From: Richard Sandiford <richard.sandiford@arm.com>
To: Martin Sebor <msebor@gmail.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [07/23] Add a class that multiplexes two pointer types
Date: Thu, 26 Nov 2020 17:06:52 +0000 [thread overview]
Message-ID: <mpty2iof3o3.fsf@arm.com> (raw)
In-Reply-To: <c2079f07-0d25-a604-0ab9-ab3314704b12@gmail.com> (Martin Sebor's message of "Wed, 25 Nov 2020 16:33:26 -0700")
Martin Sebor <msebor@gmail.com> writes:
> I do have one concern: the tendency to prioritize efficiency
> over safety (this can be said about most GCC code). Specifically
> in this class, the address bit twiddling makes me uneasy. I don't
> think the object model in either language (certainly not C but
> I don't have the impression C++ either) makes it unequivocally
> valid. On the contrary, I'd say many of us interpret the current
> rules as leaving it undefined. There are efforts to sanction
> this sort of thing under some conditions (e.g, the C object
> model proposal) but they have not been adopted yet. I think
> we should try to avoid exploiting these dark corners in new
> code.
I'd tried to stick to operations that I thought were well-defined.
The primitives being used are really:
(1) convert a T1* or T2* to char*
(2) increment an unincremented char*
(3) decrement an incremented char*
(4) convert a char* back to T1* or T2*
(5) convert a char* to an intptr_t in order to test its low bit
I thought (1) and (4) were allowed. At least, [basic.compound] says
that void* must be able to hold any object pointer and that it must have
the same representation as char*, so I thought the conversion in (1) was
guaranteed to be representable. And (4) only ever undoes (1): it only
converts the result of (1) back to the original pointer type.
For (2) and (3), the incremented pointer will still be within the
containing object, so I thought it would be well-defined. Here too,
(3) only ever undoes (2): it only decrements a pointer that had
previously been incremented.
One thing I'd deliberately tried to avoid was converting integers
“back” to pointers, because that seemed like a more dangerous thing.
That's why:
>> +template<typename T1, typename T2>
>> +inline T2 *
>> +pointer_mux<T1, T2>::second_or_null () const
>> +{
>> + // Micro optimization that's effective as of GCC 11: compute the value
>> + // of the second pointer as an integer and test that, so that the integer
>> + // result can be reused as the pointer and so that all computation can
>> + // happen before a branch on null. This reduces the number of branches
>> + // needed for loops.
>> + return uintptr_t (m_ptr - 1) & 1 ? nullptr : known_second ();
>> +}
is written in a somewhat indirect way.
Are your concerns with the primitives above, or is the problem with
something else?
Thanks,
Richard
next prev parent reply other threads:[~2020-11-26 17:06 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-13 8:10 [00/23] Make fwprop use an on-the-side RTL SSA representation Richard Sandiford
2020-11-13 8:11 ` [01/23] vec: Silence clang warning Richard Sandiford
2020-11-25 19:58 ` Jeff Law
2020-11-13 8:12 ` [02/23] rtlanal: Remove noop_move_p REG_EQUAL condition Richard Sandiford
2020-11-25 20:00 ` Jeff Law
2020-11-13 8:12 ` [03/23] reginfo: Add a global_reg_set Richard Sandiford
2020-11-25 20:01 ` Jeff Law
2020-11-13 8:13 ` [04/23] Move iterator_range to a new iterator-utils.h file Richard Sandiford
2020-11-25 20:02 ` Jeff Law
2020-11-13 8:13 ` [05/23] Add more iterator utilities Richard Sandiford
2020-11-25 20:12 ` Jeff Law
2020-11-13 8:14 ` [06/23] Add an RAII class for managing obstacks Richard Sandiford
2020-11-25 20:15 ` Jeff Law
2020-11-13 8:14 ` [07/23] Add a class that multiplexes two pointer types Richard Sandiford
2020-11-25 20:23 ` Jeff Law
2020-11-26 16:15 ` Richard Sandiford
2020-11-30 1:28 ` Jeff Law
2020-11-25 23:33 ` Martin Sebor
2020-11-26 17:06 ` Richard Sandiford [this message]
2020-11-27 18:12 ` Richard Sandiford
2020-11-28 0:17 ` Martin Sebor
2020-12-17 0:17 ` Richard Sandiford
2020-12-17 14:21 ` Tom Tromey
2020-12-17 15:38 ` Richard Sandiford
2020-12-17 15:44 ` Nathan Sidwell
2021-01-04 15:32 ` Jeff Law
2020-11-13 8:15 ` [08/23] Add an alternative splay tree implementation Richard Sandiford
2020-12-02 20:36 ` Jeff Law
2020-12-17 0:29 ` Richard Sandiford
2021-01-04 15:27 ` Jeff Law
2021-01-01 8:25 ` Andreas Schwab
2021-01-04 14:53 ` Richard Sandiford
2021-01-04 15:02 ` Andreas Schwab
2021-01-04 15:42 ` Richard Sandiford
2021-01-05 12:13 ` Richard Biener
2020-11-13 8:15 ` [09/23] Add a cut-down version of std::span (array_slice) Richard Sandiford
2020-11-30 19:56 ` Jeff Law
2022-08-03 15:13 ` Martin Jambor
2022-08-03 15:31 ` Richard Sandiford
2022-08-10 16:03 ` Martin Jambor
2022-08-11 6:58 ` Richard Biener
2022-08-16 7:59 ` Richard Sandiford
2020-11-13 8:16 ` [10/23] Tweak the way that is_a is implemented Richard Sandiford
2020-12-02 5:15 ` Jeff Law
2020-11-13 8:16 ` [11/23] Split update_cfg_for_uncondjump out of combine Richard Sandiford
2020-11-30 6:14 ` Jeff Law
2020-11-13 8:17 ` [12/23] Export print-rtl.c:print_insn_with_notes Richard Sandiford
2020-11-25 20:24 ` Jeff Law
2020-11-13 8:18 ` [13/23] recog: Split out a register_asm_p function Richard Sandiford
2020-11-25 20:24 ` Jeff Law
2020-11-13 8:18 ` [14/23] simplify-rtx: Put simplify routines into a class Richard Sandiford
2020-11-30 19:54 ` Jeff Law
2020-11-13 8:19 ` [15/23] recog: Add a validate_change_xveclen function Richard Sandiford
2020-11-30 20:03 ` Jeff Law
2020-11-13 8:19 ` [16/23] recog: Add a way of temporarily undoing changes Richard Sandiford
2020-11-25 20:27 ` Jeff Law
2020-12-17 0:22 ` Richard Sandiford
2020-11-13 8:20 ` [17/23] recog: Add a class for propagating into insns Richard Sandiford
2020-12-03 22:32 ` Jeff Law
2020-11-13 8:20 ` [18/23] recog: Add an RAII class for undoing insn changes Richard Sandiford
2020-11-25 20:27 ` Jeff Law
2020-11-13 8:20 ` [19/23] rtlanal: Add some new helper classes Richard Sandiford
2020-12-13 17:30 ` Jeff Law
2020-12-14 16:37 ` Richard Sandiford
2020-12-14 20:02 ` Jeff Law
2020-11-13 8:21 ` [20/23] rtlanal: Add simple_regno_set Richard Sandiford
2020-11-25 20:31 ` Jeff Law
2020-12-17 0:47 ` Richard Sandiford
2021-01-04 15:28 ` Jeff Law
2020-11-13 8:22 ` [21/23] doc: Add documentation for rtl-ssa Richard Sandiford
2020-11-30 6:26 ` Jeff Law
2020-11-13 8:23 ` [PATCH 22/23] Add rtl-ssa Richard Sandiford
2020-12-16 3:31 ` Jeff Law
2020-12-17 0:33 ` Richard Sandiford
2020-12-19 20:01 ` Jeff Law
2020-11-13 8:24 ` [PATCH 23/23] fwprop: Rewrite to use RTL SSA Richard Sandiford
2020-12-16 3:52 ` Jeff Law
2020-12-17 0:34 ` Richard Sandiford
2020-11-25 19:58 ` [00/23] Make fwprop use an on-the-side RTL SSA representation Jeff Law
2020-11-26 16:03 ` Richard Sandiford
2020-11-27 15:56 ` Michael Matz
2020-11-27 16:31 ` Richard Sandiford
2020-11-30 21:13 ` Jeff Law
2020-12-01 0:03 ` Michael Matz
2020-12-01 10:15 ` Richard Sandiford
2020-12-02 0:25 ` Jeff Law
2020-11-30 6:45 ` Jeff Law
2020-11-30 14:12 ` Richard Sandiford
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