Messages in this thread | | | Date | Tue, 6 Dec 2022 10:18:19 +0800 | Subject | Re: [PATCH] KVM: x86/pmu: Avoid ternary operator by directly referring to counters->type | From | Like Xu <> |
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On 6/12/2022 12:46 am, Sean Christopherson wrote: > On Mon, Dec 05, 2022, Like Xu wrote: >> From: Like Xu <likexu@tencent.com> >> >> In either case, the counters will point to fixed or gp pmc array, and >> taking advantage of the C pointer, it's reasonable to use an almost known >> mem load operation directly without disturbing the branch predictor. > > The compiler is extremely unlikely to generate a branch for this, e.g. gcc-12 uses > setne and clang-14 shifts "fixed" by 30. FWIW, clang is also clever enough to > use a cmov to load the address of counters, i.e. the happy path will have no taken > branches for either type of counter.
If so, good news for users of the new tool chain. I assume our Linux project is also to be commended when it comes to supporting legacy issues even if just a little.
> >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/vmx/pmu_intel.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c >> index e5cec07ca8d9..28b0a784f6e9 100644 >> --- a/arch/x86/kvm/vmx/pmu_intel.c >> +++ b/arch/x86/kvm/vmx/pmu_intel.c >> @@ -142,7 +142,7 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu, >> } >> if (idx >= num_counters) >> return NULL; >> - *mask &= pmu->counter_bitmask[fixed ? KVM_PMC_FIXED : KVM_PMC_GP]; >> + *mask &= pmu->counter_bitmask[counters->type]; > > In terms of readability, I have a slight preference for the current code as I > don't have to look at counters->type to understand its possible values. When someone tries to add a new type of pmc type, the code bugs up. And, this one will make all usage of pmu->counter_bitmask[] more consistent.
Please reconsider this minor diff if it does no harm.
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