Path: utzoo!telly!ddsw1!lll-winken!killer!mit-eddie!bloom-beacon!tut.cis.ohio-state.edu!PARIS.ICS.UCI.EDU!schmidt%crimee.ics.uci.edu
From: schmidt%crimee.ics.uci.edu@PARIS.ICS.UCI.EDU ("Douglas C. Schmidt")
Newsgroups: gnu.gcc.bug
Subject: asm fails with -O
Message-ID: <8901152345.aa12082@PARIS.ICS.UCI.EDU>
Date: 16 Jan 89 07:30:33 GMT
Sender: daemon@tut.cis.ohio-state.edu
Distribution: gnu
Organization: GNUs Not Usenet
Lines: 85


Hi,

   The following problem is either a gcc 1.32 bug on the sun3,
or an error in my understanding of inline asm (probably the latter!).

   I'm attempting to use an asm statement to exchange 2 values.
Here's my approach:
----------------------------------------   
main () {
   int i = 10;
   int j = 11;
   printf ( "i = %d, j = %d\n", i, j); 
   /* exchange the values of i and j */
   asm ("exg %0,%2" : "=d" (i) : "0" (i), "=d" (j), "2" (j));
   printf ( "i = %d, j = %d\n", i, j); 
}
----------------------------------------

This works fine when -O is not used.  However, when -O *is*
used, it fails.  Examining the assembly code shows why:

----------------------------------------
#NO_APP
gcc_compiled.:
.text
LC0:
	.ascii "i = %d, j = %d\12\0"
	.even
.globl _main
_main:
	link a6,#0
	moveml #0x3020,sp@-
	moveq #10,d2
	moveq #11,d3
	movel d3,sp@-
	movel d2,sp@-
	pea LC0
	lea _printf,a2
	jbsr a2@
	movel d3,sp@-  # error, this is pushed to soon onto stack before swap.
#APP
	exg d2,d3
#NO_APP
#	movel d3,sp@- however, if moved down here the program works.
	movel d2,sp@-
	pea LC0
	jbsr a2@
	moveml a6@(-12),#0x40c
	unlk a6
	rts
----------------------------------------

Now, I'm not sure if this is a bug, or if I've failed to specify the
constraints properly.  The underlying question is, ``how does one
correctly indicate that both operands to an asm statement are
`read-write'?''

thanks,

Doug
----------------------------------------

p.s. I think there's a typo in gcc.texinfo.  The following patch
     corrects it.

*** gnu/gcc-1.32/gcc.texinfo.~1~	Sat Jan 14 11:12:11 1989
--- gnu/gcc-1.32/gcc.texinfo	Sun Jan 15 23:22:47 1989
***************
*** 2205,2211 ****
  Here @code{angle} is the C expression for the input operand while
  @code{result} is that of the output operand.  Each has @samp{"f"} as its
  operand constraint, saying that a floating-point register is required.  The
! @samp{=} in @samp{=r} indicates that the operand is an output; all output
  operands' constraints must use @samp{=}.  The constraints use the same
  language used in the machine description (@pxref{Constraints}).
  
--- 2205,2211 ----
  Here @code{angle} is the C expression for the input operand while
  @code{result} is that of the output operand.  Each has @samp{"f"} as its
  operand constraint, saying that a floating-point register is required.  The
! @samp{=} in @samp{=f} indicates that the operand is an output; all output
  operands' constraints must use @samp{=}.  The constraints use the same
  language used in the machine description (@pxref{Constraints}).