tree-optimization/109434 - bogus DSE of throwing call LHS

The byte tracking of call LHS didn't properly handle possibly
throwing calls correctly which cases bogus DSE and in turn, for the
testcase a bogus uninit diagnostic and (unreliable) wrong-code.

	PR tree-optimization/109434
	* tree-ssa-dse.cc (initialize_ao_ref_for_dse): Properly
	handle possibly throwing calls when processing the LHS
	and may-defs are not OK.  Add mode to initialize a may-def.
	(dse_optimize_stmt): Query may-defs.

	* g++.dg/opt/pr109434.C: New testcase.

diff --git a/gcc/testsuite/g++.dg/opt/pr109434.C b/gcc/testsuite/g++.dg/opt/pr109434.C
new file mode 100644 (file)
index 0000000..cffa327
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr109434.C
@@ -0,0 +1,28 @@
+// { dg-do compile }
+// { dg-require-effective-target c++17 }
+// { dg-options "-O2 -Wall" }
+
+#include <optional>
+#include <stdexcept>
+
+std::optional<int> foo()
+{
+  volatile int x = 1;
+  if (x)
+    throw std::runtime_error("haha");
+  return 42;
+}
+
+int main()
+{
+  std::optional<int> optInt;
+  try {
+      // We falsely DSEd the LHS of the call even though foo throws
+      // which results in an uninitialized diagnostic
+      optInt = foo();
+  } catch (...) {
+      return optInt.has_value();
+  }
+  std::optional<double> optDbl{optInt};
+  return optDbl ? optDbl.value () : 2.0;
+}

diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index e647f0e7368c3a2191f0fe295dccc3d7c6022a47..ba448c999dd7805b07de2fe66e8548628cfd690e 100644 (file)
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -93,7 +93,9 @@ static bitmap need_eh_cleanup;
 static bitmap need_ab_cleanup;
 
 /* STMT is a statement that may write into memory.  Analyze it and
-   initialize WRITE to describe how STMT affects memory.
+   initialize WRITE to describe how STMT affects memory.  When
+   MAY_DEF_OK is true then the function initializes WRITE to what
+   the stmt may define.
 
    Return TRUE if the statement was analyzed, FALSE otherwise.
 
@@ -101,7 +103,7 @@ static bitmap need_ab_cleanup;
    can be achieved by analyzing more statements.  */
 
 static bool
-initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write)
+initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write, bool may_def_ok = false)
 {
   /* It's advantageous to handle certain mem* functions.  */
   if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
@@ -148,7 +150,8 @@ initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write)
     }
   else if (tree lhs = gimple_get_lhs (stmt))
     {
-      if (TREE_CODE (lhs) != SSA_NAME)
+      if (TREE_CODE (lhs) != SSA_NAME
+         && (may_def_ok || !stmt_could_throw_p (cfun, stmt)))
        {
          ao_ref_init (write, lhs);
          return true;
@@ -1293,8 +1296,10 @@ dse_optimize_stmt (function *fun, gimple_stmt_iterator *gsi, sbitmap live_bytes)
 
   ao_ref ref;
   /* If this is not a store we can still remove dead call using
-     modref summary.  */
-  if (!initialize_ao_ref_for_dse (stmt, &ref))
+     modref summary.  Note we specifically allow ref to be initialized
+     to a conservative may-def since we are looking for followup stores
+     to kill all of it.  */
+  if (!initialize_ao_ref_for_dse (stmt, &ref, true))
     {
       dse_optimize_call (gsi, live_bytes);
       return;