[wasm][liftoff] Add comment to reserving stack space

R=clemensb@chromium.org

Bug: v8:11074
Change-Id: I43036a826008027cf44179f1236f4feb9ef4c83b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2554608Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71372}

src/wasm/baseline/arm64/liftoff-assembler-arm64.h

@@ -182,6 +182,10 @@ inline void EmitAllTrue(LiftoffAssembler* assm, LiftoffRegister dst,
 int LiftoffAssembler::PrepareStackFrame() {
   int offset = pc_offset();
   InstructionAccurateScope scope(this, 1);
+  // Next we reserve the memory for the whole stack frame. We do not know yet
+  // how big the stack frame will be so we just emit a placeholder instruction.
+  // PatchPrepareStackFrame will patch this in order to increase the stack
+  // appropriately.
   sub(sp, sp, 0);
   return offset;
 }

src/wasm/baseline/ia32/liftoff-assembler-ia32.h

@@ -159,6 +159,10 @@ constexpr int kSubSpSize = 6;  // 6 bytes for "sub esp, <imm32>"
 
 int LiftoffAssembler::PrepareStackFrame() {
   int offset = pc_offset();
+  // Next we reserve the memory for the whole stack frame. We do not know yet
+  // how big the stack frame will be so we just emit a placeholder instruction.
+  // PatchPrepareStackFrame will patch this in order to increase the stack
+  // appropriately.
   sub_sp_32(0);
   DCHECK_EQ(liftoff::kSubSpSize, pc_offset() - offset);
   return offset;

src/wasm/baseline/x64/liftoff-assembler-x64.h

@@ -133,6 +133,10 @@ constexpr int kSubSpSize = 7;  // 7 bytes for "subq rsp, <imm32>"
 
 int LiftoffAssembler::PrepareStackFrame() {
   int offset = pc_offset();
+  // Next we reserve the memory for the whole stack frame. We do not know yet
+  // how big the stack frame will be so we just emit a placeholder instruction.
+  // PatchPrepareStackFrame will patch this in order to increase the stack
+  // appropriately.
   sub_sp_32(0);
   DCHECK_EQ(liftoff::kSubSpSize, pc_offset() - offset);
   return offset;