Date: Sat, 21 Feb 2004 12:23:43 -0800 From: Peter Wemm <peter@wemm.org> To: freebsd-amd64@freebsd.org Subject: Re: CFLAGS+= -fPIC per default? Message-ID: <200402211223.43589.peter@wemm.org> In-Reply-To: <BD6DAE5F-64A6-11D8-ACAA-000A95BAD088@raisdorf.net> References: <BD6DAE5F-64A6-11D8-ACAA-000A95BAD088@raisdorf.net>
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On Saturday 21 February 2004 11:47 am, Hendrik Scholz wrote:
> Adding CFLAGS= -fPIC to /etc/make.conf may be a local solution but
> are there any drawbacks by adding something like
> .if ${ARCH} == "amd64"
> CFLAGS+= -fPIC
> .endif
>
> to ports/Mk/bsd.port.mk?
No.. please don't. Although the AMD64 platform supports PIC addressing
modes directly, it is still a penalty. (Although thankfully, its
nowhere near as expensive as it is on i386!)
For example, in libc when built in PIC mode:
#ifdef PIC
movq PIC_GOT(HIDENAME(curbrk)),%rdx
movq (%rdx),%rax
#else
movq HIDENAME(curbrk)(%rip),%rax
#endif
The problem is that we can't be sure that everything will be in +/- 31
bit offsets of each other. This means that PIC objects have to do
indirect memory references that aren't required in no-pic mode.
I386 also loses a general purpose register (%ebx) which is why -fpic is
more expensive there. But even though we don't lose a register, its
still a cost because of the extra global-offset-table memory
references.
Footnote: you just made me wonder about some of these ifdefs.. We
shouldn't need them for intra-object references like this. I'll have
to go and look again.
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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