PR85678: Change default to -fno-common

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.

    gcc/
	PR85678
	* common.opt (fcommon): Change init to 1.
	* invoke.texi (-fcommon): Update documentation.

    testsuite/
	* g++.dg/lto/odr-6_1.c: Add -fcommon.
	* gcc.dg/alias-15.c: Likewise.
	* gcc.dg/fdata-sections-1.c: Likewise.	
	* gcc.dg/ipa/pr77653.c: Likewise.
	* gcc.dg/lto/20090729_0.c: Likewise.
	* gcc.dg/lto/20111207-1_0.c: Likewise.
	* gcc.dg/lto/c-compatible-types-1_0.c: Likewise.
	* gcc.dg/lto/pr55525_0.c: Likewise.
	* gcc.dg/lto/pr88077_0.c: Use long to avoid alignment warning.
	* gcc.dg/lto/pr88077_1.c: Add -fcommon.
	* gcc.target/aarch64/sve/peel_ind_1.c: Allow ANCHOR0.
	* gcc.target/aarch64/sve/peel_ind_2.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3.c: Likewise.
	* gcc.target/i386/volatile-bitfields-2.c: Allow movl or movq.

From-SVN: r278509

gcc/ChangeLog

+2019-11-20  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	PR85678
+	* common.opt (fcommon): Change init to 1.
+	* doc/invoke.texi (-fcommon): Update documentation.
+
 2019-11-20  Jan Hubicka  <jh@suse.cz>
 
 	* fibonacci_heap.h (fibonacci_heap<K,V>::consolidate): Turn auto_vec

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

gcc/doc/invoke.texi

@@ -569,7 +569,7 @@ 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
@@ -14112,35 +14112,27 @@ 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

gcc/testsuite/ChangeLog

+2019-11-20  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	PR85678
+	* g++.dg/lto/odr-6_1.c: Add -fcommon.
+	* gcc.dg/alias-15.c: Likewise.
+	* gcc.dg/fdata-sections-1.c: Likewise.	
+	* gcc.dg/ipa/pr77653.c: Likewise.
+	* gcc.dg/lto/20090729_0.c: Likewise.
+	* gcc.dg/lto/20111207-1_0.c: Likewise.
+	* gcc.dg/lto/c-compatible-types-1_0.c: Likewise.
+	* gcc.dg/lto/pr55525_0.c: Likewise.
+	* gcc.dg/lto/pr88077_0.c: Use long to avoid alignment warning.
+	* gcc.dg/lto/pr88077_1.c: Add -fcommon.
+	* gcc.target/aarch64/sve/peel_ind_1.c: Allow ANCHOR0.
+	* gcc.target/aarch64/sve/peel_ind_2.c: Likewise.
+	* gcc.target/aarch64/sve/peel_ind_3.c: Likewise.
+	* gcc.target/i386/volatile-bitfields-2.c: Allow movl or movq.
+
 2019-11-20  Richard Biener  <rguenther@suse.de>
 
 	* gcc.dg/vect/bb-slp-42.c: Adjust.

gcc/testsuite/g++.dg/lto/odr-6_1.c

+/* { dg-options {-fcommon} } */
 struct {} admbaserest_; // { dg-lto-message "type of " 2 }
 
 

gcc/testsuite/gcc.dg/alias-15.c

 /* { dg-do compile } */
-/* { dg-additional-options  "-O2 -fdump-ipa-cgraph" } */
+/* { dg-additional-options  "-O2 -fcommon -fdump-ipa-cgraph" } */
 
 /* RTL-level CSE shouldn't introduce LCO (for the string) into varpool */
 char *p;

gcc/testsuite/gcc.dg/fdata-sections-1.c

@@ -2,7 +2,7 @@
 /* Origin: Jonathan Larmour <jifl-bugzilla@jifvik.org> */
 
 /* { dg-do compile { target *-*-linux* *-*-gnu* *-*-uclinux* } } */
-/* { dg-options "-fdata-sections" } */
+/* { dg-options "-fcommon -fdata-sections" } */
 
 int x;
 

gcc/testsuite/gcc.dg/ipa/pr77653.c

 /* { dg-require-alias "" } */
-/* { dg-options "-O2 -fdump-ipa-icf-details"  } */
+/* { dg-options "-O2 -fcommon -fdump-ipa-icf-details"  } */
 
 int a, b, c, d, e, h, i, j, k, l;
 const int f;

gcc/testsuite/gcc.dg/lto/20090729_0.c

-/* { dg-lto-options "-w" } */
+/* { dg-lto-options { {-fcommon -w} {-fcommon} } } */
 
 double i;
 int j;

gcc/testsuite/gcc.dg/lto/20111207-1_0.c

 /* { dg-lto-do run } */
-/* { dg-lto-options { { -flto } } } */
+/* { dg-lto-options { { -flto -fcommon } {-fcommon} {-fcommon} {-fcommon} } } */
 /* { dg-require-linker-plugin "" } */
 /* { dg-extra-ld-options "-fuse-linker-plugin" } */

gcc/testsuite/gcc.dg/lto/c-compatible-types-1_0.c

 /* { dg-lto-do run } */
-/* { dg-lto-options "-O3" } */
+/* { dg-lto-options { {-O3 -fcommon} {-fcommon} } } */
 
 /* By C standard Each enumerated type shall be compatible with char, a  signed
    integer, type, or an unsigned integer type. The choice of type is

gcc/testsuite/gcc.dg/lto/pr55525_0.c

 /* { dg-lto-do link } */
-/* { dg-lto-options { { -flto -w } } } */
+/* { dg-lto-options { { -fcommon -flto -w } } } */
 
 char s[sizeof (char *)];
 int main(void)

gcc/testsuite/gcc.dg/lto/pr88077_0.c

 /* { dg-lto-do link } */
 
-int HeaderStr;
+long HeaderStr;

gcc/testsuite/gcc.dg/lto/pr88077_1.c

+/* { dg-options {-fcommon} } */
+
 char HeaderStr[1];
 
 int main()

gcc/testsuite/gcc.target/aarch64/sve/peel_ind_1.c

@@ -21,7 +21,7 @@ foo (void)
 }
 
 /* We should operate on aligned vectors.  */
-/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, x\n} } } */
+/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, (x|\.LANCHOR0)\n} } } */
 /* We should use an induction that starts at -5, with only the last
    7 elements of the first iteration being active.  */
 /* { dg-final { scan-assembler {\tindex\tz[0-9]+\.s, #-5, #5\n} } } */

gcc/testsuite/gcc.target/aarch64/sve/peel_ind_2.c

@@ -17,7 +17,7 @@ foo (void)
 }
 
 /* We should operate on aligned vectors.  */
-/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, x\n} } } */
+/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, (x|\.LANCHOR0)\n} } } */
 /* We should unroll the loop three times.  */
 /* { dg-final { scan-assembler-times "\tst1w\t" 3 } } */
 /* { dg-final { scan-assembler {\tptrue\t(p[0-9]+)\.s, vl7\n.*\teor\tp[0-7]\.b, (p[0-7])/z, (\1\.b, \2\.b|\2\.b, \1\.b)\n} } } */

gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3.c

@@ -17,5 +17,5 @@ foo (int start)
 }
 
 /* We should operate on aligned vectors.  */
-/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, x\n} } } */
+/* { dg-final { scan-assembler {\t(adrp|adr)\tx[0-9]+, (x|\.LANCHOR0)\n} } } */
 /* { dg-final { scan-assembler {\tubfx\t} } } */

gcc/testsuite/gcc.target/i386/volatile-bitfields-2.c

@@ -14,4 +14,4 @@ int foo ()
   return bits.b;
 }
 
-/* { dg-final { scan-assembler "movl.*bits" } } */
+/* { dg-final { scan-assembler "mov(q|l).*bits" } } */