https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77692
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ok, so tree_add_const_value_attribute_for_decl gives up because the decl is not
readonly. But via add_location_or_const_value_attribute rtl_for_decl_location
would get us
(const_vector:V4SF [
(const_double:SF 2.5e+8 [0x0.ee6b28p+28])
(const_double:SF 0.0 [0x0.0p+0])
(const_double:SF 0.0 [0x0.0p+0])
(const_double:SF 0.0 [0x0.0p+0])
])
which looks inconsistent. That simply does
/* A variable with no DECL_RTL but a DECL_INITIAL is a compile-time constant,
and will have been substituted directly into all expressions that use it.
C does not have such a concept, but C++ and other languages do. */
if (!rtl && TREE_CODE (decl) == VAR_DECL && DECL_INITIAL (decl))
rtl = rtl_for_decl_init (DECL_INITIAL (decl), TREE_TYPE (decl));
which may be true if called late enough.
We're invoking dwarf2out_late_global_decl for removed globals only at a very
specific point so it doesn't get called for
typedef float FloatVect __attribute__((__vector_size__(16)));
static FloatVect Foo = { 250000000.0, 0.0, 0.0, 0.0 };
int main()
{
Foo = (FloatVect) { 1., 1., 1., 1. };
}
where we also remove Foo (as write-only). For this case it would be definitely
bogus to add a DW_AT_const_value attribute. We do call it for
typedef float FloatVect __attribute__((__vector_size__(16)));
static FloatVect Foo = { 250000000.0, 0.0, 0.0, 0.0 };
static void bar ()
{
Foo = (FloatVect) { 1., 1., 1., 1. };
}
int main()
{
}
which means this case did get a DW_AT_const_value attached to Foo.
Note that on the original testcase IPA would have found Foo to be readonly,
likewise on the last.
I believe rtl_for_decl_location is bogus doing
/* A variable with no DECL_RTL but a DECL_INITIAL is a compile-time constant,
and will have been substituted directly into all expressions that use it.
C does not have such a concept, but C++ and other languages do. */
if (!rtl && TREE_CODE (decl) == VAR_DECL && DECL_INITIAL (decl))
rtl = rtl_for_decl_init (DECL_INITIAL (decl), TREE_TYPE (decl));
when it is called early before decl got a chance to even create RTL.
So -- is it worth trying to handle this special case where we know a variable
is readonly by means of nothing referencing it? Then the following fixes it:
Index: gcc/cgraphunit.c
===================================================================
--- gcc/cgraphunit.c (revision 240388)
+++ gcc/cgraphunit.c (working copy)
@@ -1194,8 +1194,15 @@ analyze_functions (bool first_time)
at looking at optimized away DECLs, since
late_global_decl will subsequently be called from the
contents of the now pruned symbol table. */
- if (!decl_function_context (node->decl))
- (*debug_hooks->late_global_decl) (node->decl);
+ if (TREE_CODE (node->decl) == VAR_DECL
+ && !decl_function_context (node->decl))
+ {
+ /* We are reclaiming totally unreachable code and variables
+ so they effectively appear as readonly. Show that to
+ the debug machinery. */
+ TREE_READONLY (node->decl) = 1;
+ (*debug_hooks->late_global_decl) (node->decl);
+ }
node->remove ();
continue;
