Added options -fcommon and -fno-common (#164)

The option -fcommon controls whether uninitialized global
variables are placed in the COMMON section. If the option is given
in the negated form, -fno-common, variables are not placed in the
COMMON section. They are placed in the same sections as gcc does.

If the variables are not placed in the COMMON section merging of
tentative definitions is inhibited and multiple definitions lead
to a linker error, as it does for gcc.

8 files changed, 41 insertions, 15 deletions

diff --git a/arm/TargetPrinter.ml b/arm/TargetPrinter.ml
index bf37b0e4..20989615 100644
--- a/arm/TargetPrinter.ml
+++ b/arm/TargetPrinter.ml
@@ -148,9 +148,9 @@ struct
   let name_of_section = function
     | Section_text -> ".text"
     | Section_data i | Section_small_data i ->
-      if i then ".data" else "COMM"
+      if i then ".data" else common_section ()
     | Section_const i | Section_small_const i ->
-      if i then ".section	.rodata" else "COMM"
+      if i || (not !Clflags.option_fcommon) then ".section	.rodata" else "COMM"
     | Section_string -> ".section	.rodata"
     | Section_literal -> ".text"
     | Section_jumptable -> ".text"
diff --git a/backend/PrintAsmaux.ml b/backend/PrintAsmaux.ml
index 0fd164b0..7e075f04 100644
--- a/backend/PrintAsmaux.ml
+++ b/backend/PrintAsmaux.ml
@@ -298,3 +298,11 @@ let print_version_and_options oc comment =
     fprintf oc " %s" Commandline.argv.(i)
   done;
   fprintf oc "\n"
+(** Get the name of the common section if it is used otherwise the given section
+    name, with bss as default *)
+
+let common_section ?(sec = ".bss") () =
+  if !Clflags.option_fcommon then
+    "COMM"
+  else
+    sec
diff --git a/doc/ccomp.1 b/doc/ccomp.1
index 36fe4e7b..1579beb9 100644
--- a/doc/ccomp.1
+++ b/doc/ccomp.1
@@ -180,6 +180,12 @@ Set alignment of function entry points to <n> bytes.
 The default alignment is 16 bytes for x86 targets and 4 bytes for ARM and PowerPC.
 .
 .TP
+.BR \-fcommon ", " \-fno\-common
+Turn on/off placement of global variables defined without an initializer (tentative definitions) in the common section.
+Disabling the use of the common section inhibits merging of tentative definitions by the linker and may lead to multiple-definition errors.
+Enabled by default.
+.
+.TP
 .BR \-ffpu ", " \-fno\-fpu
 Turn on/off use of FP registers for some integer operations.
 Enabled by default.
diff --git a/driver/Clflags.ml b/driver/Clflags.ml
index a886ee9b..fc12863d 100644
--- a/driver/Clflags.ml
+++ b/driver/Clflags.ml
@@ -31,6 +31,7 @@ let option_falignfunctions = ref (None: int option)
 let option_falignbranchtargets = ref 0
 let option_faligncondbranchs = ref 0
 let option_finline_asm = ref false
+let option_fcommon = ref true
 let option_mthumb = ref (Configuration.model = "armv7m")
 let option_Osize = ref false
 let option_finline = ref true
diff --git a/driver/Driver.ml b/driver/Driver.ml
index 5f7d0b20..50f14d13 100644
--- a/driver/Driver.ml
+++ b/driver/Driver.ml
@@ -203,6 +203,7 @@ Code generation options: (use -fno-<opt> to turn off -f<opt>)
   -falign-functions <n>  Set alignment (in bytes) of function entry points
   -falign-branch-targets <n>  Set alignment (in bytes) of branch targets
   -falign-cond-branches <n>  Set alignment (in bytes) of conditional branches
+  -fcommon       Put uninitialized globals in the common section [on].
 |} ^
  target_help ^
  toolchain_help ^
@@ -304,6 +305,7 @@ let cmdline_actions =
   Exact "-falign-functions", Integer(fun n -> check_align n; option_falignfunctions := Some n);
   Exact "-falign-branch-targets", Integer(fun n -> check_align n; option_falignbranchtargets := n);
   Exact "-falign-cond-branches", Integer(fun n -> check_align n; option_faligncondbranchs := n);] @
+      f_opt "common" option_fcommon @
 (* Target processor options *)
   (if Configuration.arch = "arm" then
     if Configuration.model = "armv6" then
diff --git a/powerpc/TargetPrinter.ml b/powerpc/TargetPrinter.ml
index c1aaa55d..0a98584b 100644
--- a/powerpc/TargetPrinter.ml
+++ b/powerpc/TargetPrinter.ml
@@ -118,13 +118,22 @@ module Linux_System : SYSTEM =
     let name_of_section = function
       | Section_text -> ".text"
       | Section_data i ->
-          if i then ".data" else "COMM"
+          if i then
+            ".data"
+          else
+            common_section ~sec:".section	.bss" ()
       | Section_small_data i ->
-          if i then ".section	.sdata,\"aw\",@progbits" else "COMM"
+          if i then
+            ".section	.sdata,\"aw\",@progbits"
+          else
+            common_section ~sec:".section	.sbss,\"aw\",@nobits" ()
       | Section_const i ->
-          if i then ".rodata" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".rodata" else "COMM"
       | Section_small_const i ->
-          if i then ".section	.sdata2,\"a\",@progbits" else "COMM"
+          if i || (not !Clflags.option_fcommon) then
+            ".section	.sdata2,\"a\",@progbits"
+          else
+            "COMM"
       | Section_string -> ".rodata"
       | Section_literal -> ".section	.rodata.cst8,\"aM\",@progbits,8"
       | Section_jumptable -> ".text"
@@ -209,7 +218,7 @@ module Diab_System : SYSTEM =
 
     let name_of_section = function
       | Section_text -> ".text"
-      | Section_data i -> if i then ".data" else "COMM"
+      | Section_data i -> if i then ".data" else common_section ()
       | Section_small_data i -> if i then ".sdata" else ".sbss"
       | Section_const _ -> ".text"
       | Section_small_const _ -> ".sdata2"
diff --git a/riscV/TargetPrinter.ml b/riscV/TargetPrinter.ml
index 19704bad..92df7a76 100644
--- a/riscV/TargetPrinter.ml
+++ b/riscV/TargetPrinter.ml
@@ -108,9 +108,9 @@ module Target : TARGET =
     let name_of_section = function
       | Section_text         -> ".text"
       | Section_data i | Section_small_data i ->
-          if i then ".data" else "COMM"
+          if i then ".data" else common_section ()
       | Section_const i | Section_small_const i ->
-          if i then ".section	.rodata" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".section	.rodata" else "COMM"
       | Section_string       -> ".section	.rodata"
       | Section_literal      -> ".section	.rodata"
       | Section_jumptable    -> ".section	.rodata"
diff --git a/x86/TargetPrinter.ml b/x86/TargetPrinter.ml
index 3ac2f36e..2e28e983 100644
--- a/x86/TargetPrinter.ml
+++ b/x86/TargetPrinter.ml
@@ -134,9 +134,9 @@ module ELF_System : SYSTEM =
     let name_of_section = function
       | Section_text -> ".text"
       | Section_data i | Section_small_data i ->
-          if i then ".data" else "COMM"
+          if i then ".data" else common_section ()
       | Section_const i | Section_small_const i ->
-          if i then ".section	.rodata" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".section	.rodata" else "COMM"
       | Section_string -> ".section	.rodata"
       | Section_literal -> ".section	.rodata.cst8,\"aM\",@progbits,8"
       | Section_jumptable -> ".text"
@@ -192,9 +192,9 @@ module MacOS_System : SYSTEM =
     let name_of_section = function
       | Section_text -> ".text"
       | Section_data i | Section_small_data i ->
-          if i then ".data" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".data" else "COMM"
       | Section_const i  | Section_small_const i ->
-          if i then ".const" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".const" else "COMM"
       | Section_string -> ".const"
       | Section_literal -> ".literal8"
       | Section_jumptable -> ".text"  (* needed in 64 bits, not a problem in 32 bits *)
@@ -269,9 +269,9 @@ module Cygwin_System : SYSTEM =
     let name_of_section = function
       | Section_text -> ".text"
       | Section_data i | Section_small_data i ->
-          if i then ".data" else "COMM"
+          if i then ".data" else common_section ()
       | Section_const i | Section_small_const i ->
-          if i then ".section	.rdata,\"dr\"" else "COMM"
+          if i || (not !Clflags.option_fcommon) then ".section	.rdata,\"dr\"" else "COMM"
       | Section_string -> ".section	.rdata,\"dr\""
       | Section_literal -> ".section	.rdata,\"dr\""
       | Section_jumptable -> ".text"