[LLVMdev] clang .code16 with -Os producing larger code that it needs to

[Craig Topper]

Does gas really relax from 16-bit addresses to 32-bit address as necessary?
I played around briefly and it looks like gas will only emit 16-bit
addresses in 16-bit mode unless addr32 is prefixed. Even for an external
symbol it only emitted a 16-bit relocation type until I added addr32.

I wonder if we shouldn't fix the x86 encoder to use 16-bit addresses in
16-bit mode. (Actually I think we're emitting 0x67 prefix because the
displacement size check in Is16BitMemOperand doesn't like cases where
displacement isExpr instead of isImm). And maybe override the mode in
SubTargetInfo around the EmitInstruction call for any that specifies
"addr32" in 16-bit mode or "addr16" in 32-bit mode?

That doesn't help with jumps though since they do need their opcode
switched to JMP_2 instead of JMP_4. Again I can't prove that gas will
further relax 2-byte to 4-byte without addr32. I think we either need to
again change SubTargetInfo and pass it into relaxInstruction OR we could
create new 16-bit mode only 1-byte jumps that we can parse based on mode
and relax to the 2 byte form.

On Mon, Feb 23, 2015 at 4:07 AM, David Woodhouse <dwmw2 at infradead.org>
wrote:

> On Fri, 2015-02-20 at 13:47 -0500, Rafael Espíndola wrote:
> > > Your task, should you choose to accept it, is to make it cope with
> other
> > > forms of relaxation where necessary.
> >
> > And if not, please open a bug :-)
>
> http://llvm.org/bugs/show_bug.cgi?id=22662
>
> FWIW I could reproduce the 'movl foo, %ebx' one but a relative jump
> *was* using 16 bits (although gas uses 8):
>
>  $ cat foo.S
> .code16
>         jae foo
>         movl (foo), %ebx
> foo:
>  $ gcc -c -oa.out foo.S   ; llvm-objdump -d -triple=i686-pc-linux-code16
>
> a.out:  file format ELF64-x86-64
>
> Disassembly of section .text:
> .text:
>        0:       73 05                                           jae     5
>        2:       66 8b 1e 00 00                                  movl    0,
> %ebx
>  $ llvm-mc -filetype=obj foo.S | llvm-objdump -d
> -triple=i686-pc-linux-code16 -
>
> <stdin>:        file format ELF64-x86-64
>
> Disassembly of section .text:
> .text:
>        0:       0f 83 08 00                                     jae     8
>        4:       67 66 8b 1d 00 00 00 00                         movl    0,
> %ebx
>
>
> --
> dwmw2
>
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>


-- 
~Craig
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