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Re: [Qemu-devel] [PATCH] tcg: try sti when moving a constant into a dead
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] tcg: try sti when moving a constant into a dead memory temp |
Date: |
Wed, 21 Sep 2016 15:37:50 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 |
On 20/09/2016 22:41, Richard Henderson wrote:
> On 09/15/2016 06:16 AM, Paolo Bonzini wrote:
>> This comes from free from unifying tcg_reg_alloc_mov and
>> tcg_reg_alloc_movi's handling of TEMP_VAL_CONST. It triggers
>> often on moves to cc_dst, such as the following translation
>> of "sub $0x3c,%esp":
>>
>> before: after:
>> subl $0x3c,%ebp subl $0x3c,%ebp
>> movl %ebp,0x10(%r14) movl %ebp,0x10(%r14)
>> movl $0x3c,%ebx movl $0x3c,0x2c(%r14)
>> movl %ebx,0x2c(%r14)
>
> What's the input here? From just this snippet it looks as if we should have
> converted both of them to an immediate store.
I don't have the exact binary but this is what I get from qemu
movi_i32 tmp1,$0x20
sub_i32 tmp0,esp,tmp1 dead: 1
mov_i32 esp,tmp0 sync: 0 dead: 1
mov_i32 cc_src,tmp1 sync: 0 dead: 0 1
mov_i32 cc_dst,esp sync: 0 dead: 0
discard cc_src2
discard cc_op
which is compiled to
0x7f98ed3ff761: mov %ebx,(%rsi) ; load esp
0x7f98ed3ff763: sub $0x20,%ebp ; sub_i32 tmp0,esp,tmp1
0x7f98ed3ff766: mov %ebp,0x10(%r14)
0x7f98ed3ff76a: mov $0x20,%ebx ; mov_i32 cc_src,tmp1
0x7f98ed3ff76f: mov %ebx,0x2c(%r14)
0x7f98ed3ff773: mov %ebp,0x28(%r14) ; mov_i32 cc_dst,esp
TCG doesn't convert the "mov_i32 cc_src,tmp1" to movi, instead it tracks
the register as a TEMP_VAL_CONST.
Paolo