Re: Recent serial port lossage

Thread View: gnu.gcc.bug

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2 total messages Started by Charles Hannum Sun, 07 Jan 1996 00:00

#6284

Author: Charles Hannum
Date: Sun, 07 Jan 1996 00:00

33 lines
1347 bytes

I've tracked this down to what I believe to be a bug in GCC 2.7.2.
Basically, if you have code of the form:

volatile int foo;

...

	foo |= 0x80000000;

...

in many (all?) cases, `gcc -O3' now compiles it as:

	movl _foo,%e??
	orl $0x80000000,%e??
	movl %e??,_foo

rather than the obvious:

	orl $0x80000000,_foo

This probably has something to do with pipeline scheduling on the
Pentium, but it's not clear to me that the new behaviour, regardless
of its relative performance, is *correct* for a `volatile' variable.
In particular, I don't see a way to make sig_atomic_t work correctly.
In general, it makes `volatile' nearly useless.

At any rate, I've worked around the problem in this case, but I
wouldn't be surprised if it causes other random bugs.

If you've been having problems, please update your kernel sources
after tonight's update, and let me know if your problems persist.

Re: Recent serial port lossage

#6283

Author: Torbjorn Granlund
Date: Sun, 07 Jan 1996 00:00

14 lines
938 bytes

This is not a bug in GCC.

You need to refresh your understanding of what `volatile' means.  In
particular, it doesn't mean that statements that both read and write a
memory position are executed with the bus locked.

Also not that x86 processors will *not* perform statements like,

	orl $0x80000000,_foo

with the bus locked between the read and the write.  For that, you need to
prefix the `orl' with a `lock' instruction.

Torbjorn

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