From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id A53463858C55 for ; Thu, 13 Oct 2022 17:57:37 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org A53463858C55 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1665683857; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=2nT/0TKj9y6QmnIlukxFgAPgBPiOPOZ5K2utG9SgvVQ=; b=gGKE8rb2muL5S/AQ3+BnOrjJi3+pN1UFP3CTl+8tuMmsBTAgdKjlPCohZ0aXlnOdk7oWCa zs8NTf1GSdTNGizD6Ao6BIkXjt8V5FMh46oYicnpoS0kDvmDXd/eAk87mBAzurux+TE1no xeXl/ygBvN2CXeriB9hima6i3VOa0FI= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-408-j49joMQBMvC_4AY8s1kS-w-1; Thu, 13 Oct 2022 13:57:36 -0400 X-MC-Unique: j49joMQBMvC_4AY8s1kS-w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id CDC748027EB for ; Thu, 13 Oct 2022 17:57:35 +0000 (UTC) Received: from tucnak.zalov.cz (unknown [10.39.192.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 89DE3112C242; Thu, 13 Oct 2022 17:57:35 +0000 (UTC) Received: from tucnak.zalov.cz (localhost [127.0.0.1]) by tucnak.zalov.cz (8.17.1/8.17.1) with ESMTPS id 29DHvXTG655347 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Thu, 13 Oct 2022 19:57:33 +0200 Received: (from jakub@localhost) by tucnak.zalov.cz (8.17.1/8.17.1/Submit) id 29DHvW8l655346; Thu, 13 Oct 2022 19:57:32 +0200 Date: Thu, 13 Oct 2022 19:57:32 +0200 From: Jakub Jelinek To: Aldy Hernandez Cc: GCC patches , Andrew MacLeod Subject: Re: [PATCH] [PR24021] Implement PLUS_EXPR range-op entry for floats. Message-ID: Reply-To: Jakub Jelinek References: <20221013123649.474497-1-aldyh@redhat.com> MIME-Version: 1.0 In-Reply-To: <20221013123649.474497-1-aldyh@redhat.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-3.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_NONE,TXREP 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: On Thu, Oct 13, 2022 at 02:36:49PM +0200, Aldy Hernandez wrote: > +// Like real_arithmetic, but round the result to INF if the operation > +// produced inexact results. > +// > +// ?? There is still one problematic case, i387. With > +// -fexcess-precision=standard we perform most SF/DFmode arithmetic in > +// XFmode (long_double_type_node), so that case is OK. But without > +// -mfpmath=sse, all the SF/DFmode computations are in XFmode > +// precision (64-bit mantissa) and only occassionally rounded to > +// SF/DFmode (when storing into memory from the 387 stack). Maybe > +// this is ok as well though it is just occassionally more precise. ?? > + > +static void > +frange_arithmetic (enum tree_code code, tree type, > + REAL_VALUE_TYPE &result, > + const REAL_VALUE_TYPE &op1, > + const REAL_VALUE_TYPE &op2, > + const REAL_VALUE_TYPE &inf) > +{ > + REAL_VALUE_TYPE value; > + enum machine_mode mode = TYPE_MODE (type); > + bool mode_composite = MODE_COMPOSITE_P (mode); > + > + bool inexact = real_arithmetic (&value, code, &op1, &op2); > + real_convert (&result, mode, &value); > + > + // If real_convert above has rounded an inexact value to towards > + // inf, we can keep the result as is, otherwise we'll adjust by 1 ulp > + // later (real_nextafter). > + bool rounding = (flag_rounding_math > + && (real_isneg (&inf) > + ? real_less (&result, &value) > + : !real_less (&value, &result))); I thought the agreement during Cauldron was that we'd do this always, regardless of flag_rounding_math. Because excess precision (the fast one like on ia32 or -mfpmath=387 on x86_64), or -frounding-math, or FMA contraction can all increase precision and worst case it all behaves like -frounding-math for the ranges. So, perhaps use: if ((mode_composite || (real_isneg (&inf) ? real_less (&result, &value) : !real_less (&value, &result)) && (inexact || !real_identical (&result, &value)))) ? No need to do the real_isneg/real_less stuff for mode_composite, then we do it always for inexacts, but otherwise we check if the rounding performed by real.cc has been in the conservative direction (for upper bound to +inf, for lower bound to -inf), if yes, we don't need to do anything, if yes, we frange_nextafter. As discussed, for mode_composite, I think we want to do the extra stuff for inexact denormals and otherwise do the nextafter unconditionally, because our internal mode_composite representation isn't precise enough. > + // Be extra careful if there may be discrepancies between the > + // compile and runtime results. > + if ((rounding || mode_composite) > + && (inexact || !real_identical (&result, &value))) > + { > + if (mode_composite) > + { > + bool denormal = (result.sig[SIGSZ-1] & SIG_MSB) == 0; Use real_isdenormal here? Though, real_iszero needs the same thing. > + if (denormal) > + { > + REAL_VALUE_TYPE tmp; And explain here why is this, that IBM extended denormals have just DFmode precision. Though, now that I think about it, while this is correct for denormals, > + real_convert (&tmp, DFmode, &value); > + frange_nextafter (DFmode, tmp, inf); > + real_convert (&result, mode, &tmp); > + } there are also the cases where the higher double exponent is in the [__DBL_MIN_EXP__, __LDBL_MIN_EXP__] aka [-1021, -968] or so. https://en.wikipedia.org/wiki/Double-precision_floating-point_format If the upper double is denormal in the DFmode sense, so smaller absolute value than __DBL_MIN__, then doing nextafter in DFmode is the right thing to do, the lower double must be always +/- zero. Now, if the result is __DBL_MIN__, the upper double is already normalized but we can add __DBL_DENORM_MIN__ to it, which will make the number have 54-bit precision. If the result is __DBL_MIN__ * 2, we can again add __DBL_DENORM_MIN__ and make it 55-bit precision. Etc. until we reach __DBL_MIN__ * 2e53 where it acts like fully normalized 106-bit precision number. I must say I'm not really sure what real_nextafter is doing in those cases, I'm afraid it doesn't handle it correctly but the only other use of real_nextafter is guarded with: /* Don't handle composite modes, nor decimal, nor modes without inf or denorm at least for now. */ if (format->pnan < format->p || format->b == 10 || !format->has_inf || !format->has_denorm) return false; so it isn't that big deal except for ranges. Jakub