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Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Keith Thompson <Keith.S.Thompson+u@gmail.com> Newsgroups: comp.lang.c Subject: Re: how cast works? Date: Fri, 09 Aug 2024 13:26:39 -0700 Organization: None to speak of Lines: 179 Message-ID: <87ikw9788w.fsf@nosuchdomain.example.com> References: <v8vlo9$2oc1v$1@dont-email.me> <slrnvb7kis.28a.dan@djph.net> <v929ah$3u7l7$1@dont-email.me> <87ttfu94yv.fsf@nosuchdomain.example.com> <v93a3t$6q7v$1@dont-email.me> <v93e2q$8put$1@dont-email.me> <v94smd$mgp8$1@dont-email.me> <v95j4r$qh1q$3@dont-email.me> <v95okr$2oa92$1@dont-email.me> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Date: Fri, 09 Aug 2024 22:26:40 +0200 (CEST) Injection-Info: dont-email.me; posting-host="153d1bcdb0929153f62e0b2f2aa4005b"; logging-data="2596224"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19FymYsOC1ySrvZGHVRBg+s" User-Agent: Gnus/5.13 (Gnus v5.13) Cancel-Lock: sha1:/MLYjIRzPPIa6fM0MwVaBmtCCeU= sha1:HWNGJ1ibAO0bN3DJLDfUv0qGUJQ= Bytes: 8616 Thiago Adams <thiago.adams@gmail.com> writes: > Em 8/9/2024 2:20 PM, David Brown escreveu: >> On 09/08/2024 12:57, Thiago Adams wrote: >>> Em 8/8/2024 6:41 PM, Bart escreveu: >>>> On 08/08/2024 21:34, Thiago Adams wrote: >>>>> On 08/08/2024 16:42, Keith Thompson wrote: >>>>>> Thiago Adams <thiago.adams@gmail.com> writes: >>>>>>> On 07/08/2024 17:00, Dan Purgert wrote: >>>>>>>> On 2024-08-07, Thiago Adams wrote: >>>>>> [...] >>>>>>>>> How about floating point? >>>>>>>> Floating point is a huge mess, and has a few variations for >>>>>>>> encoding; >>>>>>>> though I think most C implementations use the one from the >>>>>>>> IEEE on 1985 >>>>>>>> (uh, IEEE754, I think?) >>>>>>> >>>>>>> I didn't specify properly , but my question was more about floating >>>>>>> point registers. I think in this case they have specialized >>>>>>> registers. >>>>>> >>>>>> Who is "they"? >>>>>> >>>>>> Some CPUs have floating-point registers, some don't. C says nothing >>>>>> about registers. >>>>>> >>>>>> What exactly is your question? Is it not already answered by reading >>>>>> the "Conversions" section of the C standard? >>>>>> >>>>> >>>>> >>>>> This part is related with the previous question about the origins >>>>> of integer promotions. >>>>> >>>>> We don't have "char" register or signed/unsigned register. But I >>>>> believe we may have double and float registers. So float does not >>>>> need to be converted to double. >>>>> >>>>> There is no specif question here, just trying to understand the >>>>> rationally behind the conversions rules. >>>> >>>> The rules have little to do with concrete machines with registers. >>>> >>>> Your initial post showed come confusion about how conversions >>>> work. They are not performed 'in-place', any more than writing `a >>>> + 1` changes the value of `a`. >>>> >>>> Take: >>>> >>>> int a; double x; >>>> >>>> x = (double)a; >>>> >>>> The cast is implicit here but I've written it out to make it >>>> clear. My C compiler produces intermediate code like this before >>>> converting it to native code: >>>> >>>> push x r64 # r64 means float64 >>>> fix r64 -> i32 >>>> pop a i32 >>>> >>>> I could choose to interprete this code just as it is. Then, in >>>> this execution model, there are no registers at all, only a stack >>>> that can hold data of any type. >>>> >>>> The 'fix' instruction pops the double value from the stack, >>>> converts it to int (which involves changing both the bit-pattern, >>>> and the bit-width), and pushes it back onto the stack. >>>> >>>> Registers come into it when running it directly on a real >>>> machine. But you seem more concerned with safety and correctness >>>> than performance, so there's probably no real need to look at >>>> actual generated native code. >>>> >>>> That'll just be confusing (especially if you follow the advice to >>>> generate only optimised code). >>>> >>>> >>>> >>>> >>> This part was always clear to me: >>> >>> "They are not performed 'in-place', any more than writing `a + 1` >>> changes the value of `a`." >>> >>> Lets take double to int. >>> >>> In this case the bits of double needs to be reinterpreted (copied >>> to) int. >>> >>> So the answer "how it works" can be >>> >>> always/generally machine has a instruction to do this >>> >>> or.. this is defined by the IIE ... standard as ... >>> >>> >> It would be helpful if you made more of an effort to write clearly >> here. (We know you can do so when you want to.) It is very >> difficult to follow what you are referring to here - what is "this >> case" here? A conversion from a double to an int certainly does not >> re-interpret or copy bits - like other conversions, it copies the >> /value/ to the best possible extent given the limitations of the >> types. > > Everything is a bit mixed up, but I'll try to explain the part about > registers that I have in mind. Why you keep talking about registers? The C standard does not talk about registers, and the promotion rules are based on which *operations* are available, not what kinds of registers a given CPU happens to have. > In C, when you have an expression like char + char, each char is > promoted to int. The computation then occurs as int + int. Right. And the most important thing to keep in mind is that it works that way *because the C standard says so*. If you're looking for a rationale for this, it's probably because some early C compilers were for target systems that didn't provide operations on narrow types. (The PDP-11 does have instructions that operate on 8-bit bytes, but the arithmetic instructions operate only on 16-bit words.) But the C standard rules apply to all conforming C implementations. If a target CPU happens to provide a 1-byte add instruction, a C compiler can't use it unless it can prove that the result is consistent with C semantics. Knowing why the C standard says what it says can be interesting, but it's not necessarily directly useful. > On the other hand, when you have float + float, it remains as float + float. That's implementation-defined. Read the "Characteristics of floating types <float.h>" section of the C standard (it's 5.2.5.3.3 in the N3220 draft) and look for FLT_EVAL_METHOD. If FLT_EVAL_METHOD (defined in <float.h>) is 0, float operations are done in type float. If it's 1, float operations are done in type double. If it's 2, all floating-point operations are done in type long double. > My guess for this design is that computations involving char are done > using registers that are the size of an int. Who says the target CPU even has registers, or that computations can't be done directly on values in memory? > But, float + float is not promoted to double, so I assume that the > computer has specific float registers or similar operation > instructions for float. Again, some CPUs have floating-point registers and some do not. Those that don't might store floating-point values in the same registers used for integer values, applying different instructions to operate on them. Of those that do have floating-point registers, some have registers that can hold a double value (typically 64 bits); they may or may not be able to treat a half-register as a 32-bot float value. The rules in the C standard are, for the most part, based on the capabilities of CPUs that existed when the standard was written, not necessarily on modern CPUs (though there have been tweaks in later editions). > Regarding the part about signed/unsigned registers and operations, I > must admit that I'm not sure. I was planning to check on Compiler > Explorer, but I haven't done that yet. > > I can frame the question like this: Does the computer make a > distinction when adding signed versus unsigned integers? Are there > specific assembly instructions for signed versus unsigned operations, > covering all possible combinations? Maybe. What is your goal here? Are you trying to understand the history behind the rules given in the C standard, or trying to understand the rules themselves, or both? ========== REMAINDER OF ARTICLE TRUNCATED ==========