Workaround LLVM incorrectly enables NEON on armv7-a by default

Explicitly disable NEON when a generic arch is specified

(cherry picked from commit 7737162)

Author: yuyichao

src/codegen.cpp

@@ -5597,27 +5597,40 @@ static inline std::string getNativeTarget()
 #if defined(_CPU_ARM_) || defined(_CPU_AARCH64_)
 // Check if the cpu name is a ARM/AArch64 arch name and return a
 // string that can be used as LLVM feature name
-static inline std::string checkARMArchFeature(const std::string &cpu)
+static inline void checkARMArchFeature(std::string &cpu,
+                                       StringMap<bool> &HostFeatures)
 {
-    const char *prefix = "armv";
-    size_t prefix_len = strlen(prefix);
-    if (cpu.size() <= prefix_len ||
-        memcmp(cpu.data(), prefix, prefix_len) != 0 ||
-        cpu[prefix_len] < '1' || cpu[prefix_len] > '9')
-        return std::string();
 #if defined(_CPU_ARM_)
+    if (cpu == "generic") {
+        HostFeatures["neon"] = false;
+        return;
+    }
+#endif
+    StringRef cpu_s = cpu;
+    if (!cpu_s.startswith("armv"))
+        return;
+    // Generic names
+#if defined(_CPU_ARM_)
+    if (!cpu_s.startswith("armv8")) {
+        // Turn off `neon` for generic archs on ARM
+        // since LLVM seems to enable it for all armv7-a processors.
+        HostFeatures["neon"] = false;
+    }
     // "v7" and "v8" are not available in the form of `armv*`
     // in the feature list
     if (cpu == "armv7") {
-        return "v7";
+        HostFeatures["v7"] = true;
     }
     else if (cpu == "armv8") {
-        return "v8";
+        HostFeatures["v8"] = true;
+    }
+    else {
+        HostFeatures[cpu] = true;
     }
-    return cpu;
 #else
-    return cpu.substr(3);
+    HostFeatures[cpu.substr(3)] = true;
 #endif
+    cpu = "generic";
 }
 #endif
 
@@ -5667,9 +5680,25 @@ static inline SmallVector<std::string,10> getTargetFeatures(std::string &cpu)
 
     // Arch version
 #if __ARM_ARCH >= 8
+#  if defined(__ARM_ARCH_PROFILE) && __ARM_ARCH_PROFILE == 'A'
+    HostFeatures["armv8-a"] = true;
+#  else
     HostFeatures["v8"] = true;
+#  endif
 #elif __ARM_ARCH >= 7
+    // v7 + aclass emits slightly different code than armv7-a
+    // In particular LLVM does not use the armv7-a instruction for barrier
+    // with v7 + aclass.
+#  if defined(__ARM_ARCH_PROFILE) && __ARM_ARCH_PROFILE == 'A'
+    HostFeatures["armv7-a"] = true;
+#  elif defined(__ARM_ARCH_PROFILE) && __ARM_ARCH_PROFILE == 'R'
+    HostFeatures["armv7-r"] = true;
+#  elif defined(__ARM_ARCH_PROFILE) && __ARM_ARCH_PROFILE == 'M'
+    // Thumb
+    HostFeatures["armv7-m"] = true;
+#  else
     HostFeatures["v7"] = true;
+#  endif
 #else
     // minimum requirement
     HostFeatures["v6"] = true;
@@ -5704,18 +5733,21 @@ static inline SmallVector<std::string,10> getTargetFeatures(std::string &cpu)
     //
     // Supported AArch64 arch names on LLVM 3.8:
     //   armv8.1a, armv8.2a
-    std::string arm_arch = checkARMArchFeature(cpu);
-    if (!arm_arch.empty()) {
-        HostFeatures[arm_arch] = true;
-        cpu = "generic";
-    }
+    checkARMArchFeature(cpu, HostFeatures);
 #endif
 
     SmallVector<std::string,10> attr;
-    for (StringMap<bool>::const_iterator it = HostFeatures.begin(); it != HostFeatures.end(); it++) {
-        std::string att = it->getValue() ? it->getKey().str() :
-                          std::string("-") + it->getKey().str();
-        attr.append(1, att);
+    for (auto it = HostFeatures.begin(); it != HostFeatures.end(); it++) {
+        if (it->getValue()) {
+            attr.append(1, it->getKey().str());
+        }
+    }
+    // Explicitly disabled features need to be added at the end so that
+    // they are not reenabled by other features that implies them by default.
+    for (auto it = HostFeatures.begin(); it != HostFeatures.end(); it++) {
+        if (!it->getValue()) {
+            attr.append(1, std::string("-") + it->getKey().str());
+        }
     }
     return attr;
 }