rewriter: support the 6 x86 param regs for post condition functions

This is done by pushing the param regs onto the stack in the prologue
(if there's a post condition function), and then
popping them before we call the post condition function.

Next we'll work on supporting more than the 6 param regs,
as well as passing the return value.

tools/rewriter/GenCallAsm.cpp

@@ -445,7 +445,7 @@ static AsmWriter get_asmwriter(bool as_macro) {
   return {.ss = {}, .terminator = terminator};
 }
 
-static void emit_prologue(AsmWriter &aw, uint32_t caller_pkey, uint32_t target_pkey, Arch arch) {
+static void emit_prologue(AsmWriter &aw, uint32_t caller_pkey, uint32_t target_pkey, Arch arch, bool save_param_regs) {
   if (arch == Arch::X86) {
     // Save the old frame pointer and set the frame pointer for the call gate
     add_asm_line(aw, "pushq %rbp");
@@ -456,6 +456,15 @@ static void emit_prologue(AsmWriter &aw, uint32_t caller_pkey, uint32_t target_p
     for (auto &r : x86_preserved_registers) {
       add_asm_line(aw, "pushq %"s + r);
     }
+    if (save_param_regs) {
+      // Push first 6 registers for args onto the stack, too,
+      // so that we can store them for the post condition function.
+      add_comment_line(aw, "Save param regs for post condition call");
+      for (auto &r : x86_int_param_reg_order) {
+        add_asm_line(aw, "pushq %"s + r);
+      }
+    }
+    // TODO this for arm, too
   } else if (arch == Arch::Aarch64) {
     // Frame pointer and link register need to be saved first, to make backtraces work
     add_asm_line(aw, "stp x29, x30, [sp, #-16]!");
@@ -816,6 +825,13 @@ static void emit_set_return_pkru(AsmWriter &aw, uint32_t caller_pkey, Arch arch)
 
 static void emit_post_condition_fn_call(AsmWriter &aw, Arch arch, std::string_view target_post_condition_name) {
   llvm::errs() << "emitting post condition call to " << target_post_condition_name << "\n";
+  if (arch == Arch::X86) {
+    add_comment_line(aw, "Restore param regs for post condition call");
+    for (auto it = x86_int_param_reg_order.rbegin(); it != x86_int_param_reg_order.rend(); ++it) {
+      auto &r = *it;
+      add_asm_line(aw, "popq %"s + r);
+    }
+  }
   add_comment_line(aw, "Align stack");
   add_asm_line(aw, "subq $8, %rsp");
   add_comment_line(aw, "Call post condition function");
@@ -990,6 +1006,14 @@ std::string emit_asm_wrapper(AbiSignature &sig,
     stack_alignment = (compartment_stack_space + 8) % 16;
   }
 
+  // For now, we hardcode the existence and name of the post-condition function.
+  // The name is `${target_name}_post_condition`,
+  // and we only do this for `dav1d_get_picture`.
+  std::optional<std::string> target_post_condition_name = std::nullopt;
+  if (target_name && *target_name == "dav1d_get_picture") {
+    target_post_condition_name = *target_name + "_post_condition";
+  }
+
   add_comment_line(aw, "Wrapper for "s + sig_string(sig, target_name) + ":");
   add_asm_line(aw, ".text");
   if (kind != WrapperKind::PointerToStatic) {
@@ -1001,7 +1025,7 @@ std::string emit_asm_wrapper(AbiSignature &sig,
   add_asm_line(aw, ".type "s + wrapper_name + ", @function");
   add_asm_line(aw, wrapper_name + ":");
 
-  emit_prologue(aw, caller_pkey, target_pkey, arch);
+  emit_prologue(aw, caller_pkey, target_pkey, arch, target_post_condition_name.has_value());
 
   if (arch == Arch::X86) {
     add_raw_line(aw, llvm::formatv("ASSERT_PKRU({0:x8}) \"\\n\"", ~((0b11 << (2 * caller_pkey)) | 0b11)));
@@ -1041,13 +1065,6 @@ std::string emit_asm_wrapper(AbiSignature &sig,
 
   // Call the post-condition function, if one was specified.
   // The call happens in the caller's compartment.
-  // For now, we hardcode the existence and name of the post-condition function.
-  // The name is `${target_name}_post_condition`,
-  // and we only do this for `dav1d_get_picture`.
-  std::optional<std::string> target_post_condition_name = std::nullopt;
-  if (target_name && *target_name == "dav1d_get_picture") {
-    target_post_condition_name = *target_name + "_post_condition";
-  }
   if (target_post_condition_name) {
     emit_post_condition_fn_call(aw, arch, *target_post_condition_name);
   }