[Bug 264590] assembler generates wrong opcodes of instructions fdiv fdivp fdivr fdivrp fsub fsubp fsubr fsubrp

From: <bugzilla-noreply_at_freebsd.org>
Date: Fri, 10 Jun 2022 14:20:13 UTC

Mark Millard <marklmi26-fbsd@yahoo.com> changed:

           What    |Removed                     |Added
                 CC|                            |marklmi26-fbsd@yahoo.com

--- Comment #7 from Mark Millard <marklmi26-fbsd@yahoo.com> ---
Very old issue. See, for example:


Part of that has some history on the issue:

Andrew Pinski 2006-12-07 22:40:52 UTC
This is at most a GNU binutils bug.  Please file it with them at
http://sourceware.org/bugzilla/ .

Also IIRC fdivp's arguments are swapped in AT&T asm mode because of some
historical accident.
See the comment in i386.c:
          /* The SystemV/386 SVR3.2 assembler, and probably all AT&T
             derived assemblers, confusingly reverse the direction of
             the operation for fsub{r} and fdiv{r} when the
             destination register is not st(0).  The Intel assembler
             doesn't have this brain damage.  Read !SYSV386_COMPAT to
             figure out what the hardware really does.  */

#ifndef SYSV386_COMPAT
/* Set to 1 for compatibility with brain-damaged assemblers.  No-one
   wants to fix the assemblers because that causes incompatibility
   with gcc.  No-one wants to fix gcc because that causes
   incompatibility with assemblers...  You can use the option of
   -DSYSV386_COMPAT=0 if you recompile both gcc and gas this way.  */
#define SYSV386_COMPAT 1

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