In this testcase, A is for some reason considered volatile, so the first
write is not removed:
procedure Vol is
A : Integer;
pragma Import (C, A);
begin
A := 1;
A := 2;
end;
$ gcc -S -O2 vol.adb
$ grep mov vol.s
movl %esp, %ebp
movl $1, a
movl $2, a
This is because of the following in ada/decl.c:
/* Make a volatile version of this object's type if we are to
make the object volatile. Note that 13.3(19) says that we
should treat other types of objects as volatile as well. */
if ((Treat_As_Volatile (gnat_entity)
|| Is_Exported (gnat_entity)
|| Is_Imported (gnat_entity))
&& !TYPE_VOLATILE (gnu_type))
gnu_type = build_qualified_type (gnu_type,
(TYPE_QUALS (gnu_type)
| TYPE_QUAL_VOLATILE));
What does 13.3(19) actually say?
"If the Address of an object is specified, or it is imported or exported, then
the implementation should not perform optimizations based on assumptions of no
aliases."
So it looks like volatile is a hack to get the effect of "no aliasing". Surely
this can be done right nowadays?
--
Summary: Imported variables marked "volatile" in Ada
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ada
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: baldrick at gcc dot gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31877
