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Re: [PATCH v3][AArch64] Fix symbol offset limit
- From: James Greenhalgh <james dot greenhalgh at arm dot com>
- To: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, nd <nd at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, <marcus dot shawcroft at arm dot com>
- Date: Wed, 14 Jun 2017 15:07:03 +0100
- Subject: Re: [PATCH v3][AArch64] Fix symbol offset limit
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On Tue, Jun 13, 2017 at 03:00:28PM +0100, Wilco Dijkstra wrote:
I've been avoiding reviewing this patch as Richard was the last to comment
on it, and I wasn't sure that his comments had been resolved to his
satisfaction. The conversation was back in August 2016 on v1 of the patch:
> Richard Earnshaw (lists) <Richard.Earnshaw@arm.com> wrote:
> > So isn't the real bug that we've permitted the user to create an object
> > that is too large for the data model?
> No that's a different issue I'm not trying to address here. The key is that as long
> as the start of the symbol is in range, we should be able to link. Due to optimization
> the offset may be huge even when the object is tiny, so the offset must be limited.
> > Consider, for example:
> char fixed_regs[0x200000000ULL];
> char fixed_regs2;
> return fixed_regs + fixed_regs2;
> > Neither offset is too large, but we still generate relocation errors
> > when trying to reference fixed_regs2.
> But so would creating a million objects of size 1. The linker could warn about
> large objects as well as giving better error messages for relocations that are
> out of range. But that's mostly QoI, what we have here is a case where legal
> code fails to link due to optimization. The original example is from GCC itself,
> the fixed_regs array is small but due to optimization we can end up with
> &fixed_regs + 0xffffffff.
Richard, do you have anything further to say on this patch? Or can we start
progressing the review again.