DSE isn't good at identifying program points that end lifetime
of variables that are not associated with virtual operands. But
at least for those that end basic-blocks we can handle the simple
case where this ending is in the same basic-block as the definition
we want to elide. That should catch quite some common cases already.
Bootstrapped and tested on x86_64-unknown-linux-gnu.
As you can see from the testcase I had to adjust this possibly can
lead to more severe issues when one forgets a return (the C++ frontend
places builtin_unreachable () there). I'm still planning to push
this improvement unless I hear objections.
Thanks,
Richard.
* tree-ssa-dse.cc (dse_classify_store): When we found
no defs and the basic-block with the original definition
ends in __builtin_unreachable[_trap] the store is dead.
* gcc.dg/tree-ssa/ssa-dse-47.c: New testcase.
* c-c++-common/asan/pr106558.c: Avoid undefined behavior
due to missing return.
---
gcc/testsuite/c-c++-common/asan/pr106558.c | 2 +-
gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c | 17 +++++++++++++++++
gcc/tree-ssa-dse.cc | 21 ++++++++++++++++++++-
3 files changed, 38 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
diff --git a/gcc/testsuite/c-c++-common/asan/pr106558.c
b/gcc/testsuite/c-c++-common/asan/pr106558.c
index d82b2dc7a83..c8cefdf09ff 100644
--- a/gcc/testsuite/c-c++-common/asan/pr106558.c
+++ b/gcc/testsuite/c-c++-common/asan/pr106558.c
@@ -8,7 +8,7 @@ int **c = &b;
int d[1];
int *e = &d[1];
-static int f(int *g) {
+static void f(int *g) {
*b = e;
*c = e;
*b = 2;
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
new file mode 100644
index 00000000000..659f1d0d415
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1-details" } */
+
+int a;
+int b[3];
+void test()
+{
+ if (a > 0)
+ {
+ b[0] = 0;
+ b[1] = 1;
+ b[2] = 2;
+ __builtin_unreachable ();
+ }
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 3 "dse1" } } */
diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index eabe8ba4522..3c7a2e9992d 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -1118,7 +1118,26 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
if (defs.is_empty ())
{
if (ref_may_alias_global_p (ref, false))
- return DSE_STORE_LIVE;
+ {
+ basic_block def_bb = gimple_bb (SSA_NAME_DEF_STMT (defvar));
+ /* Assume that BUILT_IN_UNREACHABLE and BUILT_IN_UNREACHABLE_TRAP
+ do not need to keep (global) memory side-effects live.
+ We do not have virtual operands on BUILT_IN_UNREACHABLE
+ but we can do poor mans reachability when the last
+ definition we want to elide is in the block that ends
+ in such a call. */
+ if (EDGE_COUNT (def_bb->succs) == 0)
+ if (gcall *last = dyn_cast <gcall *> (*gsi_last_bb (def_bb)))
+ if (gimple_call_builtin_p (last, BUILT_IN_UNREACHABLE)
+ || gimple_call_builtin_p (last,
+ BUILT_IN_UNREACHABLE_TRAP))
+ {
+ if (by_clobber_p)
+ *by_clobber_p = false;
+ return DSE_STORE_DEAD;
+ }
+ return DSE_STORE_LIVE;
+ }
if (by_clobber_p)
*by_clobber_p = false;
--
2.35.3
