[Support] Don't require the XCR0 OSXSAVE bits when detecting skylake-avx512.

[fhahn]

Summary

Darwin lazily saves the AVX512 context on first use: instead of checking
that it already does to figure out if the OS supports AVX512, trust that
the kernel will do the right thing and always assume the context save
support is available.

[fhahn]

clang-format

[craig.topper]

Can we check sysctl for apple?

[fhahn]

In D70453#1752392, @craig.topper wrote:

Can we check sysctl for apple?

sysctl would return AVX512 as optional, but I am not sure if that adds any value? As mentioned in the comment, Darwin will save the AXV512 context on AVX512 hardware on-demand. More details can be found here: https://github.com/apple/darwin-xnu/blob/master/osfmk/i386/fpu.c#L174

[craig.topper]

In D70453#1752570, @fhahn wrote:

In D70453#1752392, @craig.topper wrote:

Can we check sysctl for apple?

sysctl would return AVX512 as optional, but I am not sure if that adds any value? As mentioned in the comment, Darwin will save the AXV512 context on AVX512 hardware on-demand. More details can be found here: https://github.com/apple/darwin-xnu/blob/master/osfmk/i386/fpu.c#L174

How long has that support been in Darwin. Is it possible someone might be using an old version of darwin that completely lacks avx512 support and tries to run -march=native on a recent CPU? I guess that can only happen in a hackintosh setup since any Apple system that supports avx512 would have proper kernel support.

[craig.topper]

LGTM

llvm/lib/Support/Host.cpp

if ((ECX >> 22) & 1)
     setFeature(X86::FEATURE_MOVBE);
 
   // If CPUID indicates support for XSAVE, XRESTORE and AVX, and XGETBV
   // indicates that the AVX registers will be saved and restored on context
   // switch, then we have full AVX support.
   const unsigned AVXBits = (1 << 27) | (1 << 28);
   bool HasAVX = ((ECX & AVXBits) == AVXBits) && !getX86XCR0(&EAX, &EDX) &&
                 ((EAX & 0x6) == 0x6);
+#if defined(__APPLE__)
+  // Darwin lazily saves the AVX512 context on first use: trust that the OS will
+  // save the AVX512 context if we use AVX512 instructions, even the bit is not
+  // set right now.
+  bool HasAVX512Save = true;
+#else
+  // AVX512 requires additional context to be saved by the OS.
   bool HasAVX512Save = HasAVX && ((EAX & 0xe0) == 0xe0);
+#endif
 
   if (HasAVX)
     setFeature(X86::FEATURE_AVX);
 
   bool HasLeaf7 =
       MaxLeaf >= 0x7 && !getX86CpuIDAndInfoEx(0x7, 0x0, &EAX, &EBX, &ECX, &EDX);
 
   if (HasLeaf7 && ((EBX >> 3) & 1))
bool sys::getHostCPUFeatures(StringMap<bool> &Features) {
   Features["aes"]    = (ECX >> 25) & 1;
   Features["rdrnd"]  = (ECX >> 30) & 1;
 
   // If CPUID indicates support for XSAVE, XRESTORE and AVX, and XGETBV
   // indicates that the AVX registers will be saved and restored on context
   // switch, then we have full AVX support.
   bool HasAVXSave = ((ECX >> 27) & 1) && ((ECX >> 28) & 1) &&
                     !getX86XCR0(&EAX, &EDX) && ((EAX & 0x6) == 0x6);
+#if defined(__APPLE__)
+  // Darwin lazily saves the AVX512 context on first use: trust that the OS will
+  // save the AVX512 context if we use AVX512 instructions, even the bit is not
+  // set right now.
+  bool HasAVX512Save = true;
+#else
   // AVX512 requires additional context to be saved by the OS.
   bool HasAVX512Save = HasAVXSave && ((EAX & 0xe0) == 0xe0);
+#endif
 
   Features["avx"]   = HasAVXSave;
   Features["fma"]   = ((ECX >> 12) & 1) && HasAVXSave;
   // Only enable XSAVE if OS has enabled support for saving YMM state.
   Features["xsave"] = ((ECX >> 26) & 1) && HasAVXSave;
   Features["f16c"]  = ((ECX >> 29) & 1) && HasAVXSave;
 
   unsigned MaxExtLevel;