diff -r 5d4c3724e4c7 -r 03fbcd06b4c0 src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.lir.amd64/src/org/graalvm/compiler/lir/amd64/AMD64VZeroUpper.java
--- a/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.lir.amd64/src/org/graalvm/compiler/lir/amd64/AMD64VZeroUpper.java	Thu Nov 14 11:16:14 2019 -0800
+++ b/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.lir.amd64/src/org/graalvm/compiler/lir/amd64/AMD64VZeroUpper.java	Thu Nov 14 12:21:00 2019 -0800
@@ -32,6 +32,8 @@
 
 import org.graalvm.compiler.asm.amd64.AMD64MacroAssembler;
 import org.graalvm.compiler.lir.LIRInstructionClass;
+import org.graalvm.compiler.lir.amd64.AMD64Call.ForeignCallOp;
+import org.graalvm.compiler.lir.amd64.vector.AMD64VectorInstruction;
 import org.graalvm.compiler.lir.asm.CompilationResultBuilder;
 
 import jdk.vm.ci.amd64.AMD64;
@@ -40,6 +42,48 @@
 import jdk.vm.ci.code.RegisterValue;
 import jdk.vm.ci.meta.Value;
 
+/**
+ * vzeroupper is essential to avoid performance penalty during SSE-AVX transition. Specifically,
+ * once we have executed instructions that modify the upper bits (i.e., 128+) of the YMM registers,
+ * we need to perform vzeroupper to transit the state to 128bits before executing any SSE
+ * instructions. We don't need to place vzeroupper between VEX-encoded SSE instructions and legacy
+ * SSE instructions, nor between AVX instructions and VEX-encoded SSE instructions.
+ *
+ * When running Graal on HotSpot, we emit a vzeroupper LIR operation (i.e. an instance of this
+ * class) before a foreign call to the runtime function where Graal has no knowledge. The underlying
+ * reason is that HotSpot is SSE-compiled so as to support older CPUs. We also emit a vzeroupper
+ * instruction (see {@code AMD64HotSpotReturnOp.emitCode}) upon returning, if the current LIR graph
+ * contains LIR operations that touch the upper bits of the YMM registers, including but not limited
+ * to {@link AMD64VectorInstruction}, {@link AMD64ArrayCompareToOp}, {@link AMD64ArrayEqualsOp},
+ * {@link AMD64ArrayIndexOfOp}, and {@link ForeignCallOp} that invokes to Graal-compiled stubs. For
+ * the last case, since Graal-compiled stubs is under our control, we don't emit vzeroupper upon
+ * returning of the stub, but rather do that upon returning of the current method.
+ *
+ * On JDK8, C2 does not emit many vzeroupper instructions, potentially because that YMM registers
+ * are not heavily employed (C2 vectorization starts using YMM registers in 9, source
+ * https://cr.openjdk.java.net/~vlivanov/talks/2017_Vectorization_in_HotSpot_JVM.pdf) and thus less
+ * care has been taken to place these instructions. One example is that many intrinsics employ YMM
+ * registers starting from https://bugs.openjdk.java.net/browse/JDK-8005419, but does not properly
+ * place vzeroupper upon returning of the intrinsic stub or the caller of the stub.
+ *
+ * Most vzeroupper were added in JDK 10 (https://bugs.openjdk.java.net/browse/JDK-8178811), and was
+ * later restricted on Haswell Xeon due to performance regression
+ * (https://bugs.openjdk.java.net/browse/JDK-8190934). The actual condition for placing vzeroupper
+ * is at http://hg.openjdk.java.net/jdk/jdk/file/c7d9df2e470c/src/hotspot/cpu/x86/x86_64.ad#l428. To
+ * summarize, if nmethod employs YMM registers (or intrinsics which use them, search for
+ * clear_upper_avx() in opto/library_call.cpp) vzeroupper will be generated on nmethod's exit and
+ * before any calls in nmethod, because even compiled nmethods can still use only SSE instructions.
+ *
+ * This means, if a Java method performs a call to an intrinsic that employs YMM registers,
+ * C2-compiled code will place a vzeroupper before the call, upon exit of the stub and upon exit of
+ * this method. Graal will only place the last, because it ensures that Graal-compiled Java method
+ * and stubs will be consistent on using VEX-encoding.
+ *
+ * In SubstrateVM, since the whole image is compiled consistently with or without VEX encoding (the
+ * later is the default behavior, see {@code NativeImageGenerator.createTarget}), there is no need
+ * for vzeroupper. For dynamic compilation on a SubstrateVM image, if the image is SSE-compiled, we
+ * then need vzeroupper when returning from the dynamic compiled code to the pre-built image code.
+ */
 public class AMD64VZeroUpper extends AMD64LIRInstruction {
 
     public static final LIRInstructionClass<AMD64VZeroUpper> TYPE = LIRInstructionClass.create(AMD64VZeroUpper.class);