From: "A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Date: Tue, 15 Aug 1995 13:23:39 BST
Subject: Re: inline asm?

  I quoted this:-
long _ax,_bx,_cx,_dx,_si,_di,_bp,_es; short _flags;
#define __SR() /* save the registers */         ({asm("xchgl %eax,__ax"); \
    asm("xchgl %ebx,__bx"); asm("xchgl %ecx,__cx"); asm("xchgl %edx,__dx"); \
    asm("xchgl %esi,__si"); asm("xchgl %edi,__di"); asm("xchgl %ebp,__bp"); })
#define __RR() /* restore the registers */      ({ \
    asm("pushf"); asm("popw __flags"); __SR();})

  mat AT ardi DOT com (Mat Hostetter) criticized it thus:-

> You must tell gcc about input and output values, and which registers get
> clobbered. ... This code hides from gcc the fact that __ax, etc. are really
> input and output parameters. gcc is free to assume that those variables do
> not get modified by this asm, and optimize accordingly.

  I use these two macros as a pair thus, e.g.:-
    _ax=0x1234; etc; __SR(); asm("this and that"); __RR(); use_value_of(_ax);
  The original registers are saved by __SR() and restored by __RR(), and I do
explicitly alter _ax in C text.

> This code contains a sequence of contiguous asm statements, which is a bad
> idea if you really want the asms to be contiguous ...

  Thankyou. Point taken.

> This code uses the hardcoded names "__ax", etc. for asm symbols, which
> aren't guaranteed to be correct; for example, on a Linux ELF ...

  This code is intended for djgpp on PC only.

> xchgl is really slow, at least on the Pentium.

  How many times slower than a `movl'?