cppgc: Conservative stack scan for non-Windows ia32 on clang

The difference to Chromium is that we support frame pointers and thus
integrate better with debugging in e.g. gdb.

Bug: chromium:1056170
Change-Id: I2109744d07a7b3af158ed3e0fe731f890b192eee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2129630
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66967}

BUILD.gn

@@ -352,6 +352,10 @@ config("cppgc_base_config") {
     if (is_clang) {
       defines = [ "CPPGC_SUPPORTS_CONSERVATIVE_STACK_SCAN" ]
     }
+  } else if (target_cpu == "x86") {
+    if (is_clang && !is_win) {
+      defines = [ "CPPGC_SUPPORTS_CONSERVATIVE_STACK_SCAN" ]
+    }
   }
 }
 
@@ -3967,6 +3971,10 @@ v8_source_set("cppgc_base") {
     if (is_clang) {
       sources += [ "src/heap/cppgc/asm/x64/push_registers_clang.cc" ]
     }
+  } else if (target_cpu == "x86") {
+    if (is_clang && !is_win) {
+      sources += [ "src/heap/cppgc/asm/ia32/push_registers_clang.cc" ]
+    }
   }
 
   configs = [

src/heap/cppgc/asm/ia32/push_registers_clang.cc

+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Push all callee-saved registers to get them on the stack for conservative
+// stack scanning.
+//
+// See asm/x64/push_registers_clang.cc for why the function is not generated
+// using clang.
+
+// Do not depend on V8_TARGET_OS_* defines as some embedders may override the
+// GN toolchain (e.g. ChromeOS) and not provide them.
+#ifdef _WIN32
+
+#error "Not yet supported"
+
+#else  // !_WIN32
+
+// We maintain 16-byte alignment at calls. There is an 4-byte return address
+// on the stack and we push 28 bytes which maintains 16-byte stack alignment
+// at the call.
+//
+// The following assumes cdecl calling convention.
+// Source: https://en.wikipedia.org/wiki/X86_calling_conventions#cdecl
+asm(".globl PushAllRegistersAndIterateStack             \n"
+    ".hidden PushAllRegistersAndIterateStack            \n"
+    "PushAllRegistersAndIterateStack:                   \n"
+    // [ IterateStackCallback ]
+    // [ StackVisitor*        ]
+    // [ Stack*               ]
+    // [ ret                  ]
+    // ebp is callee-saved. Maintain proper frame pointer for debugging.
+    "  push %ebp                                        \n"
+    "  movl %esp, %ebp                                  \n"
+    "  push %ebx                                        \n"
+    "  push %esi                                        \n"
+    "  push %edi                                        \n"
+    // Save 3rd parameter (IterateStackCallback).
+    "  movl 28(%esp), %ecx                              \n"
+    // Pass 3rd parameter as esp (stack pointer).
+    "  push %esp                                        \n"
+    // Pass 2nd parameter (StackVisitor*).
+    "  push 28(%esp)                                    \n"
+    // Pass 1st parameter (Stack*).
+    "  push 28(%esp)                                    \n"
+    "  call *%ecx                                       \n"
+    // Pop the callee-saved registers.
+    "  addl $24, %esp                                   \n"
+    // Restore rbp as it was used as frame pointer.
+    "  pop %ebp                                         \n"
+    "  ret                                              \n");
+
+#endif  // !_WIN32