Avoid expicit builtion list in tree-ssa-dce

while working on -fmalloc-dce I noticed that tree-ssa-dce.cc still has an
outdated list of builtions that are known to not read memory that can be
replaced by query to fnspec and modref.

If I get things right, dce does some dead store removal, but only on those
memory object that are non-aliased (automatic variabels with no address taken)
and for all other memory addresses it resorts to
mark_all_reaching_defs_necessary expecting DSE to do the rest.  So we really
want to only check if there are no memory reads at all rather then trying to
understand them by parsing fnspec or modref summary.

I did run testsuite ensuring that all builtins matched previously are
still matched.  There are few testcases where this check fails, due to
type incompatibility.  New code uses gimple_call_builtin while other
just checked callee_decl.

We test things like calling free() without parmeter which I don't think
we want to care about, but there is also testase declaring

void * calloc (long, long)

where builtin declaration expects unsigned long.  I am not sure if this
case should not be allowed by gimple_call_builtin?

Bootstrappe/regtested x86_64-linux. OK?

gcc/ChangeLog:

	* ipa-modref.cc (ipa_modref_callee_reads_no_memory_p): New function.
	* ipa-modref.h (ipa_modref_callee_reads_no_memory_p): Declare
	* tree-ssa-dce.cc (propagate_necessity): Use it.

1 files changed, 12 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr109442.C b/gcc/testsuite/g++.dg/tree-ssa/pr109442.C
new file mode 100644
index 0000000..ec40c47
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr109442.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-O1 -fdump-tree-optimized" }
+#include <vector>
+#define T int
+T vat1(std::vector<T> v1) {
+    auto v = v1;
+    return 10;
+}
+// This should compile to empty function; check that no size of
+// vector is determined and there is no allocation
+// { dg-final { scan-tree-dump-not "_M_start" "optimized" } }
+// { dg-final { scan-tree-dump-not "delete" "optimized" } }