On Mon, Nov 4, 2013 at 7:01 AM, Ian Lance Taylor <i...@google.com> wrote:
> The middle-end currently marks all the sync builtins as nothrow. That
> is incorrect for most of them when using -fnon-call-exceptions. The
> -fnon-call-exceptions option means that instructions that trap may throw
> exceptions. Most of the sync builtins access memory via pointers passed
> by user code. Therefore, they may trap. (I noticed this because Go
> uses -fnon-call-exceptions.)
>
> This patch fixes the problem by introducing a new internal function
> attribute "nothrow call". The new attribute is exactly like "nothrow",
> except that it is ignored when using -fnon-call-exceptions.
>
> It is an internal attribute because there is no reason for a user to use
> this. The problem only arises when the middle-end sees a function call
> that the backend turns into an instruction sequence that directly
> references memory. That can only happen for functions that are built in
> to the compiler.
>
> This requires changes to the C family, LTO, and Ada frontends. I think
> I'm handling the option correctly for LTO, but it would be good if
> somebody could check the logic for me.
>
> Bootstrapped and ran tests on x86_64-unknown-linux-gnu. OK for
> mainline?
Hmm. I think you can handle this in a simpler way by just
doing sth similar to
#define ATTR_MATHFN_FPROUNDING_ERRNO (flag_errno_math ? \
ATTR_NOTHROW_LEAF_LIST : ATTR_MATHFN_FPROUNDING)
that is, conditionally drop the _NOTHROW part. Btw, I also think
that if it can throw then it isn't LEAF (it may return exceptionally
into another function in the unit).
Also this whole conditional-on-a-flag thing doesn't work well with
using -fnon-call-exceptions in the optimize attribute or with
different -fnon-call-exceptions settings in different TUs we LTO
together. Because we only have a single builtin decl (so for
"proper" operation you'd have to clone builtin decls based on
the matrix of flags that generate different attributes and use the
correct one depending on context).
That said, I'd prefer a simpler approach for now, mimicing flag_errno_math
handling.
Your lto-wrapper parts are clearly "wrong" (in a conservative way
though). They follow -fexceptions so the lto-wrapper.c and
lto-opts.c parts are ok.
Fixing the LTO pieces would require to really "stream" most
builtins instead of generating them in the LTO frontend and streaming
them via DECL_FUNCTION_CODE. tree merging should get
rid of the duplicates. I suppose I should try that again ;)
Thanks,
Richard.
> Ian
>
>
> gcc/ChangeLog:
>
> 2013-11-03 Ian Lance Taylor <i...@google.com>
>
> * builtin-attrs.def (ATTR_NOTHROWCALL): Define.
> (ATTR_NOTHROWCALL_LIST, ATTR_NOTHROWCALL_LEAF_LIST): Define.
> * sync-builtins.def: Use ATTR_NOTHROWCALL_LEAF_LIST for all sync
> builtins that take pointers.
> * lto-opts.c (lto_write_options): Write -fnon-call-exceptions
> if set.
> * lto-wrapper.c (merge_and_complain): Collect
> OPT_fnon_call_exceptions.
> (run_gcc): Pass -fnon-call-exceptions.
>
> gcc/c-family/ChangeLog:
>
> 2013-11-03 Ian Lance Taylor <i...@google.com>
>
> * c-common.c (c_common_attribute_table): Add "nothrow call".
> (handle_nothrowcall_attribute): New static function.
>
> gcc/lto/ChangeLog:
>
> 2013-11-03 Ian Lance Taylor <i...@google.com>
>
> * lto-lang.c (lto_attribute_table): Add "nothrow call"
> (handle_nothrowcall_attribute): New static function.
>
> gcc/ada/ChangeLog:
>
> 2013-11-03 Ian Lance Taylor <i...@google.com>
>
> * gcc-interface/utils.c (gnat_internal_attribute_table): Add
> "nothrow call".
> (handle_nothrowcall_attribute): New static function.
>
> gcc/testsuite/ChangeLog:
>
> 2013-11-03 Ian Lance Taylor <i...@google.com>
>
> * g++.dg/ext/sync-4.C: New test.
>
>
