Subject: Re: PROBLEMS WITH GCC 2.7.2/I386 ON APPLICATIONS
To: None <email@example.com>
From: Niklas Hallqvist <firstname.lastname@example.org>
Date: 04/11/1996 08:41:21
>>>>> "Brian" == Brian Buhrow <email@example.com> writes:
Brian> There has been quite a bit of talk about the
Brian> optimization code in gcc 2.7.2 and its guilt in a number of
Brian> problems compiling and using the kernel.
"A number of problems"? This statement startles me. Actually I have
become unsure if we have *actually* seen the problem at all in the
kernel. I seem to recall that pcvt was reported to fail with -O2 and
without -fno-strength-reduce, but I have lost track of where I saw
that. I could have been imagining. However I try to watch closely
when GCC is mentioned and I can't remember any other problem. The bug
is well-known and has been in GCC for five years or so but is very
seldom triggered. It has to do with mixed-signedness arithmetic and
boundary conditions in strength reduction. It seems a 2.7.3 will come
out soon to make a stab at fixing this.
I'd be interested to hear of specific other problems.
Brian> I am having problems,
Brian> which appear to be gcc related, and am wondering if this gcc
Brian> 2.7.2 bug is more serious than we first thought. I have an
Brian> application which works fine if all optimization is turned off,
Brian> but which gets its function return values screwed up if -O or
Brian> -O and -fstrength-reduce are turned on.
This fact in itself doesn't necessarily point out that GCC is at
Brian> The problem seems to
Brian> be some code in the cleanup routine which stomps on %ax when a
Brian> function is returning. Not being a compiler expert, I'm not
Brian> sure what's going on here, but instruction-by-instruction
Brian> traces show that values are getting corrupted when functions
Brian> are returning. Things aren't consistent, but it appears as if
Brian> things go south when ever the stack crosses a page boundary. I
Brian> believe I'm also having trouble with this in the libc code as
Brian> well. Ordinary wel-tested functions like: getpwent are bombing
Brian> when compiled with -O and working just fine, with -o turned
Given this explanation, I'm very sceptic. To me it seems more like vm
is the culprit than GCC (based on the page boundary condition). I'd
be happy to look over a function (preferrably small) that you think
miscompiles. Esp. if you can accompany it with a trace that shows
where the problem occurs.
Brian> I'm running -current of about February 27, or so, just
Brian> before the bus changes went into the code. Oh, yes, this is on
Brian> the i386 port, though I don't know if this bug is machine
Brian> specific. Perhaps it would be useful either to change the
Brian> default compilation flags, or to look at backing up several
Brian> revs of gcc in the distribution.
Ehh, I think this is *much* to loose ground to do such a thing.
Niklas Hallqvist Phone: +46-(0)31-40 75 00 Home: +46-(0)31-41 93 95
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