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Re: clang .code16 with -Os producing larger code that it needs to

From: David Woodhouse
Subject: Re: clang .code16 with -Os producing larger code that it needs to
Date: Fri, 20 Feb 2015 16:18:24 +0000

On Fri, 2015-02-20 at 16:05 +0000, David Woodhouse wrote:
> It's been a while since I looked at this... but I think for the short
> jumps we just emit the 8-bit version and there's a fixup which can go
> back and re-emit the instruction in 32-bit mode if it finds it doesn't
> fit?
> Do we just need to support a similar fixup for promoting 16-bit to
> 32-bit relocations?

OK, the term I was looking for was 'relaxation'. Look in
lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp for
X86AsmBackend::relaxInstruction() and related methods.

Observe that it will cope with 'relaxing' 8-bit PC-relative relocations
to 32-bit PC-relative, but it doesn't cope with anything else.

Your task, should you choose to accept it, is to make it cope with other
forms of relaxation where necessary.

Note that the existing cases end up emitting a new instruction with a
*new* opcode. In your case it won't be doing that. It's the *same*
opcode, but you'll have to set a flag to tell the emitter to use the
32-bit addressing mode (for data and/or addr as appropriate) this time.

And while you're doing that, you should note that that's the *same* flag
that'll be needed to support explicit addr32/data32 prefixes in the asm
source. So you might as well support those too. I might suggest doing
them *first*, in fact.

David Woodhouse                            Open Source Technology Centre
address@hidden                              Intel Corporation

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