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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: net.micro.68k
Subject: Re: asm construct in PCC's: workaround
Message-ID: <716@hoptoad.uucp>
Date: Mon, 21-Apr-86 12:20:16 EST
Article-I.D.: hoptoad.716
Posted: Mon Apr 21 12:20:16 1986
Date-Received: Wed, 23-Apr-86 22:06:05 EST
References: <293@Shasta.ARPA> <581@mtxinu.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 37

In article <581@mtxinu.UUCP>, ed@mtxinu.UUCP (Ed Gould) writes:
>                             ...there is a bug in the 4.2BSD compiler,
> and possibly other PCC derivatives as well.  I don't remember the
> nature of the bug in any detail, but the workaround is to place
> a null statement (extra semicolon) before the asm():
> 
> 	;asm("whatever");

The bug occurs when the asm() is preceded by an if-statement, e.g.:

	if (flag)
		do_something();
	asm("now do this");

PCC injects the asm() into the output before it recognizes that the if
statement is done, so the asm() is effectively placed inside the if
statement.  Having it scan a null statement before the asm() works.

Now for a bit of soapboxing:  I had to maintain a large piece of code
containing asm()'s (the Sun boot proms, which developed out of code
written at Stanford by Luis Trabb-Pardo and others).  Eventually I gave
up; there were too many dependencies on the compiler: where it
allocated variables, what the peephole optimizer would accept and
understand, etc.  Every time Richard Tuck improved the Sun C compiler,
my code would break in obscure ways.  I always ended up compiling it
with both compilers and diff-ing the .s files and reading the diffs.
For each compiler release over 3-1/2 years.  While this was a great
check on the generated code and helped Richard avoid bugs, it took a
lot of time.

I finally rewrote all the asm() parts so the assembler code
was in separate foo.s files, my life became easier, my code became
more maintainable, I found a good lover, my ship came in, and my
soapboxing is over.
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   jgilmore@lll-crg.arpa
			     Post no bills.