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Date: Mon, 29 Jun 2009 08:39:31 +0200
Message-ID: <761ea48b0906282339p7276c86du80fb8d6422aaa79e@mail.gmail.com>
Subject: Re: OT: TCG SSA, speed, misc (was Re: [Qemu-devel] Re: [PATCH 08/11]
	QMP: Port balloon command)
From: Laurent Desnogues <laurent.desnogues@gmail.com>
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To: Filip Navara <filip.navara@gmail.com>
Cc: qemu-devel@nongnu.org

On Mon, Jun 29, 2009 at 1:35 AM, Filip Navara<filip.navara@gmail.com> wrote=
:
> On Mon, Jun 29, 2009 at 1:19 AM, Filip Navara<filip.navara@gmail.com> wro=
te:
>> On x86 host the register allocation still looks very pathetic, I will po=
st a follow-up
>> soon.
>
> Let's look at the very first two instructions generated by the guest:
>
> ----------------
> IN:
> 0x00200070: =A0ldr =A0 =A0 =A0 =A0r0, [pc, #108] =A0; 0x2000e4
> 0x00200074: =A0ldr =A0 =A0 =A0 =A0pc, [pc, #108] =A0; 0x2000e8
>
> OP:
> =A0movi_i32 tmp8,$0x200078
> =A0movi_i32 tmp9,$0x6c
> =A0add_i32 tmp8,tmp8,tmp9
> =A0qemu_ld32u tmp9,tmp8,$0x0
> =A0mov_i32 r0,tmp9
> =A0movi_i32 tmp9,$0x20007c
> =A0movi_i32 tmp10,$0x6c
> =A0add_i32 tmp9,tmp9,tmp10
> =A0qemu_ld32u tmp8,tmp9,$0x0
> =A0movi_i32 tmp10,$0xfffffffe
> =A0and_i32 tmp8,tmp8,tmp10
> =A0mov_i32 pc,tmp8
> =A0exit_tb $0x0
>
> OUT: [size=3D128]
> 0x03230020: =A0mov =A0 =A0$0x200078,%eax
> 0x03230025: =A0add =A0 =A0$0x6c,%eax
> 0x03230028: =A0mov =A0 =A0%eax,%ecx
> 0x0323002a: =A0mov =A0 =A0%ecx,%edx
> 0x0323002c: =A0mov =A0 =A0%ecx,%eax
>
> -- this instruction sets %eax to value that it already has
>
> 0x0323002e: =A0shr =A0 =A0$0x6,%edx
> 0x03230031: =A0and =A0 =A0$0xfffffc03,%eax
> 0x03230037: =A0and =A0 =A0$0xff0,%edx
> 0x0323003d: =A0lea =A0 =A00x540(%edx,%ebp,1),%edx
> 0x03230044: =A0cmp =A0 =A0(%edx),%eax
> 0x03230046: =A0mov =A0 =A0%ecx,%eax
> 0x03230048: =A0je =A0 =A0 0x3230053
> 0x0323004a: =A0xor =A0 =A0%edx,%edx
> 0x0323004c: =A0call =A0 0x55cbc0
> 0x03230051: =A0jmp =A0 =A00x3230058
> 0x03230053: =A0add =A0 =A00xc(%edx),%eax
> 0x03230056: =A0mov =A0 =A0(%eax),%eax
> 0x03230058: =A0mov =A0 =A0$0x20007c,%edx
> 0x0323005d: =A0add =A0 =A0$0x6c,%edx
> 0x03230060: =A0mov =A0 =A0%edx,%ecx
> 0x03230062: =A0mov =A0 =A0%eax,0x0(%ebp)
> 0x03230065: =A0mov =A0 =A0%ecx,%edx
>
> -- same here
>
> 0x03230067: =A0mov =A0 =A0%ecx,%eax
> 0x03230069: =A0shr =A0 =A0$0x6,%edx
> 0x0323006c: =A0and =A0 =A0$0xfffffc03,%eax
> 0x03230072: =A0and =A0 =A0$0xff0,%edx
> 0x03230078: =A0lea =A0 =A00x540(%edx,%ebp,1),%edx
> 0x0323007f: =A0cmp =A0 =A0(%edx),%eax
> 0x03230081: =A0mov =A0 =A0%ecx,%eax
> 0x03230083: =A0je =A0 =A0 0x323008e
> 0x03230085: =A0xor =A0 =A0%edx,%edx
> 0x03230087: =A0call =A0 0x55cbc0
> 0x0323008c: =A0jmp =A0 =A00x3230093
> 0x0323008e: =A0add =A0 =A00xc(%edx),%eax
> 0x03230091: =A0mov =A0 =A0(%eax),%eax
> 0x03230093: =A0and =A0 =A0$0xfffffffe,%eax
> 0x03230096: =A0mov =A0 =A0%eax,0x3c(%ebp)
> 0x03230099: =A0xor =A0 =A0%eax,%eax
> 0x0323009b: =A0jmp =A0 =A00x7ec928
>
> If someone can explain me why the redundant mov instructions are
> generated I'd be very happy. Thanks.

What you see here is due to hard-coded assembly instructions
used to make a load.  cf tcg_out_qemu_ld in tcg-target.c


Laurent