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Date: Fri, 16 Oct 2015 00:33:20 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: status of armhf asm with VFP instructions
it is not possible to write arm asm in a way that pleases all
assemblers, here is a summary of the problems relevant to musl:
some instructions can be written in several ways since UAL
(= unified assembler language) was introduced to unify arm
and thumb instruction mnemonics.
in case of armhf VFP instructions musl uses the following ones
in hand written asm:
hex ual old
eeb1XacX vsqrt.f32 fsqrts
eeb1XbcX vsqrt.f64 fsqrtd
eeb0XacX vabs.f64 fabss
eeb0XbcX vabs.f64 fabsd
eef1Xa10 vmrs fpscr mrc p10,7,..
eee1Xa10 vmsr fpscr mcr p10,7,..
the issues:
(1) binutils gas rejects UAL VFP instructions unless either
the -mfpu=<vfp variant> option is passed or the asm source
has '.fpu <vfp variant>' directive, the old names work though
(if the target is hard float). A gcc toolchain built
--with-float=hard but without --with-fpu=<vfp variant> does not
pass -mfpu to the assembler.
(2) Most UAL mnemonics were added in binutils 2.18, older
binutils only supports the old syntax. Some UAL mnemonics were
added later, vmrs and vmsr only appear in binutils 2.21
(released in 2010).
(3) The clang assembler does not support old mnemonics in general
but includes a few exceptions (e.g. fabs and fsqrt are supported,
but not the mrc or mcr coprocessor instructions).
Using UAL asm is the clean solution, but then to fix (1) we
should have .fpu directives in the asm files.
However the conflict between (2) and (3) means that fenv code
using vmrs and vmsr either drops support for old binutils gas
or the clang assembler.
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