[ia32] Remove invalid indirect call/jump code

Indirect calls need a scratch register to load the target address. On
ia32 there's no easily-available scratch register.

This removes invalid code and documents a potential solution. But
ideally, this will remain unreachable since all inter-builtin calls
will be pc-relative.

Bug: v8:6666
Change-Id: I19e0ac699ee4757e3d5ec130b3e34a67cd1f851c
Reviewed-on: https://chromium-review.googlesource.com/1230096Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55999}

src/ia32/macro-assembler-ia32.cc

@@ -1725,14 +1725,16 @@ void TurboAssembler::CallCFunction(Register function, int num_arguments) {
 
 void TurboAssembler::Call(Handle<Code> code_object, RelocInfo::Mode rmode) {
   if (FLAG_embedded_builtins) {
-    // TODO(jgruber): Figure out which register we can clobber here.
     // TODO(jgruber): Pc-relative builtin-to-builtin calls.
-    Register scratch = kOffHeapTrampolineRegister;
     if (root_array_available_ && options().isolate_independent_code) {
-      IndirectLoadConstant(scratch, code_object);
-      lea(scratch, FieldOperand(scratch, Code::kHeaderSize));
-      call(scratch);
-      return;
+      // TODO(jgruber): There's no scratch register on ia32. Any call that
+      // requires loading a code object from the builtins constant table must:
+      // 1) spill two scratch registers, 2) load the target into scratch1, 3)
+      // store the target into a virtual register on the isolate using scratch2,
+      // 4) restore both scratch registers, and finally 5) call through the
+      // virtual register. All affected call sites should vanish once all
+      // builtins are embedded on ia32.
+      UNREACHABLE();
     } else if (options().inline_offheap_trampolines) {
       int builtin_index = Builtins::kNoBuiltinId;
       if (isolate()->builtins()->IsBuiltinHandle(code_object, &builtin_index) &&
@@ -1753,14 +1755,16 @@ void TurboAssembler::Call(Handle<Code> code_object, RelocInfo::Mode rmode) {
 
 void TurboAssembler::Jump(Handle<Code> code_object, RelocInfo::Mode rmode) {
   if (FLAG_embedded_builtins) {
-    // TODO(jgruber): Figure out which register we can clobber here.
     // TODO(jgruber): Pc-relative builtin-to-builtin calls.
-    Register scratch = kOffHeapTrampolineRegister;
     if (root_array_available_ && options().isolate_independent_code) {
-      IndirectLoadConstant(scratch, code_object);
-      lea(scratch, FieldOperand(scratch, Code::kHeaderSize));
-      jmp(scratch);
-      return;
+      // TODO(jgruber): There's no scratch register on ia32. Any call that
+      // requires loading a code object from the builtins constant table must:
+      // 1) spill two scratch registers, 2) load the target into scratch1, 3)
+      // store the target into a virtual register on the isolate using scratch2,
+      // 4) restore both scratch registers, and finally 5) call through the
+      // virtual register. All affected call sites should vanish once all
+      // builtins are embedded on ia32.
+      UNREACHABLE();
     } else if (options().inline_offheap_trampolines) {
       int builtin_index = Builtins::kNoBuiltinId;
       if (isolate()->builtins()->IsBuiltinHandle(code_object, &builtin_index) &&