+object of one type is assumed never to reside at the same address as an
+object of a different type, unless the types are almost the same.  For
+example, an @code{unsigned int} can alias an @code{int}, but not a
+@code{void*} or a @code{double}.  A character type may alias any other
+type.  
+
+Pay special attention to code like this:
+@example
+union a_union @{ 
+  int i;
+  double d;
+@};
+
+int f() @{
+  a_union t;
+  t.d = 3.0;
+  return t.i;
+@}
+@end example
+The practice of reading from a different union member than the one most
+recently written to (called ``type-punning'') is common.  Even with
+@samp{-fstrict-aliasing}, type-punning is allowed, provided the memory
+is accessed through the union type.  So, the code above will work as
+expected.  However, this code might not:
+@example
+int f() @{ 
+  a_union t;
+  int* ip;
+  t.d = 3.0;
+  ip = &t.i;
+  return *ip;
+@}
+@end example
+
+This option is not enabled by default at any optimization level because
+it is new and has yet to be subjected to thorough testing.  You may
+of course enable it manually with @samp{-fstrict-aliasing}.
+
+@ifset INTERNALS
+Every language that wishes to perform language-specific alias analysis
+should define a function that computes, given an @code{tree}
+node, an alias set for the node.  Nodes in different alias sets are not
+allowed to alias.  For an example, see the C front-end function
+@code{c_get_alias_set}.
+@end ifset
+
 @end table
 
 @node Preprocessor Options