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

From: Craig Topper
Subject: Re: [LLVMdev] clang .code16 with -Os producing larger code that it needs to
Date: Tue, 24 Feb 2015 00:42:13 -0800

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 <address@hidden> 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 :-)

FWIW I could reproduce the 'movl foo, %ebx' one but a relative jump
*was* using 16 bits (although gas uses 8):

 $ cat foo.S
        jae foo
        movl (foo), %ebx
 $ 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:
       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:
       0:       0f 83 08 00                                     jae     8
       4:       67 66 8b 1d 00 00 00 00                         movl    0, %ebx


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