commit fc25eb4380d0
Author: Lars T Hansen <lhansen@mozilla.com>
Date:   Wed Feb 28 13:57:52 2018 +0100

    Bug 1375074 - Save and restore non-volatile x28 on ARM64 for generated unboxed object constructor.  r=sstangl
    
    --HG--
    extra : rebase_source : 4a2824b23cf7453d07c962123f780c0ff2fd5292
    extra : source : dec6ab6522a4eb4b3c2defc5078b38ed904f3383
---
 js/src/jit-test/tests/bug1375074.js | 18 ++++++++++++++++++
 js/src/vm/UnboxedObject.cpp         | 25 ++++++++++++++++++++++++-
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git js/src/jit-test/tests/bug1375074.js js/src/jit-test/tests/bug1375074.js
new file mode 100644
index 0000000000000..8bf01d96903fc
--- /dev/null
+++ js/src/jit-test/tests/bug1375074.js
@@ -0,0 +1,18 @@
+// This forces the VM to start creating unboxed objects and thus stresses a
+// particular path into generated code for a specialized unboxed object
+// constructor.
+
+var K = 2000;			// 2000 should be plenty
+var s = "[";
+var i;
+for ( i=0; i < K-1; i++ )
+    s = s + `{"i":${i}},`;
+s += `{"i":${i}}]`;
+var v = JSON.parse(s);
+
+assertEq(v.length == K, true);
+
+for ( i=0; i < K; i++) {
+    assertEq(v[i] instanceof Object, true);
+    assertEq(v[i].i, i);
+}
diff --git js/src/vm/UnboxedObject.cpp js/src/vm/UnboxedObject.cpp
index c8c178965c488..5badf328ede4f 100644
--- js/src/vm/UnboxedObject.cpp
+++ js/src/vm/UnboxedObject.cpp
@@ -95,7 +95,15 @@ UnboxedLayout::makeConstructorCode(JSContext* cx, HandleObjectGroup group)
 #endif
 
 #ifdef JS_CODEGEN_ARM64
-    // ARM64 communicates stack address via sp, but uses a pseudo-sp for addressing.
+    // ARM64 communicates stack address via sp, but uses a pseudo-sp (PSP) for
+    // addressing.  The register we use for PSP may however also be used by
+    // calling code, and it is nonvolatile, so save it.  Do this as a special
+    // case first because the generic save/restore code needs the PSP to be
+    // initialized already.
+    MOZ_ASSERT(PseudoStackPointer64.Is(masm.GetStackPointer64()));
+    masm.Str(PseudoStackPointer64, vixl::MemOperand(sp, -16, vixl::PreIndex));
+
+    // Initialize the PSP from the SP.
     masm.initStackPtr();
 #endif
 
@@ -233,7 +241,22 @@ UnboxedLayout::makeConstructorCode(JSContext* cx, HandleObjectGroup group)
         masm.pop(ScratchDoubleReg);
     masm.PopRegsInMask(savedNonVolatileRegisters);
 
+#ifdef JS_CODEGEN_ARM64
+    // Now restore the value that was in the PSP register on entry, and return.
+
+    // Obtain the correct SP from the PSP.
+    masm.Mov(sp, PseudoStackPointer64);
+
+    // Restore the saved value of the PSP register, this value is whatever the
+    // caller had saved in it, not any actual SP value, and it must not be
+    // overwritten subsequently.
+    masm.Ldr(PseudoStackPointer64, vixl::MemOperand(sp, 16, vixl::PostIndex));
+
+    // Perform a plain Ret(), as abiret() will move SP <- PSP and that is wrong.
+    masm.Ret(vixl::lr);
+#else
     masm.abiret();
+#endif
 
     masm.bind(&failureStoreOther);