ipa-sra: Do not remove return values needed because of non-call EH

IPA-SRA already contains a check to figure out that an otherwise dead
parameter is actually required because of non-call exceptions, but it
is not present at the equivalent spot where SRA figures out whether
the return statement is used for anything useful.  This patch adds
that condition there.

Unfortunately, even though this patch should be good enough for any
normal (I'd even say reasonable) use of the compiler, it hints that
when the user manually switches all sorts of DCE, IPA-SRA would
probably leave behind problematic statements manipulating what
originally were return values, just like it does for parameters (PR
93385).  Fixing this properly might unfortunately be a separate issue
from the mentioned bug because the LHS of a call is changed during
call redirection and the caller often is not a clone.  But I'll see
what I can do.

Meanwhile, the patch below has been bootstrapped and tested on x86_64.

gcc/ChangeLog:

2021-01-18  Martin Jambor  <mjambor@suse.cz>

	PR ipa/98690
	* ipa-sra.c (ssa_name_only_returned_p): New parameter fun.  Check
	whether non-call exceptions allow removal of a statement.
	(isra_analyze_call): Pass the appropriate function to
	ssa_name_only_returned_p.

gcc/testsuite/ChangeLog:

2021-01-18  Martin Jambor  <mjambor@suse.cz>

	PR ipa/98690
	* g++.dg/ipa/pr98690.C: New test.

diff --git a/gcc/ipa-sra.c b/gcc/ipa-sra.c
index 5d2c0dfce533e956d85b28a6f303a96256f6674f..1571921cb4840649f0e9b0e4024ee1c7b48d2fb6 100644 (file)
--- a/gcc/ipa-sra.c
+++ b/gcc/ipa-sra.c
@@ -1952,13 +1952,13 @@ scan_function (cgraph_node *node, struct function *fun)
     }
 }
 
-/* Return true if SSA_NAME NAME is only used in return statements, or if
-   results of any operations it is involved in are only used in return
-   statements.  ANALYZED is a bitmap that tracks which SSA names we have
-   already started investigating.  */
+/* Return true if SSA_NAME NAME of function described by FUN is only used in
+   return statements, or if results of any operations it is involved in are
+   only used in return statements.  ANALYZED is a bitmap that tracks which SSA
+   names we have already started investigating.  */
 
 static bool
-ssa_name_only_returned_p (tree name, bitmap analyzed)
+ssa_name_only_returned_p (function *fun, tree name, bitmap analyzed)
 {
   bool res = true;
   imm_use_iterator imm_iter;
@@ -1978,8 +1978,9 @@ ssa_name_only_returned_p (tree name, bitmap analyzed)
              break;
            }
        }
-      else if ((is_gimple_assign (stmt) && !gimple_has_volatile_ops (stmt))
-              || gimple_code (stmt) == GIMPLE_PHI)
+      else if (!stmt_unremovable_because_of_non_call_eh_p (fun, stmt)
+              && ((is_gimple_assign (stmt) && !gimple_has_volatile_ops (stmt))
+                  || gimple_code (stmt) == GIMPLE_PHI))
        {
          /* TODO: And perhaps for const function calls too?  */
          tree lhs;
@@ -1995,7 +1996,7 @@ ssa_name_only_returned_p (tree name, bitmap analyzed)
            }
          gcc_assert (!gimple_vdef (stmt));
          if (bitmap_set_bit (analyzed, SSA_NAME_VERSION (lhs))
-             && !ssa_name_only_returned_p (lhs, analyzed))
+             && !ssa_name_only_returned_p (fun, lhs, analyzed))
            {
              res = false;
              break;
@@ -2049,7 +2050,8 @@ isra_analyze_call (cgraph_edge *cs)
       if (TREE_CODE (lhs) == SSA_NAME)
        {
          bitmap analyzed = BITMAP_ALLOC (NULL);
-         if (ssa_name_only_returned_p (lhs, analyzed))
+         if (ssa_name_only_returned_p (DECL_STRUCT_FUNCTION (cs->caller->decl),
+                                       lhs, analyzed))
            csum->m_return_returned = true;
          BITMAP_FREE (analyzed);
        }

diff --git a/gcc/testsuite/g++.dg/ipa/pr98690.C b/gcc/testsuite/g++.dg/ipa/pr98690.C
new file mode 100644 (file)
index 0000000..004418e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ipa/pr98690.C
@@ -0,0 +1,27 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fnon-call-exceptions" } */
+
+int g;
+volatile int v;
+
+static int * __attribute__((noinline))
+almost_useless_return (void)
+{
+  v = 1;
+  return &g;
+}
+
+static void __attribute__((noinline))
+foo (void)
+{
+  int *p = almost_useless_return ();
+  int i = *p;
+  v = 2;
+}
+
+int
+main (int argc, char *argv[])
+{
+  foo ();
+  return 0;
+}