[FFmpeg-devel] Why 'You can only build one library type at once on MinGW'?
Mon May 14 02:11:13 CEST 2007
On Sun, May 13, 2007 at 04:51:28PM -0700, Trent Piepho wrote:
> > > Do this:
> > > leaq 0x01020304(%rip), %rdx # link time constant, no textrel
> > > movl (%rdx), %eax
> > Forget about ELF "textrel" terminology. The rip-relative address is
> > not known until the relocation is resolved by the linker. It might not
> > be within 4gig of %rip, and then the linker will fail.
> That's pretty much the same thing as saying objects larger than 4GB aren't
> supported, isn't it?
Yes, and it is entirely practical. It's impossible for humans to make
a program larger than even a few hundred megs of code, much less 4
gigs. The human brain cannot deal with that much complexity, and
should not have to; something that complex should be broken into many
> You claimed that not supporting text relocations on x86-64 was an assembler
> bug. It's not an assembler bug, it's a limitation of the architecture.
Indeed, I'm sorry. It's a compiler bug to generate code that way, if
there's any intent that the code could be relocated beyond 4gig.
> If you want to support text relocations on x86-64, without limiting the total
> address range across all objects to 32-bits, you must use an alternate and
> much larger and slower sequence to access global data. Call this a "long
> access" if you want.
> Provided one accepts the limitation that a single object must fit in 4GB,
This is not necessary. The compiler deals with a whole object at once,
so it can choose to generate code with "long accesses" if it needs to
for a larger program. But the idea of an object >4gig is pure pedantry
since it does not exist and will never exist.
> there exists an alternative to the "long access", rip-relative addressing,
> that is smaller, faster, and, being position independent, avoids the text
> "long accesses" would allow relocations above 32-bits, but rip-relative
> addressing allows the same thing and is superior in every way. So, other
It does not allow the same thing. rip-relative addressing cannot
access addresses further away than ?2gig, and it's impossible to know
when the code is generated how far away the address will be, except
for module-local (static linkage) symbols.
> than to allow objects larger than 4GB, what possible reason is there for a
> compiler to support "long accesses"?
I agree, none. Since you've elaborated, it's clear: the bug is that
the dynamic linker loads libraries at inaccessible addresses. If it
loaded them under 4gig like it should then textrels would work just
> C89 doesn't say anything about inline asm, can that be used?
No, never as the only implementation of something. Only as an optional
optimization for use when compiling on a particular compiler/host
> C89 doesn't
> say anything about shared libraries or dynamic linking or memory models,
> so any optimization relating to those can't be used?
A good program cannot be dependent on them, no.
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