Messages in this thread | | | Date | Wed, 06 May 2015 13:41:47 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86: Deinline cpuid_eax and friends |
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On 05/06/2015 12:09 PM, Denys Vlasenko wrote: >> >> How on Earth does it make 44 bytes? Is this due to paravirt_fail? > > No, just this construct > > unsigned int eax, ebx, ecx, edx; > cpuid(op, &eax, &ebx, &ecx, &edx); > > is not really that cheap to set up. You need to allocate > variables on stack and take address of each: > > ffffffff81063668 <cpuid_eax>: > ffffffff81063668: 55 push %rbp > ffffffff81063669: 48 89 e5 mov %rsp,%rbp > ffffffff8106366c: 48 83 ec 10 sub $0x10,%rsp > ffffffff81063670: 48 8d 4d fc lea -0x4(%rbp),%rcx > ffffffff81063674: 89 7d f0 mov %edi,-0x10(%rbp) > ffffffff81063677: 48 8d 55 f8 lea -0x8(%rbp),%rdx > ffffffff8106367b: 48 8d 75 f4 lea -0xc(%rbp),%rsi > ffffffff8106367f: 48 8d 7d f0 lea -0x10(%rbp),%rdi > ffffffff81063683: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%rbp) > ffffffff8106368a: e8 3c ff ff ff callq ffffffff810635cb <__cpuid> > ffffffff8106368f: 8b 45 f0 mov -0x10(%rbp),%eax > ffffffff81063692: c9 leaveq > ffffffff81063693: c3 retq >
That almost certainly is due to paravirt_fail, because otherwise cpuid would be inline, and gcc actually knows how to optimize around the cpuid instruction to the point of eliminating the temporaries.
That being said, it would have been better to use a structure.
-hpa
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