Re: [PULL 00/51] Build system, i386 changes for 2023-09-07

[Philippe Mathieu-Daudé]

On 11/9/23 12:11, Philippe Mathieu-Daudé wrote:
> On 8/9/23 21:21, Paolo Bonzini wrote:
> > On Fri, Sep 8, 2023 at 7:28 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > > Maybe the calls aren't eliminated because --enable-debug implies -O0?
> >
> > My experience is that it will still fold simple dead code like "0 &&
> > foo()" or even "if (0) { ... }", but maybe it's a GCC vs. clang
> > difference. Philippe, I take it that you are looking at it?

FTR preprocessed code is indeed:

# 5682 "../../target/i386/cpu.c"
uint64_t x86_cpu_get_supported_feature_word(FeatureWord w,
                                            _Bool migratable_only)
{
    FeatureWordInfo *wi = &feature_word_info[w];
    uint64_t r = 0;

    if ((0)) {
        switch (wi->type) {
        case CPUID_FEATURE_WORD:
            r = kvm_arch_get_supported_cpuid(kvm_state, wi->cpuid.eax,
                                                        wi->cpuid.ecx,
                                                        wi->cpuid.reg);
            break;
        case MSR_FEATURE_WORD:
            r = kvm_arch_get_supported_msr_feature(kvm_state,
                        wi->msr.index);
            break;
        }
    } else if (0) {
        if (wi->type != CPUID_FEATURE_WORD) {
            return 0;
        }
        r = 0;


    } else if ((tcg_allowed)) {
        r = wi->tcg_features;
    } else {
        return ~0;
    }
# 5725 "../../target/i386/cpu.c"
    if (migratable_only) {
        r &= x86_cpu_get_migratable_flags(w);
    }
    return r;
}

static void x86_cpu_get_supported_cpuid(uint32_t func, uint32_t index,
                                        uint32_t *eax, uint32_t *ebx,
                                        uint32_t *ecx, uint32_t *edx)
{
    if ((0)) {
        *eax = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EAX);
        *ebx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EBX);
        *ecx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_ECX);
        *edx = kvm_arch_get_supported_cpuid(kvm_state, func, index, R_EDX);
    } else if (0) {
        *eax = 0;
        *ebx = 0;
        *ecx = 0;
        *edx = 0;
    } else {
        *eax = 0;
        *ebx = 0;
        *ecx = 0;
        *edx = 0;
    }
}

Which made me wonder about this sysemu code use from useremu,
in particular because the link failure comes from
libqemu-x86_64-linux-user.fa.p. This pair of #ifdef'ry to
restrict system-specific code seems sufficient to link:

-- >8 --
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 24ee67b42d..83914d5d1b 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6163,6 +6163,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t 
index, uint32_t count,
             }
             *edx = env->features[FEAT_7_0_EDX]; /* Feature flags */

+#ifndef CONFIG_USER_ONLY
             /*
              * SGX cannot be emulated in software.  If hardware does not
              * support enabling SGX and/or SGX flexible launch control,
@@ -6181,6 +6182,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t 
index, uint32_t count,
                     CPUID_7_0_ECX_SGX_LC))) {
                 *ecx &= ~CPUID_7_0_ECX_SGX_LC;
             }
+#endif
         } else if (count == 1) {
             *eax = env->features[FEAT_7_1_EAX];
             *edx = env->features[FEAT_7_1_EDX];
@@ -7152,6 +7154,7 @@ static void x86_cpu_filter_features(X86CPU *cpu, 
bool verbose)
         mark_unavailable_features(cpu, w, unavailable_features, prefix);
     }

+#ifndef CONFIG_USER_ONLY
     if ((env->features[FEAT_7_0_EBX] & CPUID_7_0_EBX_INTEL_PT) &&
         kvm_enabled()) {
         KVMState *s = CPU(cpu)->kvm_state;
@@ -7179,6 +7182,7 @@ static void x86_cpu_filter_features(X86CPU *cpu, 
bool verbose)
             mark_unavailable_features(cpu, FEAT_7_0_EBX, 
CPUID_7_0_EBX_INTEL_PT, prefix);
         }
     }
+#endif
 }
---

So I'll send a v3.