[gcc(refs/users/aoliva/heads/testme)] harden conds: detach without decls

[gcc(refs/users/aoliva/heads/testme)] harden conds: detach without decls

[Alexandre Oliva]

https://gcc.gnu.org/g:28cab866b951826f22390088b4cd7cf9a4c11806

commit 28cab866b951826f22390088b4cd7cf9a4c11806
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Fri Nov 19 08:09:30 2021 -0300

    harden conds: detach without decls
    
    When we create copies of SSA_NAMEs to hold "detached" copies of the
    values for the hardening tests, we end up with assignments to
    SSA_NAMEs that refer to the same decls.  That would be generally
    desirable, since it enables the variable to be recognized in dumps,
    and makes coalescing more likely if the original variable dies at that
    point.  When the decl is a DECL_BY_REFERENCE, the SSA_NAME holds the
    address of a parm or result, and it's read-only, so we shouldn't
    create assignments to it.  Gimple checkers flag at least the case of
    results.
    
    This patch arranges for us to avoid referencing the same decls, which
    cures the problem, but retaining the visible association between the
    SSA_NAMEs, by using the same identifier for the copy.
    
    
    for  gcc/ChangeLog
    
            PR tree-optimization/102988
            * gimple-harden-conditionals.cc (detach_value): Copy SSA_NAME
            without decl sharing.
    
    for  gcc/testsuite/ChangeLog
    
            PR tree-optimization/102988
            * g++.dg/pr102988.C: New.

Diff:
---
 gcc/gimple-harden-conditionals.cc |  9 ++++++++-
 gcc/testsuite/g++.dg/pr102988.C   | 17 +++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-harden-conditionals.cc b/gcc/gimple-harden-conditionals.cc
index 8916420d7df..cfa2361d65b 100644
--- a/gcc/gimple-harden-conditionals.cc
+++ b/gcc/gimple-harden-conditionals.cc
@@ -123,7 +123,14 @@ detach_value (location_t loc, gimple_stmt_iterator *gsip, tree val)
       return val;
     }
 
-  tree ret = copy_ssa_name (val);
+  /* Create a SSA "copy" of VAL.  This could be an anonymous
+     temporary, but it's nice to have it named after the corresponding
+     variable.  Alas, when VAL is a DECL_BY_REFERENCE RESULT_DECL,
+     setting (a copy of) it would be flagged by checking, so we don't
+     use copy_ssa_name: we create an anonymous SSA name, and then give
+     it the same identifier (rather than decl) as VAL.  */
+  tree ret = make_ssa_name (TREE_TYPE (val));
+  SET_SSA_NAME_VAR_OR_IDENTIFIER (ret, SSA_NAME_IDENTIFIER (val));
 
   /* Output asm ("" : "=g" (ret) : "0" (val));  */
   vec<tree, va_gc> *inputs = NULL;
diff --git a/gcc/testsuite/g++.dg/pr102988.C b/gcc/testsuite/g++.dg/pr102988.C
new file mode 100644
index 00000000000..05a1a8f67ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr102988.C
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fharden-conditional-branches -fchecking=1" } */
+
+/* DECL_BY_REFERENCE RESULT_DECL is read-only, we can't create a copy
+   of its (address) default def and set it.  */
+
+void ll();
+struct k {
+  ~k();
+};
+k ice(k *a)
+{
+  k v;
+  if (&v!= a)
+    ll();
+  return v;
+}