Bug 1656335 - Improve wasm gating on ARM. r=jseward

Generalizes wasm::HasSupport() on 32-bit ARM so that we check whether
the atomic operations are available in all cases, not just when
running on the simulator.  This was a dumb bug, but an old one.
Effectively this means we get wasm only with ARMv7, which we've been
assuming anyway.  Now we gate it properly.

Changes a whitebox test case that explicitly sets the ARM hw
capabilities to set capabilities that will pass the new test even on
non-simulator builds, and handles the run-time error that results when
command line switches select a compiler that has stronger requirements
than that general requirement, leaving an empty set of available
compilers.  (The baseline compiler requires IDIV support.)

Renames the test case appropriately.

Differential Revision: https://phabricator.services.mozilla.com/D85547

js/src/jit-test/tests/wasm/arm-hwcap-madness.js

@@ -0,0 +1,32 @@
+// |jit-test| skip-if: !this.setARMHwCapFlags
+
+// This will disable the idiv instruction and thus make the baseline compiler
+// unavailable, so we must be prepared for a run-time error below in the
+// baseline-only configuration.
+setARMHwCapFlags('vfp,armv7');
+
+// The flags above should be sufficient for there to be a WebAssembly object.
+assertEq(typeof WebAssembly, "object");
+
+try {
+    var i = new WebAssembly.Instance(
+        new WebAssembly.Module(
+            wasmTextToBinary('(module (func (export "") (result i32) (i32.const 42)))')));
+    assertEq(i.exports[""](), 42);
+} catch (e) {
+    if (String(e).match(/no WebAssembly compiler available/)) {
+        switch (wasmCompileMode()) {
+        case "none":
+            // This is fine: the limited feature set combined with command line
+            // compiler selection caused all compilers to be disabled.
+            break;
+        default:
+            // This is not fine: if there's a compiler available then we should
+            // not get an error.
+            throw e;
+        }
+    } else {
+        // Some other error, propagate it.
+        throw e;
+    }
+}

js/src/jit-test/tests/wasm/no-movwt.js

@@ -1,10 +0,0 @@
-// |jit-test| skip-if: !this.setARMHwCapFlags
-
-setARMHwCapFlags('vfp');
-
-if (typeof WebAssembly !== "undefined") {
-    var i = new WebAssembly.Instance(
-                new WebAssembly.Module(
-                    wasmTextToBinary('(module (func (export "") (result i32) (i32.const 42)))')));
-    assertEq(i.exports[""](), 42);
-}

js/src/jit/arm/Simulator-arm.h

@@ -197,8 +197,6 @@ class Simulator {
   Simulator();
   ~Simulator();
 
-  static bool supportsAtomics() { return HasLDSTREXBHD(); }
-
   // The currently executing Simulator instance. Potentially there can be one
   // for each native thread.
   static Simulator* Current();

js/src/jit/arm64/vixl/Simulator-vixl.h

@@ -529,9 +529,6 @@ class Simulator : public DecoderVisitor {
   static uintptr_t StackLimit() {
     return Simulator::Current()->stackLimit();
   }
-  static bool supportsAtomics() {
-    return true;
-  }
   template<typename T> T Read(uintptr_t address);
   template <typename T> void Write(uintptr_t address_, T value);
   JS::ProfilingFrameIterator::RegisterState registerState();

js/src/jit/mips32/Simulator-mips32.h

@@ -213,8 +213,6 @@ class Simulator {
   Simulator();
   ~Simulator();
 
-  static bool supportsAtomics() { return true; }
-
   // The currently executing Simulator instance. Potentially there can be one
   // for each native thread.
   static Simulator* Current();

js/src/jit/mips64/Simulator-mips64.h

@@ -215,8 +215,6 @@ class Simulator {
   Simulator();
   ~Simulator();
 
-  static bool supportsAtomics() { return true; }
-
   // The currently executing Simulator instance. Potentially there can be one
   // for each native thread.
   static Simulator* Current();

js/src/wasm/WasmJS.cpp

@@ -384,17 +384,18 @@ bool wasm::HasPlatformSupport(JSContext* cx) {
     return false;
   }
 
+#ifdef JS_CODEGEN_ARM
+  // Wasm threads require support for atomic operations.
+  if (!HasLDSTREXBHD()) {
+    return false;
+  }
+#endif
+
   // Wasm threads require 8-byte lock-free atomics.
   if (!jit::AtomicOperations::isLockfree8()) {
     return false;
   }
 
-#ifdef JS_SIMULATOR
-  if (!Simulator::supportsAtomics()) {
-    return false;
-  }
-#endif
-
   // Test only whether the compilers are supported on the hardware, not whether
   // they are enabled.
   return BaselinePlatformSupport() || IonPlatformSupport() ||