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Date: Tue, 29 Jul 2003 13:22:00 -0000
From: Richard Guenther <rguenth@tat.physik.uni-tuebingen.de>
To: Paolo Carlini <pcarlini@unitus.it>
cc: Steven Bosscher <s.bosscher@student.tudelft.nl>,
   Gabriel Dos Reis <gdr@integrable-solutions.net>, <gcc@gcc.gnu.org>
Subject: Re: std::pow implementation
In-Reply-To: <3F266DD6.30607@unitus.it>
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On Tue, 29 Jul 2003, Paolo Carlini wrote:

> Steven Bosscher wrote:
>
> >I would expect that the RTL inliner can deduce the number of iterations
> >and unroll the loop.  All other code is dead at that point and you'd end
> >up with x*x.
> >
> Cool!

Unfortunately not,

double foo2(double x)
{
  pow(x, 2);
}

gets to (with gcc3.4, __cmath_power declared inline, -O2 -funroll-loops
-ffast-math)

_Z4foo2d:
.LFB12:
        pushl   %ebp    #
.LCFI4:
        movl    %esp, %ebp      #,
.LCFI5:
        fldl    8(%ebp) # x
        fld1
        movl    $1, %eax        #, __n
        jmp     .L62    #
        .p2align 4,,7
.L73:
        fxch    %st(1)  #
.L62:
        fxch    %st(1)  #
        testb   $1, %al #, __n
        fmul    %st(0), %st     #,
        je      .L59    #,
        fmul    %st, %st(1)     #,
.L59:
        shrl    %eax    # __n
        jne     .L73    #,
        fstp    %st(0)  #
        popl    %ebp    #
        ret

yay!

Richard.

--
Richard Guenther <richard dot guenther at uni-tuebingen dot de>
WWW: http://www.tat.physik.uni-tuebingen.de/~rguenth/