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From: Denis Zaitsev <zzz@anda.ru>
To: gcc@gcc.gnu.org, linux-gcc@vger.kernel.org
Subject: Why -fPIC stops some optimization?
Date: Fri, 9 Jul 2004 21:02:30 +0600	[thread overview]
Message-ID: <20040709210230.C7162@natasha.ward.six> (raw)

I have met such a behaviour while compiling GLIBC for x86.  A
construct which suffers looks like:


#define __xyz(x,y,z) ({ \
    ...                 \
    size_t __n= (z);    \
    ...                 \
    switch (__n) {      \
        case ...        \
        ...             \
    }                   \
    ...                 \
})


This macro is intended for use with some constant z
(__builtin_constant_p(z) is true).  And when this macro is used such a
way, GCC should unroll (or what is the word for this?) the switch into
the only branch which is needed.  And GCC does this optimization, but
only not for the -fPIC case.  There GCC emits the whole body for the
switch operator and then does just a jump for the correct branch.
Why?  Or it is some expected misbehaviour for the -fPIC case?

GCC-3.3.2.

Thanks in advance.

next             reply	other threads:[~2004-07-09 15:02 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2004-07-09 15:02 Denis Zaitsev [this message]
2004-07-09 20:45 ` Why -fPIC stops some optimization? Michel Lespinasse
2004-07-09 21:58   ` Denis Zaitsev
2004-07-11 22:28     ` Michel Lespinasse

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