[ia32] Change descriptor of frame dropper trampoline

This changes the interface descriptor for the frame dropper trampoline
to not use the kRootRegister (ebx) anymore.

Change-Id: I15df6af57f70d8aeea6c97298b437ad96af19cd4
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/1193242
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55484}

src/debug/ia32/debug-ia32.cc

@@ -28,19 +28,19 @@ void DebugCodegen::GenerateHandleDebuggerStatement(MacroAssembler* masm) {
 
 void DebugCodegen::GenerateFrameDropperTrampoline(MacroAssembler* masm) {
   // Frame is being dropped:
-  // - Drop to the target frame specified by ebx.
+  // - Drop to the target frame specified by eax.
   // - Look up current function on the frame.
   // - Leave the frame.
   // - Restart the frame by calling the function.
-  __ mov(ebp, ebx);
+  __ mov(ebp, eax);
   __ mov(edi, Operand(ebp, JavaScriptFrameConstants::kFunctionOffset));
   __ leave();
 
-  __ mov(ebx, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
+  __ mov(eax, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
   __ movzx_w(
-      ebx, FieldOperand(ebx, SharedFunctionInfo::kFormalParameterCountOffset));
+      eax, FieldOperand(eax, SharedFunctionInfo::kFormalParameterCountOffset));
 
-  ParameterCount dummy(ebx);
+  ParameterCount dummy(eax);
   __ InvokeFunction(edi, dummy, dummy, JUMP_FUNCTION);
 }
 

src/ia32/interface-descriptors-ia32.cc

@@ -266,7 +266,7 @@ void ResumeGeneratorDescriptor::InitializePlatformSpecific(
 void FrameDropperTrampolineDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
   Register registers[] = {
-      ebx,  // loaded new FP
+      eax,  // loaded new FP
   };
   data->InitializePlatformSpecific(arraysize(registers), registers);
 }

src/ia32/macro-assembler-ia32.cc

@@ -410,8 +410,8 @@ void MacroAssembler::MaybeDropFrames() {
   // Check whether we need to drop frames to restart a function on the stack.
   ExternalReference restart_fp =
       ExternalReference::debug_restart_fp_address(isolate());
-  mov(ebx, StaticVariable(restart_fp));
-  test(ebx, ebx);
+  mov(eax, StaticVariable(restart_fp));
+  test(eax, eax);
   j(not_zero, BUILTIN_CODE(isolate(), FrameDropperTrampoline),
     RelocInfo::CODE_TARGET);
 }