One of the micro-optimizations that may be done inside the tree optimizer
involves disregarding V_MAY_DEF/V_MUST_DEF operands for read-only
globals.
So, if a symbol is marked read-only and the operand scanner
requests a V_MAY_DEF or V_MUST_DEF operand for it, we replace it
with a VUSE.
This works fine, except that I was having comparison errors with
SPEC2000's eon. I tracked it down to the C++ FE emitting an
assignment instruction for a global const variable.
The source code declares
const double ggPi = 3.14159265358979323846;
double const divPi = 1 / ggPi;
And since divPi is initialized to an expression, I guess it needs
to compute it at runtime, so it emits an initialization function:
void __static_initialization_and_destruction_0(int, int) (__initialize_p,
__priority)
{
...
if (D.55019)
{
ggPi.319 = ggPi;
D.55021 = 1.0e+0 / ggPi.319;
divPi = D.55021;
}
...
}
So, we now have an assignment for divPi in the IL stream. This
throws a monkey wrench into this micro-optimization because it
shouldn't really have ignored that V_MUST_DEF (the tree optimizers
end up removing the assignment).
This optimization is currently disabled because of this. If the C++
FE is fixed to address this problem, the following patch will re-enable
the optimization:
--- tree-ssa-operands.c 9 Apr 2005 01:37:24 -0000 2.75
+++ tree-ssa-operands.c 9 Apr 2005 01:51:02 -0000
@@ -1803,14 +1803,8 @@ add_stmt_operand (tree *var_p, stmt_ann_
it into a VUSE. This happens when read-only variables are marked
call-clobbered and/or aliased to writeable variables. So we only
check that this only happens on stores, and not writes to GIMPLE
- registers.
-
- FIXME: The C++ FE is emitting assignments in the IL stream for
- read-only globals. This is wrong, but for the time being disable
- this transformation on V_MUST_DEF operands (otherwise, we
- mis-optimize SPEC2000's eon). */
+ registers. */
if ((flags & opf_is_def)
- && !(flags & opf_kill_def)
&& unmodifiable_var_p (var))
{
gcc_assert (!is_real_op);
--
Summary: C++ FE emitting assignments to read-only global symbols
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dnovillo at gcc dot gnu dot org
CC: gcc-bugs at gcc dot gnu dot org,mark at codesourcery dot
com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20912
