ARM: Changed the handling of compiletime CPU feature detection

Always use CpuFeaturesImpliedByCompiler() when selecting CPU features. This checks both for CAN_USE_ARMV7_INSTRUCTIONS and CAN_USE_VFP_INSTRUCTIONS and for GCC preprocessor symbols. This will support using the CAN_USE_XXX for a simulator build used for generating a snapshot followed by a crosscompile using -march= and -mfpu= for selecting the (minimal) target device CPU features. The snapshot will use instructions based on the CAN_USE_XXX whereas the target will at least use features based on both CAN_USE_XXX and -march= and -mfpu=, but will try runtime CPU feature detection a well looking for somethis better.

Remove the compiler based CPU feature detection from the OS::CpuFeaturesImpliedByPlatform() as it did not belong there. Also was already in the CpuFeaturesImpliedByCompiler().

Add the variable 'v8_can_use_vfp_instructions' to the GYP file which can be used to turn on CAN_USE_VFP_INSTRUCTIONS when building V8. I did not add any -mfpu= cflags for this, as there are several options here (e.g. vfp and neon).

R=erik.corry@gmail.com, karlklose@chromium.org

BUG=none
TEST=none

Review URL: http://codereview.chromium.org//6904164

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7754 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

.gitignore

@@ -31,3 +31,5 @@ shell_g
 /tools/visual_studio/Release
 /xcodebuild/
 TAGS
+Makefile
+*.Makefile

src/arm/assembler-arm.cc

@@ -51,24 +51,30 @@ unsigned CpuFeatures::supported_ = 0;
 unsigned CpuFeatures::found_by_runtime_probing_ = 0;
 
 
-#ifdef __arm__
+// Get the CPU features enabled by the build. For cross compilation the
+// preprocessor symbols CAN_USE_ARMV7_INSTRUCTIONS and CAN_USE_VFP_INSTRUCTIONS
+// can be defined to enable ARMv7 and VFPv3 instructions when building the
+// snapshot.
 static uint64_t CpuFeaturesImpliedByCompiler() {
   uint64_t answer = 0;
 #ifdef CAN_USE_ARMV7_INSTRUCTIONS
   answer |= 1u << ARMv7;
 #endif  // def CAN_USE_ARMV7_INSTRUCTIONS
+#ifdef CAN_USE_VFP_INSTRUCTIONS
+  answer |= 1u << VFP3 | 1u << ARMv7;
+#endif  // def CAN_USE_VFP_INSTRUCTIONS
+
+#ifdef __arm__
   // If the compiler is allowed to use VFP then we can use VFP too in our code
   // generation even when generating snapshots.  This won't work for cross
   // compilation. VFPv3 implies ARMv7, see ARM DDI 0406B, page A1-6.
 #if defined(__VFP_FP__) && !defined(__SOFTFP__)
   answer |= 1u << VFP3 | 1u << ARMv7;
 #endif  // defined(__VFP_FP__) && !defined(__SOFTFP__)
-#ifdef CAN_USE_VFP_INSTRUCTIONS
-  answer |= 1u << VFP3 | 1u << ARMv7;
-#endif  // def CAN_USE_VFP_INSTRUCTIONS
+#endif  // def __arm__
+
   return answer;
 }
-#endif  // def __arm__
 
 
 void CpuFeatures::Probe() {
@@ -76,6 +82,18 @@ void CpuFeatures::Probe() {
 #ifdef DEBUG
   initialized_ = true;
 #endif
+
+  // Get the features implied by the OS and the compiler settings. This is the
+  // minimal set of features which is also alowed for generated code in the
+  // snapshot.
+  supported_ |= OS::CpuFeaturesImpliedByPlatform();
+  supported_ |= CpuFeaturesImpliedByCompiler();
+
+  if (Serializer::enabled()) {
+    // No probing for features if we might serialize (generate snapshot).
+    return;
+  }
+
 #ifndef __arm__
   // For the simulator=arm build, use VFP when FLAG_enable_vfp3 is
   // enabled. VFPv3 implies ARMv7, see ARM DDI 0406B, page A1-6.
@@ -87,13 +105,8 @@ void CpuFeatures::Probe() {
     supported_ |= 1u << ARMv7;
   }
 #else  // def __arm__
-  if (Serializer::enabled()) {
-    supported_ |= OS::CpuFeaturesImpliedByPlatform();
-    supported_ |= CpuFeaturesImpliedByCompiler();
-    return;  // No features if we might serialize.
-  }
-
-  if (OS::ArmCpuHasFeature(VFP3)) {
+  // Probe for additional features not already known to be available.
+  if (!IsSupported(VFP3) && OS::ArmCpuHasFeature(VFP3)) {
     // This implementation also sets the VFP flags if runtime
     // detection of VFP returns true. VFPv3 implies ARMv7, see ARM DDI
     // 0406B, page A1-6.
@@ -101,7 +114,7 @@ void CpuFeatures::Probe() {
     found_by_runtime_probing_ |= 1u << VFP3 | 1u << ARMv7;
   }
 
-  if (OS::ArmCpuHasFeature(ARMv7)) {
+  if (!IsSupported(ARMv7) && OS::ArmCpuHasFeature(ARMv7)) {
     supported_ |= 1u << ARMv7;
     found_by_runtime_probing_ |= 1u << ARMv7;
   }

src/platform-linux.cc

@@ -110,14 +110,7 @@ void OS::Setup() {
 
 
 uint64_t OS::CpuFeaturesImpliedByPlatform() {
-#if (defined(__VFP_FP__) && !defined(__SOFTFP__))
-  // Here gcc is telling us that we are on an ARM and gcc is assuming
-  // that we have VFP3 instructions.  If gcc can assume it then so can
-  // we. VFPv3 implies ARMv7, see ARM DDI 0406B, page A1-6.
-  return 1u << VFP3 | 1u << ARMv7;
-#elif CAN_USE_ARMV7_INSTRUCTIONS
-  return 1u << ARMv7;
-#elif(defined(__mips_hard_float) && __mips_hard_float != 0)
+#if(defined(__mips_hard_float) && __mips_hard_float != 0)
     // Here gcc is telling us that we are on an MIPS and gcc is assuming that we
     // have FPU instructions.  If gcc can assume it then so can we.
     return 1u << FPU;

tools/gyp/v8.gyp

@@ -32,6 +32,7 @@
     'gcc_version%': 'unknown',
     'v8_compress_startup_data%': 'false',
     'v8_target_arch%': '<(target_arch)',
+    'v8_can_use_vfp_instructions%': 'false',
     'v8_use_snapshot%': 'true',
     'v8_use_liveobjectlist%': 'false',
   },
@@ -56,6 +57,13 @@
                 'defines': [
                   'V8_TARGET_ARCH_ARM',
                 ],
+                'conditions': [
+                  [ 'v8_can_use_vfp_instructions=="true"', {
+                    'defines': [
+                      'CAN_USE_VFP_INSTRUCTIONS',
+                    ],
+                  }],
+                ],
               }],
               ['v8_target_arch=="ia32"', {
                 'defines': [