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Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory bar
From: |
Pranith Kumar |
Subject: |
Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier |
Date: |
Fri, 3 Jun 2016 14:27:01 -0400 |
On Thu, Jun 2, 2016 at 5:18 PM, Richard Henderson <address@hidden> wrote:
>
> Hum. That does seem helpful-ish. But I'm not certain how helpful it is to
> complicate the helper functions even further.
>
> What if we have tcg_canonicalize_memop (or some such) split off the barriers
> into separate opcodes. E.g.
>
> MO_BAR_LD_B = 32 // prevent earlier loads from crossing current op
> MO_BAR_ST_B = 64 // prevent earlier stores from crossing current op
> MO_BAR_LD_A = 128 // prevent later loads from crossing current op
> MO_BAR_ST_A = 256 // prevent later stores from crossing current op
> MO_BAR_LDST_B = MO_BAR_LD_B | MO_BAR_ST_B
> MO_BAR_LDST_A = MO_BAR_LD_A | MO_BAR_ST_A
> MO_BAR_MASK = MO_BAR_LDST_B | MO_BAR_LDST_A
>
> // Match Sparc MEMBAR as the most flexible host.
> TCG_BAR_LD_LD = 1 // #LoadLoad barrier
> TCG_BAR_ST_LD = 2 // #StoreLoad barrier
> TCG_BAR_LD_ST = 4 // #LoadStore barrier
> TCG_BAR_ST_ST = 8 // #StoreStore barrier
> TCG_BAR_SYNC = 64 // SEQ_CST barrier
I really like this format. I would also like to add to the frontend:
MO_BAR_ACQUIRE
MO_BAR_RELEASE
and the following to the backend:
TCG_BAR_ACQUIRE
TCG_BAR_RELEASE
since these are one-way barriers and the previous barrier types do not
cover them.
>
> where
>
> tcg_gen_qemu_ld_i32(x, y, i, m | MO_BAR_LD_BEFORE | MO_BAR_ST_AFTER)
>
> emits
>
> mb TCG_BAR_LD_LD
> qemu_ld_i32 x, y, i, m
> mb TCG_BAR_LD_ST
>
> We can then add an optimization pass which folds barriers with no memory
> operations in between, so that duplicates are eliminated.
>
Yes, folding/eliding these barriers in an optimization pass sounds
like a good idea.
Thanks,
--
Pranith
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, (continued)
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/02
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Richard Henderson, 2016/06/02
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Richard Henderson, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Pranith Kumar, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Peter Maydell, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Alex Bennée, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier,
Pranith Kumar <=
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/03
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Pranith Kumar, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Pranith Kumar, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Pranith Kumar, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Richard Henderson, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Pranith Kumar, 2016/06/06
- Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier, Sergey Fedorov, 2016/06/06