On 20/02/19 12:59, Kamil Rytarowski wrote: > Ping, still valid. Sorry, I missed your email.
> On 15.02.2019 00:38, Kamil Rytarowski wrote: >> I consider it as fragile hack and certainly not something to depend on. >> Also in some circumstances of such code, especially "if (zero0)" we want >> to enable disabled code under a debugger. That's a good objection, but certainly does not apply to KVM on NetBSD. >> There were also kernel backdoors due to this optimization. Citation please? >> Requested cpu.i (hopefully correctly generated) >> >> http://netbsd.org/~kamil/qemu/cpu.i.bz2 So, first thing first I can reproduce clang's behavior with this .i file and also with this reduced test case. extern void f(void); int i, j; int main() { if (0 && i) f(); if (j && 0) f(); } The first is eliminated but the second is not, just like in QEMU where this works: if (kvm_enabled() && cpu->enable_pmu) { KVMState *s = cs->kvm_state; *eax = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EAX); *ebx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EBX); *ecx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_ECX); *edx = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EDX); } else if (hvf_enabled() && cpu->enable_pmu) { *eax = hvf_get_supported_cpuid(0xA, count, R_EAX); *ebx = hvf_get_supported_cpuid(0xA, count, R_EBX); *ecx = hvf_get_supported_cpuid(0xA, count, R_ECX); *edx = hvf_get_supported_cpuid(0xA, count, R_EDX); while this doesn't: if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) && kvm_enabled()) { KVMState *s = CPU(cpu)->kvm_state; uint32_t eax_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_EAX); uint32_t ebx_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_EBX); uint32_t ecx_0 = kvm_arch_get_supported_cpuid(s, 0x14, 0, R_ECX); uint32_t eax_1 = kvm_arch_get_supported_cpuid(s, 0x14, 1, R_EAX); uint32_t ebx_1 = kvm_arch_get_supported_cpuid(s, 0x14, 1, R_EBX); But, that's okay, it's -O0 so we give clang a pass for that Note that clang does do the optimization even in more complex cases like extern _Bool f(void); int main() { if (!0) return 0; if (!f()) return 0; } The problem is that there is a kvm-stub.c entry for that, and in fact my compilation passes and the symbol is resolved correctly: $ nm target/i386/cpu.o |grep kvm_.*get_sup U kvm_arch_get_supported_cpuid $ nm target/i386/kvm-stub.o|grep kvm_.*get_sup 0000000000000030 T kvm_arch_get_supported_cpuid $ nm qemu-system-x86_64 |grep kvm_.*get_sup 000000000046eab0 T kvm_arch_get_supported_cpuid As expected, something much less obvious is going on for you, in particular __OPTIMIZE__seems not to be working properly. However, that would also be very surprising. Please: 1) run the last two "nm" commands on your build (wthout grep). 2) do the same exercise to get a .i for target/i386/kvm-stub.c 3) try removing the "#ifndef __OPTIMIZE__" and leave everything else as is, see if it works. No need to play with macros, which also goes to show that you didn't really understand what's going on---that's fine, but then please refrain from making summary judgments which only lengthen the discussion. Thanks, Paolo
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