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Sat, 23 May 2020 09:44:16 -0700 (PDT) Subject: Re: stack module To: =?UTF-8?Q?Marc_Nieper-Wi=c3=9fkirchen?= References: <1665251.0HJpBWKgsO@omega> <2943012.YJjvApnCys@omega> From: Paul Eggert Autocrypt: addr=eggert@cs.ucla.edu; prefer-encrypt=mutual; keydata= LS0tLS1CRUdJTiBQR1AgUFVCTElDIEtFWSBCTE9DSy0tLS0tCgptUUlOQkV5QWNtUUJFQURB QXlIMnhvVHU3cHBHNUQzYThGTVpFb243NGRDdmM0K3ExWEEySjJ0QnkycHdhVHFmCmhweHhk R0E5Smo1MFVKM1BENGJTVUVnTjh0TFowc2FuNDdsNVhUQUZMaTI0NTZjaVNsNW04c0thSGxH ZHQ5WG0KQUF0bVhxZVpWSVlYL1VGUzk2ZkR6ZjR4aEVtbS95N0xiWUVQUWRVZHh1NDd4QTVL aFRZcDVibHRGM1dZRHoxWQpnZDdneDA3QXV3cDdpdzdlTnZub0RUQWxLQWw4S1lEWnpiRE5D UUdFYnBZM2VmWkl2UGRlSStGV1FONFcra2doCnkrUDZhdTZQcklJaFlyYWV1YTdYRGRiMkxT MWVuM1NzbUUzUWpxZlJxSS9BMnVlOEpNd3N2WGUvV0szOEV6czYKeDc0aVRhcUkzQUZINmls QWhEcXBNbmQvbXNTRVNORnQ3NkRpTzFaS1FNcjlhbVZQa25qZlBtSklTcWRoZ0IxRApsRWR3 MzRzUk9mNlY4bVp3MHhmcVQ2UEtFNDZMY0ZlZnpzMGtiZzRHT1JmOHZqRzJTZjF0azVlVThN Qml5Ti9iClowM2JLTmpOWU1wT0REUVF3dVA4NGtZTGtYMndCeHhNQWhCeHdiRFZadWR6eERa SjFDMlZYdWpDT0pWeHEya2wKakJNOUVUWXVVR3FkNzVBVzJMWHJMdzYrTXVJc0hGQVlBZ1Jy NytLY3dEZ0JBZndoUEJZWDM0blNTaUhsbUxDKwpLYUhMZUNMRjVaSTJ2S20zSEVlQ1R0bE9n N3haRU9OZ3d6TCtmZEtvK0Q2U29DOFJSeEpLczhhM3NWZkk0dDZDCm5yUXp2SmJCbjZneGRn Q3U1aTI5SjFRQ1lyQ1l2cWwyVXlGUEFLK2RvOTkvMWpPWFQ0bTI4MzZqMXdBUkFRQUIKdENC UVlYVnNJRVZuWjJWeWRDQThaV2RuWlhKMFFHTnpMblZqYkdFdVpXUjFQb2tDUGdRVEFRSUFL QVVDVElCeQpaQUliQXdVSkVzd0RBQVlMQ1FnSEF3SUdGUWdDQ1FvTEJCWUNBd0VDSGdFQ0Y0 QUFDZ2tRN1pmcERtS3FmalJSCkd3LytJajAzZGhZZllsL2dYVlJpdXpWMWdHcmJIayt0bmZy SS9DN2ZBZW9GelE1dFZnVmluU2hhUGtabzBIVFAKZjE4eDZJREVkQWlPOE1xbzF5cDBDdEht ekdNQ0o1MG80R3JnZmpscjZnLyt2dEVPS2JobGVzek4yWHBKdnB3TQoyUWdHdm4vbGFUTFV1 OFBIOWFSV1RzN3FKSlpLS0tBYjRzeFljOTJGZWhQdTZGT0QwZERpeWhsREFxNGxPVjJtCmRC cHpRYmlvam9aelFMTVF3anBnQ1RLMjU3MmVLOUVPRVF5U1VUaFhyU0l6NkFTZW5wNE5ZVEZI czl0dUpRdlgKazlnWkRkUFNsM2JwKzQ3ZEd4bHhFV0xwQklNN3pJT053NGtzNGF6Z1Q4bnZE WnhBNUlaSHR2cUJsSkxCT2JZWQowTGU2MVdwMHkzVGxCRGgycWRLOGVZTDQyNlc0c2NFTVN1 aWc1Z2I4T0F0UWlCVzZrMnNHVXh4ZWl2OG92V3U4CllBWmdLSmZ1b1dJK3VSbk1FZGRydVk4 SnNvTTU0S2FLdlppa2tLczJiZzFuZHRMVnpIcEo2cUZaQzdRVmplSFUKaDYvQm1ndmRqV1Ba WUZUdE4rS0E5Q1dYM0dRS0tnTjN1dTk4OHl6bkQ3TG5COThUNEVVSDFIQS9HbmZCcU1WMQpn cHpUdlBjNHFWUWluQ21Ja0VGcDgzemwrRzVmQ2pKSjNXN2l2ekNuWW80S2hLTHBGVW05N29r VEtSMkxXM3haCnpFVzRjTFNXTzM4N01USzNDekRPeDVxZTZzNGE5MVp1Wk0vai9UUWRUTERh cU5uODNrQTRIcTQ4VUhYWXhjSWgKK05kOGsvM3c2bEZ1b0swd3JPRml5d2pMeCswdXI1am1t YmVjQkdIYzF4ZGhBRkc1QWcwRVRJQnlaQUVRQUthRgo2NzhUOXd5SDR3alRyVjFQejNjREVv U25WLzBaVXJPVDM3cDFkY0d5ai9JWHExeDY3MEhSVmFoQW1rMHNacFljCjI1UEY5RDVHUFlI RldsTmp1UFU5NnJEbmRYQjNoZWRtQlJoTGRDNGJBWGpJNERWK2JtZFZlK3EvSU1ubFpSYVYK bG05RWlNQ1ZBUjZ3MTNzUmV1N3FYa1c5cjNSd1kyQXpYc2twL3RBZTRCUktyMVptYnZpMm5i blE2ZXBFQzQycgpSYngwQjFFaGpiSVFaNUpIR2syNGlQVDdMZEJnbk5tb3M1d1lqendObGtN UUQ1VDBZZHpoazdKK1V4d0E1bTQ2Cm1PaFJEQzJyRlYvQTBnbTVUTHk4RFhqdi9Fc2M0Z1lu WWFpNlNRcW5VRVZoNUx1VjhZQ0pCbmlqcytUaXc3MXgKMWljbW42eEdJNDVFdWdKT2dlYyty THlwWWdwVnA0eDBISTVUODhxQlJZQ2t4SDNLZzhRbytFV05BOUE0TFJROQpEWDhuam9uYTBn ZjBzMDN0b2NLOGtCTjY2VW9xcVB0SEJuYzRlTWdCeW1DZmxLMTJlS2ZkMllZeG55ZzljWmF6 CldBNVZzbHZUeHBtNzZoYmc1b2lBRUgvVmcvOE14SHlBblBoZnJnd3lQcm1KRWNWQmFmZHNw Sm5ZUXhCWU5jbzIKTEZQSWhsT3ZXaDhyNGF0K3MrTTNMYjI2b1VUY3psZ2RXMVNmM1NEQTc3 Qk1SbkYwRlF5RSs3QXpWNzlNQk40eQpraXFhZXpReHRhRjFGeS90dmtoZmZTbzh1K2R3RzBF Z0poK3RlMzhnVGNJU1ZyMEdJUHBsTHo2WWhqcmJIclBSCkYxQ041VXVMOURCR2p4dU4zNVJM TlZFZnRhNlJVRmxSNk5jdFRqdnJBQkVCQUFHSkFpVUVHQUVDQUE4RkFreUEKY21RQ0d3d0ZD UkxNQXdBQUNna1E3WmZwRG1LcWZqU3JIQS8rS3pBS3ZUeFJoQTlNV05MeEl5SjdTNXVKMTZn cwpUM29DalpyQktHRWhLTU9HWDRPMEdBNlZPRXJ5TzdRUkNDWWFoM294U0czOElBbk5laXdK WGdVOUJ6a2s4NVVHCmJQRWQ3SEdGL1ZTZUhDUXdXb3U2anFVRFRTRHZuOVloTlRkRzBLWFBN NzRhQyt4cjJab3cxTzJtaFhpaGdXS0QKMER3KzBMWVBuVU9zUTBLT0Z4SFhYWUhtUnJTMU9a UFU1OUJMdmMrVFJoSWhhZlNIS0x3YlhLKzZja2t4Qng2aAo4ejVjY3BHMFFzNGJGaGRGWW5G ckVpZURMb0dtbkUyWUxoZFY2c3dKOVZOQ1M2cExpRW9oVDNmbTdhWG0xNXRaCk9JeXpNWmhI UlNBUGJsWHhRMFpTV2pxOG9ScmNZTkZ4YzRXMVVScEFrQkNPWUpvWHZRZkQ1TDNscUFsOFRD cUQKVXpZeGhIL3RKaGJEZEhycUhINzY3amFEYVRCMStUYWxwLzJBTUt3Y1hOT2Rpa2xHeGJt SFZHNllHbDZnOExyYgpzdTlOWkVJNHlMbEh6dWlrdGhKV2d6KzN2WmhWR3lObHQrSE5Jb0Y2 Q2pETDJvbXU1Y0VxNFJESE00NFFxUGs2Cmw3TzBwVXZOMW1UNEIrUzFiMDhSS3BxbS9mZjAx NUUzN0hOVi9waUl2Smx4R0FZejhQU2Z1R0NCMXRoTVlxbG0KZ2RoZDkvQmFiR0ZiR0dZSEE2 VTQvVDV6cVUrZjZ4SHkxU3NBUVoxTVNLbEx3ZWtCSVQrNC9jTFJHcUNIam5WMApxNUgvVDZh N3Q1bVBrYnpTck9MU280cHVqK0lUb05qWXlZSURCV3pobEExOWF2T2ErcnZVam1IdEQzc0ZO N2NYCld0a0dvaThidU5jYnk0VT0KPUFMNm8KLS0tLS1FTkQgUEdQIFBVQkxJQyBLRVkgQkxP Q0stLS0tLQo= Organization: UCLA Computer Science Department Message-ID: <8ac85bb6-22ed-761d-f86a-a37bca490480@cs.ucla.edu> Date: Sat, 23 May 2020 09:44:16 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/mixed; boundary="------------CACDAA585AAFA57C98CCEE49" Content-Language: en-US Received-SPF: pass client-ip=131.179.128.68; envelope-from=eggert@cs.ucla.edu; helo=zimbra.cs.ucla.edu X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/23 11:33:18 X-ACL-Warn: Detected OS = Linux 3.1-3.10 X-Spam_score_int: -41 X-Spam_score: -4.2 X-Spam_bar: ---- X-Spam_report: (-4.2 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: bug-gnulib@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Gnulib discussion list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: bug-gnulib@gnu.org Errors-To: bug-gnulib-bounces+normalperson=yhbt.net@gnu.org Sender: "bug-gnulib" This is a multi-part message in MIME format. --------------CACDAA585AAFA57C98CCEE49 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 5/23/20 8:55 AM, Marc Nieper-Wi=C3=9Fkirchen wrote: > A combination of assure and assume would be helpful: >=20 > #define checked_assume(X) do { assure (X); assume (X); } while (0) No, because the compiler is entitled to optimize away the 'assure (X)' in= this case. I installed the attached to try to explain this better. --------------CACDAA585AAFA57C98CCEE49 Content-Type: text/x-patch; charset=UTF-8; name="0001-verify-document-assume-better.patch" Content-Disposition: attachment; filename="0001-verify-document-assume-better.patch" Content-Transfer-Encoding: quoted-printable >From b2c8d02c9750f335549781d20fa37415c9a1edb3 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Sat, 23 May 2020 09:41:54 -0700 Subject: [PATCH] =3D?UTF-8?q?verify:=3D20document=3D20=3DE2=3D80=3D98assu= me=3DE2=3D80=3D99?=3D =3D?UTF-8?q?=3D20better?=3D MIME-Version: 1.0 Content-Type: text/plain; charset=3DUTF-8 Content-Transfer-Encoding: 8bit * lib/verify.h (assume): Say it=E2=80=99s for static analysis, not dynami= c. --- ChangeLog | 5 +++++ lib/verify.h | 20 ++++++++++++++++---- 2 files changed, 21 insertions(+), 4 deletions(-) diff --git a/ChangeLog b/ChangeLog index a4473284c..44450a354 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,8 @@ +2020-05-23 Paul Eggert + + verify: document =E2=80=98assume=E2=80=99 better + * lib/verify.h (assume): Say it=E2=80=99s for static analysis, not dyna= mic. + 2020-05-22 Asher Gordon =20 gendocs: Clarify licenses for templates. diff --git a/lib/verify.h b/lib/verify.h index d9ab89a57..f10976127 100644 --- a/lib/verify.h +++ b/lib/verify.h @@ -277,10 +277,22 @@ template #endif =20 /* Assume that R always holds. Behavior is undefined if R is false, - fails to evaluate, or has side effects. Although assuming R can - help a compiler generate better code or diagnostics, performance - can suffer if R uses hard-to-optimize features such as function - calls not inlined by the compiler. */ + fails to evaluate, or has side effects. + + 'assume (R)' is a directive from the programmer telling the + compiler that R is true so the compiler needn't generate code to + test R. This is why 'assume' is in verify.h: it's related to + static checking (in this case, static checking done by the + programmer), not dynamic checking. + + 'assume (R)' can affect compilation of all the code, not just code + that happens to be executed after the assume (R) is "executed". + For example, if the code mistakenly does 'assert (R); assume (R);' + the compiler is entitled to optimize away the 'assert (R)'. + + Although assuming R can help a compiler generate better code or + diagnostics, performance can suffer if R uses hard-to-optimize + features such as function calls not inlined by the compiler. */ =20 #if _GL_HAS_BUILTIN_UNREACHABLE # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ()) --=20 2.17.1 --------------CACDAA585AAFA57C98CCEE49--