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Re: strict aliasing question
Joe Buck wrote:
On Fri, Nov 10, 2006 at 04:18:25PM -0800, Howard Chu wrote:
Richard Guenther wrote:
If you compile with -O3 -combine *.c -o alias it will break.
Hm, actually it still prints the correct result for me. What platform
are you using where it actually makes a difference?
Rather, he is saying that, with those flags, it is possible that gcc
will do optimizations that breaks the code, but these optimizations
might show up only on some platforms, with some releases, under
some conditions. You might luck out, but it is possible that a future
gcc will do an optimization that changes the meaning of the code.
OK, that's fair.
The compiler is allowed to reason roughly as follows: "I have a copy of
foo in register R1. foo is of type long. There have been no writes,
since foo was loaded into R1, for any types compatible with type long.
Therefore the copy of foo in R1 is still good, so I don't have to reload
it from memory." The C standard has rules of this form because, without
them, it can be hard to do decent loop optimization. The reason Richard
mentioned the options he did was that they enable some optimization across
function boundaries, meaning that the compiler is more likely to see that
there can't have been any legal modifications to some objects.
I understand that logic, in the general case. In this specific example,
none of those conditions apply. foo is an uninitialized local variable.
Therefore the compiler cannot know that it has a valid copy of it in any
register. In fact what it should know is that it has no valid copy of
it. And of course, there are no loops to consider here, so that type of
reload optimization isn't relevant. As such, the compiler has no choice
but to do the right thing, and load the value from memory, thus getting
the correct result. Which it does.
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc
OpenLDAP Core Team http://www.openldap.org/project/