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Date: Sun, 02 Oct 2011 00:59:00 -0000
Message-ID: <CAFkk2KTGL53aWgNiNQm8+Yt_6SK2nXjPDonpH4fT9Mp_PWoJTw@mail.gmail.com>
Subject: Suboptimal __restrict optimization?
From: Ulf Magnusson <ulfalizer@gmail.com>
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Hi,

Given the code

class C { void f(int *p); int q; };

void C::f(int * __restrict p) __restrict {
    q += 10;
    *p = 7;
    q += 10;
}

g++ 4.5.2 with -O3 generates the following for C::f() (prologue and
epilogue omitted):

mov    0x8(%ebp),%eax // eax = this (= &q)
mov    0xc(%ebp),%ecx // ecx = p
mov    (%eax),%edx    // edx = q
movl   $0x7,(%ecx)    // *p = 7
add    $0x14,%edx     // q += 20
mov    %edx,(%eax)    // save q

If C::f() is rearranged as

void C::f(int * __restrict p) __restrict {
    *p = 7;
    q += 10;
    q += 10;
}

the following is generated instead:

mov    0x8(%ebp),%eax // eax = this (= &q)
mov    0xc(%ebp),%edx // edx = p
movl   $0x7,(%edx)    // *p = 7
addl   $0x14,(%eax)   // q += 20

Is there some reason why GCC couldn't generate this code for the first
version of C::f()? Is this a failure of optimization, or am I missing
something in how __restricted works?

/Ulf