[X86] Explicitly add popcnt feature to Intel CPUs with SSE4.2 in the frontend.

Previously we inferred it if sse4.2 ended up being enabled after
all feature processing. But writing -march=nehalem -mno-sse4.2
should have popcnt enabled.

Author: topperc

clang/lib/Basic/Targets/X86.cpp

@@ -224,6 +224,7 @@ bool X86TargetInfo::initFeatureMap(
     LLVM_FALLTHROUGH;
   case CK_Nehalem:
     setFeatureEnabledImpl(Features, "sse4.2", true);
+    setFeatureEnabledImpl(Features, "popcnt", true);
     LLVM_FALLTHROUGH;
   case CK_Penryn:
     setFeatureEnabledImpl(Features, "sse4.1", true);
@@ -281,6 +282,7 @@ bool X86TargetInfo::initFeatureMap(
     setFeatureEnabledImpl(Features, "rdrnd", true);
     setFeatureEnabledImpl(Features, "pclmul", true);
     setFeatureEnabledImpl(Features, "sse4.2", true);
+    setFeatureEnabledImpl(Features, "popcnt", true);
     setFeatureEnabledImpl(Features, "prfchw", true);
     LLVM_FALLTHROUGH;
   case CK_Bonnell:
@@ -307,6 +309,7 @@ bool X86TargetInfo::initFeatureMap(
     setFeatureEnabledImpl(Features, "rdseed", true);
     setFeatureEnabledImpl(Features, "adx", true);
     setFeatureEnabledImpl(Features, "lzcnt", true);
+    setFeatureEnabledImpl(Features, "popcnt", true);
     setFeatureEnabledImpl(Features, "bmi", true);
     setFeatureEnabledImpl(Features, "bmi2", true);
     setFeatureEnabledImpl(Features, "fma", true);

clang/test/Preprocessor/x86_target_features.c

@@ -269,6 +269,12 @@
 
 // NOSSE42POPCNT: #define __POPCNT__ 1
 
+// RUN: %clang -target i386-unknown-unknown -march=nehalem -mno-sse4.2 -x c -E -dM -o - %s | FileCheck -match-full-lines --check-prefix=CPUPOPCNT %s
+// RUN: %clang -target i386-unknown-unknown -march=silvermont -mno-sse4.2 -x c -E -dM -o - %s | FileCheck -match-full-lines --check-prefix=CPUPOPCNT %s
+// RUN: %clang -target i386-unknown-unknown -march=knl -mno-sse4.2 -x c -E -dM -o - %s | FileCheck -match-full-lines --check-prefix=CPUPOPCNT %s
+
+// CPUPOPCNT: #define __POPCNT__ 1
+
 // RUN: %clang -target i386-unknown-unknown -march=pentium -msse -x c -E -dM -o - %s | FileCheck -match-full-lines --check-prefix=SSEMMX %s
 
 // SSEMMX: #define __MMX__ 1