[PATCH] PR85678: Change default to -fno-common

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

GCC currently defaults to -fcommon.  As discussed in the PR, this is an ancient
C feature which is not conforming with the latest C standards.  On many targets
this means global variable accesses have a codesize and performance penalty.
This applies to C code only, C++ code is not affected by -fcommon.  It is about
time to change the default.

OK for commit?

ChangeLog
2019-10-25  Wilco Dijkstra  <wdijkstr@arm.com>

	PR85678
	* common.opt (fcommon): Change init to 1.

doc/
	* invoke.texi (-fcommon): Update documentation.
---

diff --git a/gcc/common.opt b/gcc/common.opt
index 0195b0cb85a06dd043fd0412b42dfffddfa2495b..b0840f41a5e480f4428bd62724b0dc3d54c68c0b 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1131,7 +1131,7 @@ Common Report Var(flag_combine_stack_adjustments) Optimization
 Looks for opportunities to reduce stack adjustments and stack references.
 
 fcommon
-Common Report Var(flag_no_common,0)
+Common Report Var(flag_no_common,0) Init(1)
 Put uninitialized globals in the common section.
 
 fcompare-debug
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 857d9692729e503657d0d0f44f1f6252ec90d49a..5b4ff66015f5f94a5bd89e4dc3d2d53553cc091e 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -568,7 +568,7 @@ Objective-C and Objective-C++ Dialects}.
 -fnon-call-exceptions  -fdelete-dead-exceptions  -funwind-tables @gol
 -fasynchronous-unwind-tables @gol
 -fno-gnu-unique @gol
--finhibit-size-directive  -fno-common  -fno-ident @gol
+-finhibit-size-directive  -fcommon  -fno-ident @gol
 -fpcc-struct-return  -fpic  -fPIC  -fpie  -fPIE  -fno-plt @gol
 -fno-jump-tables @gol
 -frecord-gcc-switches @gol
@@ -14050,35 +14050,27 @@ useful for building programs to run under WINE@.
 code that is not binary compatible with code generated without that switch.
 Use it to conform to a non-default application binary interface.
 
-@item -fno-common
-@opindex fno-common
+@item -fcommon
 @opindex fcommon
+@opindex fno-common
 @cindex tentative definitions
-In C code, this option controls the placement of global variables 
-defined without an initializer, known as @dfn{tentative definitions} 
-in the C standard.  Tentative definitions are distinct from declarations 
+In C code, this option controls the placement of global variables
+defined without an initializer, known as @dfn{tentative definitions}
+in the C standard.  Tentative definitions are distinct from declarations
 of a variable with the @code{extern} keyword, which do not allocate storage.
 
-Unix C compilers have traditionally allocated storage for
-uninitialized global variables in a common block.  This allows the
-linker to resolve all tentative definitions of the same variable
+The default is @option{-fno-common}, which specifies that the compiler places
+uninitialized global variables in the BSS section of the object file.
+This inhibits the merging of tentative definitions by the linker so you get a
+multiple-definition error if the same variable is accidentally defined in more
+than one compilation unit.
+
+The @option{-fcommon} places uninitialized global variables in a common block.
+This allows the linker to resolve all tentative definitions of the same variable
 in different compilation units to the same object, or to a non-tentative
-definition.  
-This is the behavior specified by @option{-fcommon}, and is the default for 
-GCC on most targets.  
-On the other hand, this behavior is not required by ISO
-C, and on some targets may carry a speed or code size penalty on
-variable references.
-
-The @option{-fno-common} option specifies that the compiler should instead
-place uninitialized global variables in the BSS section of the object file.
-This inhibits the merging of tentative definitions by the linker so
-you get a multiple-definition error if the same 
-variable is defined in more than one compilation unit.
-Compiling with @option{-fno-common} is useful on targets for which
-it provides better performance, or if you wish to verify that the
-program will work on other systems that always treat uninitialized
-variable definitions this way.
+definition.  This behavior does not conform to ISO C, is inconsistent with C++,
+and on many targets implies a speed and code size penalty on global variable
+references.  It is mainly useful to enable legacy code to link without errors.
 
 @item -fno-ident
 @opindex fno-ident