[llvm-dev] retpoline mitigation and 6.0

[David Woodhouse via llvm-dev]

On Mon, 2018-02-19 at 16:18 +0100, Hans Wennborg wrote:
> Reid, is this what you fixed with r325049 that's also merged to 6.0?

That's working for me, yes.

> If so, do we have everything that's needed, or is there anything else
> I should wait for?

I wouldn't mind an opinion on 
https://bugs.llvm.org/show_bug.cgi?id=33587

Clang for x86_32 will happily compile this:

	long long ret;
	asm ("movl $5, %0" : "=r" (ret)); // 32-bit reg for 64-bit val is fine.

... but not this:

	long long ret;
	asm ("movb $5, %0" : "=q" (ret)); // 8-bit reg for 64-bit val not fine.

In practice, those cases go away in optimisation but the problem for us
in building Linux is that the 8-bit case causes a build failure
*before* the optimiser realises it'd never have to emit it anyway.

I can hack that up so that we're pretending to have 8-bit variables:
https://lkml.org/lkml/2018/2/9/529

But before I go further with that rather icky approach I'd like to know
why it doesn't break for the "r" case, and if it's going to *keep*
working. And if we can make the "q" case behave the same...
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