It turns out the SSA var defs/uses generated by the gimplifier can break
apart in a way no longer satisfying the dominance requirement of SSA
uses vs. their defs by means of CFG construction adding abnormal edges
for stuff like setjmp (but also non-local gotos I guess).
This would be quite costly to overcome in gimplification - one needs
to check whether a (part?) of an expression to be gimplified may
produce such edges and disable SSA name generation for them. With
the recursive nature of the gimplifier plus the general complexity
of GENERIC I can't see how to do this.
Thus the following patch "recovers" from the extra abnormal edges
by effectively treating SSA vars pre into-SSA as "non-SSA" and thus
doing PHI insertion for them when rewriting the function into SSA.
Implementation-wise the easiest thing was to re-write the affected
SSA vars out-of-SSA (replace them by a decl).
The out-of-SSA rewriting is placed in insert_phi_nodes because thats
the first point in time we have immediate uses present (otherwise
it could be done at any point after CFG construction).
I'm not 100% happy with this but after trying for a day I can't come
up with a better solution - it has the chance of papering over
bogus gimplifications but OTOH that we only do this for functions
calling setjmp or having non-local labels would make those trigger
SSA verification in the other cases.
Bootstrap and regtest running on x86_64-unknown-linux-gnu.
Any comments / ideas? Anyone with good arguments that this is
even conceptually the correct thing to do?
I'm bootstrapping this with the calls_setjmp/has_nonlocal_label
short-cutted with a 1 || as well as without (to increase testing
coverage).
Thanks,
Richard.
2017-02-10 Richard Biener <rguent...@suse.de>
PR middle-end/79432
* tree-into-ssa.c (insert_phi_nodes): When the function can
have abnormal edges rewrite SSA names with broken use-def
dominance out of SSA and register them for PHI insertion.
* gcc.dg/torture/pr79432.c: New testcase.
Index: gcc/tree-into-ssa.c
===================================================================
--- gcc/tree-into-ssa.c (revision 245323)
+++ gcc/tree-into-ssa.c (working copy)
@@ -1066,6 +1066,59 @@ insert_phi_nodes (bitmap_head *dfs)
timevar_push (TV_TREE_INSERT_PHI_NODES);
+ /* When the gimplifier introduces SSA names it cannot easily avoid
+ situations where abnormal edges added by CFG construction break
+ the use-def dominance requirement. For this case rewrite SSA
+ names with broken use-def dominance out-of-SSA and register them
+ for PHI insertion. We only need to do this if abnormal edges
+ can appear in the function. */
+ tree name;
+ if (cfun->calls_setjmp
+ || cfun->has_nonlocal_label)
+ FOR_EACH_SSA_NAME (i, name, cfun)
+ {
+ gimple *def_stmt = SSA_NAME_DEF_STMT (name);
+ if (SSA_NAME_IS_DEFAULT_DEF (name))
+ continue;
+
+ basic_block def_bb = gimple_bb (def_stmt);
+ imm_use_iterator it;
+ gimple *use_stmt;
+ bool need_phis = false;
+ FOR_EACH_IMM_USE_STMT (use_stmt, it, name)
+ {
+ basic_block use_bb = gimple_bb (use_stmt);
+ if (use_bb != def_bb
+ && ! dominated_by_p (CDI_DOMINATORS, use_bb, def_bb))
+ need_phis = true;
+ }
+ if (need_phis)
+ {
+ tree var = create_tmp_reg (TREE_TYPE (name));
+ use_operand_p use_p;
+ FOR_EACH_IMM_USE_STMT (use_stmt, it, name)
+ {
+ basic_block use_bb = gimple_bb (use_stmt);
+ FOR_EACH_IMM_USE_ON_STMT (use_p, it)
+ SET_USE (use_p, var);
+ update_stmt (use_stmt);
+ set_livein_block (var, use_bb);
+ set_rewrite_uses (use_stmt, true);
+ bitmap_set_bit (interesting_blocks, use_bb->index);
+ }
+ def_operand_p def_p;
+ ssa_op_iter dit;
+ FOR_EACH_SSA_DEF_OPERAND (def_p, def_stmt, dit, SSA_OP_DEF)
+ if (DEF_FROM_PTR (def_p) == name)
+ SET_DEF (def_p, var);
+ update_stmt (def_stmt);
+ set_def_block (var, def_bb, false);
+ set_register_defs (def_stmt, true);
+ bitmap_set_bit (interesting_blocks, def_bb->index);
+ release_ssa_name (name);
+ }
+ }
+
auto_vec<var_info *> vars (var_infos->elements ());
FOR_EACH_HASH_TABLE_ELEMENT (*var_infos, info, var_info_p, hi)
if (info->info.need_phi_state != NEED_PHI_STATE_NO)
Index: gcc/testsuite/gcc.dg/torture/pr79432.c
===================================================================
--- gcc/testsuite/gcc.dg/torture/pr79432.c (nonexistent)
+++ gcc/testsuite/gcc.dg/torture/pr79432.c (working copy)
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+
+int fn1 (void);
+int __attribute__((returns_twice)) vfork (void);
+
+void fn2 ()
+{
+ int a;
+ a = fn1() + 2 + (vfork() + 1 + vfork());
+}
+void fn3 ()
+{
+ int a;
+ a = fn1() + 1 + vfork();
+}
+void fn4 ()
+{
+ int a;
+ a = fn1() + vfork();
+}
