From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-x42f.google.com (mail-wr1-x42f.google.com [IPv6:2a00:1450:4864:20::42f]) by sourceware.org (Postfix) with ESMTPS id 24ECB3858D33; Sun, 10 Dec 2023 11:31:37 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 24ECB3858D33 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com ARC-Filter: OpenARC Filter v1.0.0 sourceware.org 24ECB3858D33 Authentication-Results: server2.sourceware.org; arc=none smtp.remote-ip=2a00:1450:4864:20::42f ARC-Seal: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1702207901; cv=none; b=wfQrv/9LFyYS067dGstomcpnKQhtrSh10KTlVg15g6mwnpfeImaEiS+XFO+2ZaPZfqBB5GQT4xf8oSp56nHRBqj69/BIznv4cqfliJfHAT1cCS6Ug+Xl9RQinIwkL1Kam1/6VFPjgEkm4AYHg3M1cXYVjALhqwtml9qrinjh+vw= ARC-Message-Signature: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1702207901; c=relaxed/simple; bh=3vexuYDe1Lk2Khi2XP0TYEkHGcph8Yf1JjjCCRDRQ44=; h=DKIM-Signature:From:To:Subject:Date:Message-ID:MIME-Version; b=UOKMbsJggrAZLjdMf9A4ToaUmZXztivEaTlYnVK+xHiGvoeMSFpq2uADshrTSyVW5wYm2MalTAeP9m7KY3D70fJtqrUv3qykD+DPfLuZZeW5L/9m5yaWVA4CH2kqwE3EhRIcyzNUiqtoft1HMu2p6UjHizKXIDghbytBD93kqMo= ARC-Authentication-Results: i=1; server2.sourceware.org Received: by mail-wr1-x42f.google.com with SMTP id ffacd0b85a97d-332fd78fa9dso3330544f8f.3; Sun, 10 Dec 2023 03:31:37 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1702207895; x=1702812695; darn=gcc.gnu.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=6IUsInv8BE6zs14d/R3BRIYTzCAuHfgPsStrqnfSg1M=; b=Y27O3MIMp0WenHJW2DyDTtciUdAaaTlAspIYEn7qYcf60i7C5w2ZYY60clNGZUqp07 zEqryPoGa+fp4/PzfOAx8gNPA7AcLcqiZesEOJTljY5Q3i5n0ysxiZkwneAvzCosXIhK 8BlEUFduuWAe2fvla1Hu8+lJxl9KoRY9/16IvzQ97KgC6w+LPK3yIdbzv8nfdKZ3YrGV 6b+0MFU1wJurnIO9Rmeq7XKKQ3sk6CbbElU/CoksV0qT92J1LanALR1+noYpuLXW6lHN Y97TZlM/kv619fTThRnF4KLC28jkV1MMW/PJp9rM7Jr1P/hfxkKX4OeQ1RRjfSh6yc9Q TcJg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1702207895; x=1702812695; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=6IUsInv8BE6zs14d/R3BRIYTzCAuHfgPsStrqnfSg1M=; b=SXyLYTWoRn6ysu9eU6xTzIRx84XWLmtJmWbWaBz4lLU+s4YFPQsMKxm50z/F7/MSqT Gya0WMFNDKcmijTotfQoAN7whC+s5ZZjfCm7O2Hn7ECc/BFezwwqBjNTIBXS6GW0OQ9F gbu5Ya0rQkcgbERhtWxeu1nvRUkHkKWXAebi3bKcq96R8wKkldJD1ODvQ7ovWZrWmqfi EWMVwyRx+Ux2qo0PqqO00fs9VZnzP0jViyQ/bF3UhTDCy1MJe6/oXKSo1jeeyT8oNfxf QBip1WBc3kqLq87ZOBDguy4B1uHGdQNOVLQrSeUpiT0N9now9adpFUhcFZknW+OzVyQV Qndg== X-Gm-Message-State: AOJu0YxDoR+H1/BVg+Il/fP/iMJQyCaoqb7StnbHe2imsjZcosLtdM+d YpKvXuXpXLkwjCkQy+qrXz3zdFO34Hs= X-Google-Smtp-Source: AGHT+IEdmKMWrYzUfcmkgr1CAXIvkmXjoJRbCBvQFkekNhBONJaCRZ5PGh+mEvCDQJg6HbzBC+yaHg== X-Received: by 2002:a05:600c:215a:b0:40c:3efe:639b with SMTP id v26-20020a05600c215a00b0040c3efe639bmr491223wml.274.1702207894792; Sun, 10 Dec 2023 03:31:34 -0800 (PST) Received: from othello.cust.communityfibre.co.uk ([2a02:6b64:8086:0:f9dd:28dd:c9b9:d8f4]) by smtp.gmail.com with ESMTPSA id v6-20020a05600c444600b0040c46ba7b66sm553831wmn.48.2023.12.10.03.31.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 10 Dec 2023 03:31:33 -0800 (PST) From: Cassio Neri To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Cc: Cassio Neri Subject: [PING,PATCH v5] libstdc++: Remove UB from month and weekday additions and subtractions. Date: Sun, 10 Dec 2023 11:31:31 +0000 Message-ID: <20231210113131.3977-1-cassio.neri@gmail.com> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20231125135217.7180-1-cassio.neri@gmail.com> References: <20231125135217.7180-1-cassio.neri@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-12.2 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: The following invoke signed integer overflow (UB) [1]: month + months{MAX} // where MAX is the maximum value of months::rep month + months{MIN} // where MIN is the maximum value of months::rep month - months{MIN} // where MIN is the minimum value of months::rep weekday + days {MAX} // where MAX is the maximum value of days::rep weekday - days {MIN} // where MIN is the minimum value of days::rep For the additions to MAX, the crux of the problem is that, in libstdc++, months::rep and days::rep are int64_t. Other implementations use int32_t, cast operands to int64_t and perform arithmetic operations without risk of overflowing. For month + months{MIN}, the implementation follows the Standard's "returns clause" and evaluates: modulo(static_cast(unsigned{__x}) + (__y.count() - 1), 12); Overflow occurs when MIN - 1 is evaluated. Casting to a larger type could help but, unfortunately again, this is not possible for libstdc++. For the subtraction of MIN, the problem is that -MIN is not representable. It's fair to say that the intention is for these additions/subtractions to be performed in modulus (12 or 7) arithmetic so that no overflow is expected. To fix these UB, this patch implements: template unsigned __add_modulo(unsigned __x, _T __y); template unsigned __sub_modulo(unsigned __x, _T __y); which respectively, returns the remainder of Euclidean division of, __x + __y and __x - __y by __d without overflowing. These functions replace constexpr unsigned __modulo(long long __n, unsigned __d); which also calculates the reminder of __n, where __n is the result of the addition or subtraction. Hence, these operations might invoke UB before __modulo is called and thus, __modulo can't do anything to remediate the issue. In addition to solve the UB issues, __add_modulo and __sub_modulo allow better codegen (shorter and branchless) on x86-64 and ARM [2]. [1] https://godbolt.org/z/a9YfWdn57 [2] https://godbolt.org/z/Gh36cr7E4 libstdc++-v3/ChangeLog: * include/std/chrono: Fix + and - for months and weekdays. * testsuite/std/time/month/1.cc: Add constexpr tests against overflow. * testsuite/std/time/month/2.cc: New test for extreme values. * testsuite/std/time/weekday/1.cc: Add constexpr tests against overflow. * testsuite/std/time/weekday/2.cc: New test for extreme values. --- Ping. Good for trunk? Changes with respect to previous versions: v5: Fix typos in commit message. v4: Remove UB also from operator-. v3: Fix screwed up email send with v2. v2: Replace _T with _Tp and _U with _Up. Remove copyright+license from test. libstdc++-v3/include/std/chrono | 79 +++++++++++++------- libstdc++-v3/testsuite/std/time/month/1.cc | 19 +++++ libstdc++-v3/testsuite/std/time/month/2.cc | 32 ++++++++ libstdc++-v3/testsuite/std/time/weekday/1.cc | 16 +++- libstdc++-v3/testsuite/std/time/weekday/2.cc | 32 ++++++++ 5 files changed, 151 insertions(+), 27 deletions(-) create mode 100644 libstdc++-v3/testsuite/std/time/month/2.cc create mode 100644 libstdc++-v3/testsuite/std/time/weekday/2.cc diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index e4ba6eafceb..1b7cdb96e1c 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -501,18 +501,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION namespace __detail { - // Compute the remainder of the Euclidean division of __n divided by __d. - // Euclidean division truncates toward negative infinity and always - // produces a remainder in the range of [0,__d-1] (whereas standard - // division truncates toward zero and yields a nonpositive remainder - // for negative __n). + // Helper to __add_modulo and __sub_modulo. + template + constexpr auto + __modulo_offset() + { + using _Up = make_unsigned_t<_Tp>; + auto constexpr __a = _Up(-1) - _Up(255 + __d - 2); + auto constexpr __b = _Up(__d * (__a / __d) - 1); + // Notice: b <= a - 1 <= _Up(-1) - (255 + d - 1) and b % d = d - 1. + return _Up(-1) - __b; // >= 255 + d - 1 + } + + // Compute the remainder of the Euclidean division of __x + __y divided by + // __d without overflowing. Typically, __x <= 255 + d - 1 is sum of + // weekday/month with a shift in [0, d - 1] and __y is a duration count. + template + constexpr unsigned + __add_modulo(unsigned __x, _Tp __y) + { + using _Up = make_unsigned_t<_Tp>; + // For __y >= 0, _Up(__y) has the same mathematical value as __y and + // this function simply returns (__x + _Up(__y)) % d. Typically, this + // doesn't overflow since the range of _Up contains many more positive + // values than _Tp's. For __y < 0, _Up(__y) has a mathematical value in + // the upper-half range of _Up so that adding a positive value to it + // might overflow. Moreover, most likely, _Up(__y) != __y mod d. To + // fix both issues we subtract from _Up(__y) an __offset >= + // 255 + d - 1 to make room for the addition to __x and shift the modulo + // to the correct value. + auto const __offset = __y >= 0 ? _Up(0) : __modulo_offset<__d, _Tp>(); + return (__x + _Up(__y) - __offset) % __d; + } + + // Similar to __add_modulo but for __x - __y. + template constexpr unsigned - __modulo(long long __n, unsigned __d) + __sub_modulo(unsigned __x, _Tp __y) { - if (__n >= 0) - return __n % __d; - else - return (__d + (__n % __d)) % __d; + using _Up = make_unsigned_t<_Tp>; + auto const __offset = __y <= 0 ? _Up(0) : __modulo_offset<__d, _Tp>(); + return (__x - _Up(__y) - __offset) % __d; } inline constexpr unsigned __days_per_month[12] @@ -704,8 +733,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr month operator+(const month& __x, const months& __y) noexcept { - auto __n = static_cast(unsigned{__x}) + (__y.count() - 1); - return month{__detail::__modulo(__n, 12) + 1}; + // modulo(x + (y - 1), 12) = modulo(x + (y - 1) + 12, 12) + // = modulo((x + 11) + y , 12) + return month{1 + __detail::__add_modulo<12>( + unsigned{__x} + 11, __y.count())}; } friend constexpr month @@ -714,7 +745,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr month operator-(const month& __x, const months& __y) noexcept - { return __x + -__y; } + { + // modulo(x + (-y - 1), 12) = modulo(x + (-y - 1) + 12, 12) + // = modulo((x + 11) - y , 12) + return month{1 + __detail::__sub_modulo<12>( + unsigned{__x} + 11, __y.count())}; + } friend constexpr months operator-(const month& __x, const month& __y) noexcept @@ -934,15 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION static constexpr weekday _S_from_days(const days& __d) { - using _Rep = days::rep; - using _URep = make_unsigned_t<_Rep>; - const auto __n = __d.count(); - const auto __m = static_cast<_URep>(__n); - - // 1970-01-01 (__n = 0, __m = 0 ) -> Thursday (4) - // 1969-31-12 (__n = -1, __m = _URep(-1)) -> Wednesday (3) - const auto __offset = __n >= 0 ? _URep(4) : 3 - _URep(-1) % 7 - 7; - return weekday((__m + __offset) % 7); + return weekday{__detail::__add_modulo<7>(4, __d.count())}; } public: @@ -1032,8 +1060,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr weekday operator+(const weekday& __x, const days& __y) noexcept { - auto __n = static_cast(__x._M_wd) + __y.count(); - return weekday{__detail::__modulo(__n, 7)}; + return weekday{__detail::__add_modulo<7>(__x._M_wd, __y.count())}; } friend constexpr weekday @@ -1042,7 +1069,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr weekday operator-(const weekday& __x, const days& __y) noexcept - { return __x + -__y; } + { + return weekday{__detail::__sub_modulo<7>(__x._M_wd, __y.count())}; + } friend constexpr days operator-(const weekday& __x, const weekday& __y) noexcept diff --git a/libstdc++-v3/testsuite/std/time/month/1.cc b/libstdc++-v3/testsuite/std/time/month/1.cc index 773f772bf07..1b3612f27e3 100644 --- a/libstdc++-v3/testsuite/std/time/month/1.cc +++ b/libstdc++-v3/testsuite/std/time/month/1.cc @@ -20,6 +20,7 @@ // Class template day [time.cal.month] #include +#include constexpr void constexpr_month() @@ -34,6 +35,24 @@ constexpr_month() dm += months{3}; dm -= months{3}; + // Test for UB (overflow). + { + using rep = months::rep; + using std::numeric_limits; + + auto constexpr months_min = months{numeric_limits::min()}; + auto constexpr month_000_plus_months_min = month{ 0 } + months_min; + auto constexpr month_255_plus_months_min = month{255} + months_min; + auto constexpr month_000_minus_months_min = month{ 0 } - months_min; + auto constexpr month_255_minus_months_min = month{255} - months_min; + + auto constexpr months_max = months{numeric_limits::max()}; + auto constexpr month_000_plus_months_max = month{ 0 } + months_max; + auto constexpr month_255_plus_months_max = month{255} + months_max; + auto constexpr month_000_minus_months_max = month{ 0 } - months_max; + auto constexpr month_255_minus_months_max = month{255} - months_max; + } + static_assert(February + months{11} == January); static_assert(January + months{1200} == January); static_assert(January + months{1201} == February); diff --git a/libstdc++-v3/testsuite/std/time/month/2.cc b/libstdc++-v3/testsuite/std/time/month/2.cc new file mode 100644 index 00000000000..3bcefa60003 --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/month/2.cc @@ -0,0 +1,32 @@ +// { dg-do run { target c++20 } } + +// Class month [time.cal.month] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(months extreme) +{ + auto const count = extreme.count(); + auto const safe = count < 0 ? count + 12 : count; + auto const mod = safe - 12 * ((safe < 0 ? safe - 11 : safe) / 12); + + for (unsigned m = 0; m < 256; ++m) + { + auto const month_plus_extreme = month{m} + extreme; + VERIFY(unsigned{month_plus_extreme } == (m + 11 + mod) % 12 + 1); + + auto const month_minus_extreme = month{m} - extreme; + VERIFY(unsigned{month_minus_extreme} == (m + 11 - mod) % 12 + 1); + } +} + +int main() +{ + test_extreme_values(months{std::numeric_limits::max()}); + test_extreme_values(months{std::numeric_limits::min()}); + return 0; +} diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc b/libstdc++-v3/testsuite/std/time/weekday/1.cc index e89fca47d4b..2f0249b28a4 100644 --- a/libstdc++-v3/testsuite/std/time/weekday/1.cc +++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc @@ -42,8 +42,20 @@ constexpr_weekday() { using rep = days::rep; using std::numeric_limits; - constexpr weekday max{sys_days{days{numeric_limits::max()}}}; - constexpr weekday min{sys_days{days{numeric_limits::min()}}}; + + auto constexpr days_min = days{numeric_limits::min()}; + auto constexpr weekday_from_sysdays_min = weekday{sys_days{days_min}}; + auto constexpr weekday_000_plus_days_min = weekday{ 0 } + days_min; + auto constexpr weekday_255_plus_days_min = weekday{255} + days_min; + auto constexpr weekday_000_minus_days_min = weekday{ 0 } - days_min; + auto constexpr weekday_255_minus_days_min = weekday{255} - days_min; + + auto constexpr days_max = days{numeric_limits::max()}; + auto constexpr weekday_from_sysdays_max = weekday{sys_days{days_max}}; + auto constexpr weekday_000_plus_days_max = weekday{ 0 } + days_max; + auto constexpr weekday_255_plus_days_max = weekday{255} + days_max; + auto constexpr weekday_000_minus_days_max = weekday{ 0 } - days_max; + auto constexpr weekday_255_minus_days_max = weekday{255} - days_max; } static_assert(weekday{sys_days{1900y/January/1}} == Monday); diff --git a/libstdc++-v3/testsuite/std/time/weekday/2.cc b/libstdc++-v3/testsuite/std/time/weekday/2.cc new file mode 100644 index 00000000000..924709321e5 --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/weekday/2.cc @@ -0,0 +1,32 @@ +// { dg-do run { target c++20 } } + +// Class weekday [time.cal.wd] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(days extreme) +{ + auto const count = extreme.count(); + auto const safe = count < 0 ? count + 7 : count; + auto const mod = safe - 7 * ((safe < 0 ? safe - 6 : safe) / 7); + + for (unsigned d = 0; d < 254; ++d) + { + auto const weekday_plus_extreme = weekday{d} + extreme; + VERIFY(weekday_plus_extreme.c_encoding() == (d + mod) % 7); + + auto const weekday_minus_extreme = weekday{d} - extreme; + VERIFY(weekday_minus_extreme.c_encoding() == (d + 7 - mod) % 7); + } +} + +int main() +{ + test_extreme_values(days{std::numeric_limits::max()}); + test_extreme_values(days{std::numeric_limits::min()}); + return 0; +} -- 2.41.0