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Re: Problem with GAS with X86 fdiv, fdivr, etc.
- From: Alan Modra <amodra at bigpond dot net dot au>
- To: jerry dot williamson at amd dot com
- Cc: gcc-bugs at gcc dot gnu dot org
- Date: Thu, 1 May 2003 08:31:32 +0930
- Subject: Re: Problem with GAS with X86 fdiv, fdivr, etc.
- References: <99F2150714F93F448942F9A9F112634C43D916@txexmtae.amd.com>
On Wed, Apr 30, 2003 at 04:24:42PM -0500, jerry.williamson@amd.com wrote:
> ##### fdiv %st, %st(3) # Assembler bug -- incorrectly generates fdivr
> .word 0xfbdc # need to manually encode the instruction
Extract from include/opcode/i386.h
/* The SystemV/386 SVR3.2 assembler, and probably all AT&T derived
ix86 Unix assemblers, generate floating point instructions with
reversed source and destination registers in certain cases.
Unfortunately, gcc and possibly many other programs use this
reversed syntax, so we're stuck with it.
eg. `fsub %st(3),%st' results in st = st - st(3) as expected, but
`fsub %st,%st(3)' results in st(3) = st - st(3), rather than
the expected st(3) = st(3) - st
This happens with all the non-commutative arithmetic floating point
operations with two register operands, where the source register is
%st, and destination register is %st(i). See FloatDR below.
The affected opcode map is dceX, dcfX, deeX, defX. */
#ifndef SYSV386_COMPAT
/* Set non-zero for broken, compatible instructions. Set to zero for
non-broken opcodes at your peril. gcc generates SystemV/386
compatible instructions. */
#define SYSV386_COMPAT 1
#endif
--
Alan Modra
IBM OzLabs - Linux Technology Centre