Not using DECL_VALUE_EXPR for callee-copied parameters?

[Martin Jambor <mjambor at suse dot cz>]

Hi,

we sometimes currently mess up when compiling callee-copied parameters
on architectures that use it (so, as far as I know, on hppa).  The
manifestation of the issue is e.g. PR 40464 but I believe there maight
be others.

The real problem is that even though a comment in tree.h says that
declarations with a DECL_VALUE_EXPR "once this field has been set, the
decl itself may not legitimately appear in the function," they do
because gimplify_parameters() in function.c generates an explicit copy
statements.  This then has unintended wrong consequences when such a
declaration is later fed to the gimplifier again (for example through
force_gimple_operand) because they are replaced with their value_expr.

For reference, this is PR 41250.

Because DECL_VALUE_EXPRs affect debug info I attempted to simply not
doing the replacements in gimplifier but ran into all sorts of asserts
in expand and later even to execution failures which altogether looked
too overwhelming and so I gave up.  At the moment I intend to simply
stop making these callee-copied parameters value_expr ones since the
convention is implemented explicitly - this amounts to the two line
patch below.  Obviously, this will impair debug info but I hope that
for this purpose we can represent the relationship in another way.
Specifically, I was hoping to be able to use BLOCK_NONLOCALIZED_VARS
in the form in which they are in pretty-ipa where they essentially map
no longer existing decls to expressions evaluating their values.
Honza, do you think they will work also for an existing PARM_DECL?

I have run the testsuite on hppa with the following patch and I
successfully bootstrapped and tested it on x86_64.  Unless someone
objects, I will bootstrap it on hppa and commit it in a week or two
along with Richi's patch verifying such decls don't leak to the IL
(and then try to use the mechanism above to keep the debug info).

Thanks,

Martin


2009-09-11  Martin Jambor  <mjambor@suse.cz>

	* function.c (gimplify_parameters): Do not set a value-expr on
	callee-copied parm decls.


Index: small/gcc/function.c
===================================================================
--- small.orig/gcc/function.c
+++ small/gcc/function.c
@@ -3425,9 +3425,6 @@ gimplify_parameters (void)
 		}
 
 	      gimplify_assign (local, parm, &stmts);
-
-	      SET_DECL_VALUE_EXPR (parm, local);
-	      DECL_HAS_VALUE_EXPR_P (parm) = 1;
 	    }
 	}
     }


PS:  If you are interested what other purposes value_exprs are used
to, here is a list of places where SET_DECL_VALUE_EXPR is called, it
is such a weird thing in gimple that it might be a worthy goal to
replace them all with BLOCK_NONLOCALIZED_VARS:

*** function.c:
gimplify_parameters[3429]      SET_DECL_VALUE_EXPR (parm, local);

*** gimplify.c:
gimplify_vla_decl[1320]        SET_DECL_VALUE_EXPR (decl, t);
gimplify_var_or_parm_decl[1900] SET_DECL_VALUE_EXPR (copy, unshare_expr (value_expr));

*** omp-low.c:
fixup_remapped_decl[1029]      SET_DECL_VALUE_EXPR (new_decl, ve);
lower_rec_input_clauses[2382]  SET_DECL_VALUE_EXPR (new_var, x);
lower_rec_input_clauses[2438]  SET_DECL_VALUE_EXPR (new_var, x);
lower_rec_input_clauses[2465]  SET_DECL_VALUE_EXPR (placeholder, x);
lower_reduction_clauses[2672]  SET_DECL_VALUE_EXPR (placeholder, ref);

*** tree-cfg.c:
replace_block_vars_by_duplicates[6183] SET_DECL_VALUE_EXPR (t, DECL_VALUE_EXPR (*tp));

*** tree-nested.c:
get_nonlocal_debug_decl[839]   SET_DECL_VALUE_EXPR (new_decl, x);
get_local_debug_decl[1324]     SET_DECL_VALUE_EXPR (new_decl, x);
remap_vla_decls[2159]          SET_DECL_VALUE_EXPR (var, val);

*** tree.c:
copy_node_stat[955]            SET_DECL_VALUE_EXPR (t, DECL_VALUE_EXPR (node));