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Date: Wed, 13 Jul 2016 00:50:12 +0800 From: Chen Gang <chengang@...ndsoft.com.cn> To: Michal Hocko <mhocko@...nel.org> CC: Minchan Kim <minchan@...nel.org>, akpm@...ux-foundation.org, vbabka@...e.cz, mgorman@...hsingularity.net, gi-oh.kim@...fitbricks.com, iamjoonsoo.kim@....com, hillf.zj@...baba-inc.com, rientjes@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Chen Gang <gang.chen.5i5j@...il.com> Subject: Re: [PATCH] mm: migrate: Use bool instead of int for the return value of PageMovable On 7/12/16 15:48, Michal Hocko wrote: > On Tue 12-07-16 03:47:42, Chen Gang wrote: > [...] >> In our case, the 2 output size are same, but under x86_64, the insns are >> different. After uses bool, it uses push/pop instead of branch, for me, >> it should be a little better for catching. > > The code generated for bool version looks much worse. Look at the fast > path. Gcc tries to reuse the retq from the fast path in the bool case > and so it has to push rbp and rbx on the stack. > > That being said, gcc doesn't seem to generate a better code for bool so > I do not think this is really worth it. > The code below also merge 3 statements into 1 return statement, although for me, it is a little more readable, it will generate a little bad code. That is the reason why the output looks a little bad. In our case, for gcc 6.0, using bool instead of int for bool function will get the same output under x86_64. In our case, for gcc 4.8, using bool instead of int for bool function will get a little better output under x86_64. Thanks. >> The orig: >> >> 0000000000001290 <PageMovable>: >> 1290: 48 8b 47 08 mov 0x8(%rdi),%rax >> 1294: 83 e0 03 and $0x3,%eax >> 1297: 48 83 f8 02 cmp $0x2,%rax >> 129b: 74 03 je 12a0 <__SetPageMovable+0x12a0> >> 129d: 31 c0 xor %eax,%eax >> 129f: c3 retq >> 12a0: 55 push %rbp >> 12a1: 48 89 e5 mov %rsp,%rbp >> 12a4: e8 00 00 00 00 callq 12a9 <__SetPageMovable+0x12a9> >> 12a9: 48 85 c0 test %rax,%rax >> 12ac: 74 17 je 12c5 <__SetPageMovable+0x12c5> >> 12ae: 48 8b 50 68 mov 0x68(%rax),%rdx >> 12b2: 48 85 d2 test %rdx,%rdx >> 12b5: 74 0e je 12c5 <__SetPageMovable+0x12c5> >> 12b7: 48 83 7a 68 00 cmpq $0x0,0x68(%rdx) >> 12bc: b8 01 00 00 00 mov $0x1,%eax >> 12c1: 74 02 je 12c5 <__SetPageMovable+0x12c5> >> 12c3: 5d pop %rbp >> 12c4: c3 retq >> 12c5: 31 c0 xor %eax,%eax >> 12c7: 5d pop %rbp >> 12c8: c3 retq >> 12c9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) >> >> The new: >> >> 0000000000001290 <PageMovable>: >> 1290: 48 8b 47 08 mov 0x8(%rdi),%rax >> 1294: 55 push %rbp >> 1295: 48 89 e5 mov %rsp,%rbp >> 1298: 53 push %rbx >> 1299: 31 db xor %ebx,%ebx >> 129b: 83 e0 03 and $0x3,%eax >> 129e: 48 83 f8 02 cmp $0x2,%rax >> 12a2: 74 05 je 12a9 <__SetPageMovable+0x12a9> >> 12a4: 89 d8 mov %ebx,%eax >> 12a6: 5b pop %rbx >> 12a7: 5d pop %rbp >> 12a8: c3 retq >> 12a9: e8 00 00 00 00 callq 12ae <__SetPageMovable+0x12ae> >> 12ae: 48 85 c0 test %rax,%rax >> 12b1: 74 f1 je 12a4 <__SetPageMovable+0x12a4> >> 12b3: 48 8b 40 68 mov 0x68(%rax),%rax >> 12b7: 48 85 c0 test %rax,%rax >> 12ba: 74 e8 je 12a4 <__SetPageMovable+0x12a4> >> 12bc: 48 83 78 68 00 cmpq $0x0,0x68(%rax) >> 12c1: 0f 95 c3 setne %bl >> 12c4: 89 d8 mov %ebx,%eax >> 12c6: 5b pop %rbx >> 12c7: 5d pop %rbp >> 12c8: c3 retq >> 12c9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) >> >> Thanks. >> -- >> Chen Gang (陈刚) >> >> Managing Natural Environments is the Duty of Human Beings. > -- Chen Gang (陈刚) Managing Natural Environments is the Duty of Human Beings.
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