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Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics

From: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
To: Nicolas Pitre <nico at fluxnic dot net>
Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Umesh Kalappa <umesh dot kalappa0 at gmail dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, nd <nd at arm dot com>, Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>
Date: Fri, 27 Jul 2018 16:04:56 +0000
Subject: Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
References: <CAGfacvSoQyq-9PGFmEntmH5QHSMc2HpYsoJTSf1eAMPgZZK_EQ@mail.gmail.com>,<nycvar.YSQ.7.76.1807261213390.2327@knanqh.ubzr> <DB5PR08MB1030498B09A69B37DC8D43EC832A0@DB5PR08MB1030.eurprd08.prod.outlook.com>,<nycvar.YSQ.7.76.1807271132190.13885@knanqh.ubzr>

Nicolas Pitre wrote:

>> However if r4 is non-zero, the carry will be set, and the tsths will be executed. This
>> clears the carry and sets the Z flag based on bit 20.
>
> No, not at all. The carry is not affected. And that's the point of the 
> tst instruction here rather than a cmp: it sets the N and Z flags but 
> leaves C alone as there is no shifter involved.

No, the carry is always set by logical operations with a shifted immediate. It is only
unchanged if the immediate uses a zero rotate. So any shifted immediate > 255
will set the carry. This is detailed in the Arm Architecture Reference Manual, eg.
see the pseudo code for A32ExpandImm_C in LSL (immediate).

Wilco

Follow-Ups:
- Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
  - From: Nicolas Pitre

References:
- Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
  - From: Umesh Kalappa
- Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
  - From: Nicolas Pitre
- Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
  - From: Wilco Dijkstra
- Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
  - From: Nicolas Pitre

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