Deal with macOS not enabling AVX512 state in XCR0

All Mac Pros launched since 2019 have AVX512 support (CLX processor) but
the Darwin kernel does not enable the state bits in the XCR0 register
for all processes by default. Instead, it traps the #UD exception and
decodes the instruction to find out if it was an AVX512 instruction. If
so, it adds memory to the thread's context switch space. See [1].

Good solution, but forces us to have OS-specific code to determine if
the OS really supports AVX512. No other OS appears to require this. For
future features (namely, Intel Advanced Matrix Extensions), there's an
official way of implementing this, via the eXtended Feature Disable
register.

Qt has no AVX512 code yet, so this is just being future-proof. Apple has
yet to announce when or if their Mac Pros will switch from Intel Xeon to
ARM.

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

Change-Id: Ieffc3453b88c4517a1dbfffd162175ead343e622
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 50 insertions, 18 deletions

diff --git a/src/corelib/global/qsimd.cpp b/src/corelib/global/qsimd.cpp
index 6ad6795fec..d91c1542c2 100644
--- a/src/corelib/global/qsimd.cpp
+++ b/src/corelib/global/qsimd.cpp
@@ -323,23 +323,49 @@ static void xgetbv(uint in, uint &eax, uint &edx)
 #endif
 }
 
+// Flags from the XCR0 state register
+enum XCR0Flags {
+    X87             = 1 << 0,
+    XMM0_15         = 1 << 1,
+    YMM0_15Hi128    = 1 << 2,
+    BNDRegs         = 1 << 3,
+    BNDCSR          = 1 << 4,
+    OpMask          = 1 << 5,
+    ZMM0_15Hi256    = 1 << 6,
+    ZMM16_31        = 1 << 7,
+
+    SSEState        = XMM0_15,
+    AVXState        = XMM0_15 | YMM0_15Hi128,
+    AVX512State     = AVXState | OpMask | ZMM0_15Hi256 | ZMM16_31
+};
+
+static quint64 adjustedXcr0(quint64 xcr0)
+{
+    /*
+     * Some OSes hide their capability of context-switching the AVX512 state in
+     * the XCR0 register. They do that so the first time we execute an
+     * instruction that may access the AVX512 state (requiring the EVEX prefix)
+     * they allocate the necessary context switch space.
+     *
+     * This behavior is deprecated with the XFD (Extended Feature Disable)
+     * register, but we can't change existing OSes.
+     */
+#ifdef Q_OS_DARWIN
+    // from <machine/cpu_capabilities.h> in xnu
+    // <https://github.com/apple/darwin-xnu/blob/xnu-4903.221.2/osfmk/i386/cpu_capabilities.h>
+    constexpr quint64 kHasAVX512F = Q_UINT64_C(0x0000004000000000);
+    constexpr quintptr commpage = sizeof(void *) > 4 ? Q_UINT64_C(0x00007fffffe00000) : 0xffff0000;
+    constexpr quintptr cpu_capabilities64 = commpage + 0x10;
+    quint64 capab = *reinterpret_cast<quint64 *>(cpu_capabilities64);
+    if (capab & kHasAVX512F)
+        xcr0 |= AVX512State;
+#endif
+
+    return xcr0;
+}
+
 static quint64 detectProcessorFeatures()
 {
-    // Flags from the CR0 / XCR0 state register
-    enum XCR0Flags {
-        X87             = 1 << 0,
-        XMM0_15         = 1 << 1,
-        YMM0_15Hi128    = 1 << 2,
-        BNDRegs         = 1 << 3,
-        BNDCSR          = 1 << 4,
-        OpMask          = 1 << 5,
-        ZMM0_15Hi256    = 1 << 6,
-        ZMM16_31        = 1 << 7,
-
-        SSEState        = XMM0_15,
-        AVXState        = XMM0_15 | YMM0_15Hi128,
-        AVX512State     = AVXState | OpMask | ZMM0_15Hi256 | ZMM16_31
-    };
     static const quint64 AllAVX2 = CpuFeatureAVX2 | AllAVX512;
     static const quint64 AllAVX = CpuFeatureAVX | AllAVX2;
 
@@ -367,16 +393,22 @@ static quint64 detectProcessorFeatures()
     }
 
     // now check the AVX state
-    uint xgetbvA = 0, xgetbvD = 0;
+    quint64 xcr0 = 0;
     if (results[Leaf1ECX] & (1u << 27)) {
         // XGETBV enabled
+        uint xgetbvA = 0, xgetbvD = 0;
         xgetbv(0, xgetbvA, xgetbvD);
+
+        xcr0 = xgetbvA;
+        if (sizeof(XCR0Flags) > sizeof(xgetbvA))
+            xcr0 |= quint64(xgetbvD) << 32;
+        xcr0 = adjustedXcr0(xcr0);
     }
 
-    if ((xgetbvA & AVXState) != AVXState) {
+    if ((xcr0 & AVXState) != AVXState) {
         // support for YMM registers is disabled, disable all AVX
         features &= ~AllAVX;
-    } else if ((xgetbvA & AVX512State) != AVX512State) {
+    } else if ((xcr0 & AVX512State) != AVX512State) {
         // support for ZMM registers or mask registers is disabled, disable all AVX512
         features &= ~AllAVX512;
     }