cppgc: Maintain frame pointer for x64 stack scan

Maintain a proper frame pointer in rbp when going through the
trampoline. This ensures that e.g. gdb can properly unwind the
stack.

Bug: chromium:1056170
Change-Id: I7a9252316e7ce8d83cf54e1efc801b92476bc4ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2128050Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66920}

src/heap/cppgc/asm/x64/push_registers.S

@@ -27,9 +27,12 @@ PushAllRegistersAndIterateStack:
     // on the stack and we push 56 bytes which maintains 16-byte stack alignment
     // at the call.
     // Source: https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf
+    //
+    // rbp is callee-saved. Maintain proper frame pointer for debugging.
+    push %rbp
+    mov %rsp, %rbp
     push $0xCDCDCD  // Dummy for alignment.
     push %rbx
-    push %rbp
     push %r12
     push %r13
     push %r14
@@ -42,7 +45,8 @@ PushAllRegistersAndIterateStack:
     mov %rsp, %rdx
     // Call the callback.
     call *%r8
-    // Pop the callee-saved registers. None of them were modified so no
-    // restoring is needed.
-    add $56, %rsp
+    // Pop the callee-saved registers.
+    add $48, %rsp
+    // Restore rbp as it was used as frame pointer.
+    pop %rbp
     ret

src/heap/cppgc/asm/x64/push_registers_win.S

@@ -16,11 +16,14 @@ PushAllRegistersAndIterateStack:
     ;; on the stack and we push 72 bytes which maintains 16-byte stack alignment
     ;; at the call.
     ;; Source: https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention
+    ;;
+    ;; rbp is callee-saved. Maintain proper frame pointer for debugging.
+    push rbp
+    mov rbp, rsp
     push 0CDCDCDh  ;; Dummy for alignment.
     push rsi
     push rdi
     push rbx
-    push rbp
     push r12
     push r13
     push r14
@@ -33,9 +36,10 @@ PushAllRegistersAndIterateStack:
     mov r8, rsp
     ;; Call the callback.
     call r9
-    ;; Pop the callee-saved registers. None of them were modified so no
-    ;; restoring is needed.
-    add rsp, 72
+    ;; Pop the callee-saved registers.
+    add rsp, 64
+    ;; Restore rbp as it was used as frame pointer.
+    pop rbp
     ret
 
 end