From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lj1-f182.google.com (mail-lj1-f182.google.com [209.85.208.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 20BE833D2 for ; Wed, 8 Jun 2022 19:35:42 +0000 (UTC) Received: by mail-lj1-f182.google.com with SMTP id s13so23820073ljd.4 for ; Wed, 08 Jun 2022 12:35:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=w5G4Fbpw7okaDvJbNf0MUPyOzWFJdsVep+P+r/ikKsg=; b=e6MlxKPNbaQV4c4Oo53CyLESL6PSi5lSi46bOHBKBNvjX7e5Kphi0hmQokyPRJxAHs avoNesKdfLcgD4ut3r6If0laOOrw+zL2Oh9bpPwSVYsIeyOutzFB6elsI8iheEJghu2b aAfhHtH2KFgZ2MfHA+woAIYo8fondvI7mlhWwEXsFeK84ciyLYX76HYY53hsMOgYcuXC pwbkY+dv241fFgmbMkV2AD90dSMJQFyo696Uv7f7mwT4ejIUjP36UtTS9afYnDIIpefV CNt8MEu/zhuKsNW7At8b0azUSJw7hIayNdrenx8WkuzBAdozllkbnUbVXGvCoPH7DxGu uz7w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=w5G4Fbpw7okaDvJbNf0MUPyOzWFJdsVep+P+r/ikKsg=; b=ShzDQ+CHRN87TBIry8hsFfuYAMyLYXTX/dHgml1UJ1NKs9dx5c+n0tvGgWcFFYoYxC FGPymjwRjP6ERSEubQGyT+WZHzEK2xldqY+lGO3iAzMPFO7kIcDOLT1HqmpP+CcSe9iZ YL8bgjbbU6kXuEF+Y5gIa3V3De13AG9BcpyycUN5HZudbKi+dCD0I3sT+ANVt3fLLrl4 +eUOaMPlPIifaDkf4gFbEHSZz16WSlHBhwBoGwMwj0nnzIOM/yu61D/KgnsbTpoPwsB1 oWmSISlufcxDDndHTtCvIVSfcdGMoLCt6Cw1qDzEvwAoZIWuKjI8s6oMIzqC0pfUFT8V Pgjw== X-Gm-Message-State: AOAM5309GQCQoTGdujVWBX7OIWg739nlkVFHQKb8sZXHIbBdtoEa+kfW /ZN1evs5ZSEYt39poQUNz+LxcRJOagEPiWTmcveB8A== X-Google-Smtp-Source: ABdhPJwbJnHAdMV0IB/VPzx2hwY+/y+dUcwA0O9PFdif5ICszkdwEtpDKgAA2Pd14WKKHoo69GKiOtz7wz6Gs5eCUgI= X-Received: by 2002:a2e:8715:0:b0:255:a7e5:d50d with SMTP id m21-20020a2e8715000000b00255a7e5d50dmr6419645lji.26.1654716939750; Wed, 08 Jun 2022 12:35:39 -0700 (PDT) Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20220607222006.22719-1-jstitt007@gmail.com> <20220607152744.d7c801d092529309500ac9a6@linux-foundation.org> <20220607162128.b5d4aa70f4a8a7610ce29250@linux-foundation.org> In-Reply-To: From: Nick Desaulniers Date: Wed, 8 Jun 2022 12:35:27 -0700 Message-ID: Subject: Re: [PATCH] include/uapi/linux/swab.h: add __u16 cast to __swab16 conditional To: Al Viro Cc: Andrew Morton , Justin Stitt , Nathan Chancellor , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, Richard Smith Content-Type: text/plain; charset="UTF-8" On Tue, Jun 7, 2022 at 9:54 PM Al Viro wrote: > > On Tue, Jun 07, 2022 at 04:21:28PM -0700, Andrew Morton wrote: > > > > 6.5.15/5 > > > >> If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result. > > > 6.3.1.8/1 > > > >> Otherwise, the integer promotions are performed on both operands. > > > 6.3.1.1/2 > > > >> If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. > > > > Geeze. Can we please turn this into English and add it to the changelog? > > > > Is it saying that an expression > > > > int ? u16 : u16 > > > > has type int? Or something else? What did we do wrong here and is it > > possible to correct our types rather than adding a cast? > > Not quite. Same rules as u16 + u16 - on architectures where int is wider > than 16 bits it's (int)u16 + (int)u16 and yields int, on 16bit ones it's > (unsigned int)u16 + (unsigned int)u16 and yields unsigned int. > > You *can't* get smaller-than-int out of ? :, same as you can't get it > out of addition, etc. Exactly, and well put. More concise than I was able to express. I think that description will satisfy Andrew's request for additional context, so I'll recommend Justin add a blurb derived from what you said when sending a v3. > > __builtin_choose_expr() would do it, but I would take a cast over that > ugliness. > > FWIW, it might make sense for clang to keep track of the following > property: expression has the same value as it would if integer promotions > in it had been replaced with integer promotion of result. I'm not sure that's precisely the same issue here. The issue we're facing is more so that `ntohs` is being used in printf-like expressions; clang's -Wformat warns about default argument promotion so we need to clean up cases where smaller-than-int format flags are being used for promoted-to-int params. While looking at that, Nathan noticed that __swab16 will return either a __u16 or an int based on whether __HAVE_BUILTIN_BSWAP16__ is defined, which depends on BOTH the compiler being used and target architecture. This patch from Justin just cleans that up. > > Example: with > unsigned short x, y, mask; > > expresion "x & y" is interpreted as and_int((int)x, (int)y), which is equal > to (int)and_u16(x, y), so that expression has the property in question. > "x != 12 ? x : y" has the same property. "x + y", OTOH, doesn't - if x and y > are both 32768, x + y is add_int((int)x, (int)y), i.e. 65536, while > (int)add_u16(x, y) would be 0. > > For a somewhat more subtle example, > (x & ~mask) | (y & mask) > is interpreted as > or_int(and_int((int)x, not_int((int)mask)), and_int((int)y, (int)mask)) > which is equal to > (int)or_u16(and_u16(x,not_u16(mask)), and_u16(y, mask)) > IOW, the property in question holds for that one, despite having a subexpression > (~mask) that does *NOT* have that property. (int)not_u16(0) is 0xffff and > not_int((int)0) is (assuming 32bit int) 0xffffffff. Upper 16 bits get fouled; > applying & with known-16bit launders them off... > > That predicate is behind the handling of small bitwise types in sparse; > otherwise all operations on __be16 would trigger warnings due to promotions > from __be16 to int. And aforementioned subtle example is common enough, so we > had to deal with it. See commit d24967cb847b "[PATCH] handle fouled-bitwise" > in sparse git... https://git.kernel.org/pub/scm/devel/sparse/sparse.git/commit/?id=d24967cb847b7a04920698a9053ea8195046a831 (For others' reference) -- Thanks, ~Nick Desaulniers