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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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.