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

[Georg-Johann Lay]

Wilco Dijkstra schrieb:
> 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?

IIRC using -fno-common might lead to some testsuit fallout because
some optimizations / test cases are sensitive to -f[no-]common.
So I wonder that no adjustments to test cases are needed?

> 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.

IMO "uninitialized" is confusing because the variables actually
*are* initialized: with zero.  It's just that the variables don't have
explicit initializers.  Dito for "uninitialized" in the --help message.

Johann


> +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
>