[compiler-rt] a70c3f9 - [compiler-rt] Don't check XCR0 when detecting avx512 on Darwin.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2019-11-21T09:19:17Z
New Revision: a70c3f9f45c8db3092db44110a992e9fd8ee776e

URL: https://github.com/llvm/llvm-project/commit/a70c3f9f45c8db3092db44110a992e9fd8ee776e
DIFF: https://github.com/llvm/llvm-project/commit/a70c3f9f45c8db3092db44110a992e9fd8ee776e.diff

LOG: [compiler-rt] Don't check XCR0 when detecting avx512 on Darwin.

Darwin lazily saves the AVX512 context on first use [1]: 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.

[1] https://github.com/apple/darwin-xnu/blob/xnu-4903.221.2/osfmk/i386/fpu.c#L174

Reviewers: ab, RKSimon, craig.topper

Reviewed By: craig.topper

Subscribers: dberris, JDevlieghere, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D70454

Added: 
    

Modified: 
    compiler-rt/lib/builtins/cpu_model.c

Removed: 
    


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diff  --git a/compiler-rt/lib/builtins/cpu_model.c b/compiler-rt/lib/builtins/cpu_model.c
index cdd632a612bc..fb619037d398 100644
--- a/compiler-rt/lib/builtins/cpu_model.c
+++ b/compiler-rt/lib/builtins/cpu_model.c
@@ -532,7 +532,15 @@ static void getAvailableFeatures(unsigned ECX, unsigned EDX, unsigned MaxLeaf,
   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(FEATURE_AVX);