Bug 1489994 - On ARM64, make sure to switch to the PSP for the prebarrier stub when calling from wasm. r=bbouvier

The Jit stubs, of which the prebarrier is one, require Jit calling
conventions.  On ARM64 this means that the SP and the PSP (x28) must
track each other.  Wasm does not use the PSP for that purpose, so must
set it up specially when calling directly to jitted code.  The
prebarrier path did not do so.

--HG--
extra : rebase_source : 72eb810f7aa80452c919c529fc9b9bf6de7af512

js/src/wasm/WasmBaselineCompile.cpp

@@ -5655,6 +5655,12 @@ class BaseCompiler final : public BaseCompilerInterface
         masm.loadWasmTlsRegFromFrame(scratch);
         masm.loadPtr(Address(scratch, offsetof(TlsData, instance)), scratch);
         masm.loadPtr(Address(scratch, Instance::offsetOfPreBarrierCode()), scratch);
+# ifdef JS_CODEGEN_ARM64
+        // The prebarrier stub assumes the PseudoStackPointer is set up.  We do
+        // not need to save and restore x28 because it is not yet allocatable.
+        MOZ_ASSERT(!GeneralRegisterSet::All().hasRegisterIndex(x28.asUnsized()));
+        masm.Mov(x28, sp);
+# endif
         masm.call(scratch);
 
         masm.bind(&skipBarrier);