On 11/27/2017 02:22 AM, Dominik Inführ wrote:
Thanks for all the reviews! I’ve revised the patch, the operator_delete_flag is
now stored in tree_decl_with_vis (there already seem to be some
FUNCTION_DECL-flags in there). I’ve also added the option -fallocation-dce to
disable this optimization. It bootstraps and no regressions on aarch64 and
x86_64.
It's great to be able to eliminate pairs of these calls. For
unpaired calls, though, I think it would be even more useful to
also issue a warning. Otherwise the elimination will mask bugs
that might only show up without optimization, or with other
compilers (such as older versions of GCC). I realize GCC doesn't
warn for these bugs involving malloc, but I think it should for
the same reason.
The problem with this patch is what Marc noticed: it omits too many allocations. The
C++ standard seems to only allow to omit "replaceable global allocation
functions (18.6.1.1, 18.6.1.2)”. So e.g. no class-specific or user-defined
allocations. I am not sure what’s the best way to implement this. Just checking the
function declarations might not be enough and seems more like a hack. The better way
seems to introduce a __builtin_operator_new like Marc mentioned. In which way would
you implement this? Could you please give me some pointers here to look at?
I'm not sure there is a way to do it other than by comparing
the name. To see where to insert the built-in and when, you
might want to look at cp/init.c.
Martin
Thanks,
Dominik
On 22 Nov 2017, at 11:37, Jakub Jelinek <ja...@redhat.com> wrote:
On Wed, Nov 22, 2017 at 10:30:29AM +0100, Richard Biener wrote:
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1787,7 +1787,9 @@ struct GTY(()) tree_function_decl {
unsigned has_debug_args_flag : 1;
unsigned tm_clone_flag : 1;
unsigned versioned_function : 1;
- /* No bits left. */
+
+ unsigned operator_delete_flag : 1;
+ /* 31 bits left. */
while it looks bad reality is that on 64bit pointer hosts we had 32 bits left.
But we can just add it to say tree_decl_common which has 14 spare bits or
tree_decl_with_vis which has another 14 spare bits. By just noting the flag
applies only to FUNCTION_DECLs and enforcing it in the tree.h macros,
it will be easy to reuse that bit for something different for trees other
than FUNCTION_DECL.
Jakub
