[Liftoff] Optimize function prologue for arm64

Instead of spilling x0 and x1 to the stack, use temp registers which
should always be available in this situation. This avoids the spill and
load.
Also, switch from holding {start} and {end} in the register to holding
{start} and {count}. This make it easier to correctly initialize the
registers.

R=joey.gouly@arm.com

Bug: v8:9830
Change-Id: I5fc8b8603afaf12fad2ba66cd7a75555ef31bd4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859618
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64332}

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

@@ -406,9 +406,9 @@ void LiftoffAssembler::FillStackSlotsWithZero(uint32_t index, uint32_t count) {
   RecordUsedSpillSlot(last_stack_slot);
 
   int max_stp_offset = -liftoff::GetStackSlotOffset(index + count - 1);
-  if (count <= 20 && IsImmLSPair(max_stp_offset, kXRegSizeLog2)) {
-    // Special straight-line code for up to 20 slots. Generates one
-    // instruction per two slots (<= 10 instructions total).
+  if (count <= 12 && IsImmLSPair(max_stp_offset, kXRegSizeLog2)) {
+    // Special straight-line code for up to 12 slots. Generates one
+    // instruction per two slots (<= 6 instructions total).
     for (; count > 1; count -= 2) {
       STATIC_ASSERT(kStackSlotSize == kSystemPointerSize);
       stp(xzr, xzr, liftoff::GetStackSlot(index + count - 1));
@@ -416,19 +416,19 @@ void LiftoffAssembler::FillStackSlotsWithZero(uint32_t index, uint32_t count) {
     DCHECK(count == 0 || count == 1);
     if (count) str(xzr, liftoff::GetStackSlot(index));
   } else {
-    // General case for bigger counts (7 instructions).
-    // Use x0 for start address (inclusive), x1 for end address (exclusive).
-    Push(x1, x0);
-    Sub(x0, fp, Operand(liftoff::GetStackSlotOffset(last_stack_slot)));
-    Sub(x1, fp, Operand(liftoff::GetStackSlotOffset(index) - kStackSlotSize));
+    // General case for bigger counts (5-8 instructions).
+    UseScratchRegisterScope temps(this);
+    Register address_reg = temps.AcquireX();
+    // This {Sub} might use another temp register if the offset is too large.
+    Sub(address_reg, fp, liftoff::GetStackSlotOffset(last_stack_slot));
+    Register count_reg = temps.AcquireX();
+    Mov(count_reg, count);
 
     Label loop;
     bind(&loop);
-    str(xzr, MemOperand(x0, /* offset */ kSystemPointerSize, PostIndex));
-    cmp(x0, x1);
-    b(&loop, ne);
-
-    Pop(x0, x1);
+    sub(count_reg, count_reg, 1);
+    str(xzr, MemOperand(address_reg, kSystemPointerSize, PostIndex));
+    cbnz(count_reg, &loop);
   }
 }