[liftoff][cleanup] Replace loop by straight-line code

This removes a loop that runs exactly one or two times by straight-line
code for the two cases. This should make it more readable and easier to
maintain.

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: I242bc4a7f7333b04ec39e79dc530625fb5e2305f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2287505Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68800}

src/wasm/baseline/liftoff-compiler.cc

@@ -467,32 +467,34 @@ class LiftoffCompiler {
 
   // Returns the number of inputs processed (1 or 2).
   uint32_t ProcessParameter(ValueType type, uint32_t input_idx) {
-    const int num_lowered_params = 1 + needs_gp_reg_pair(type);
-    ValueType lowered_type = needs_gp_reg_pair(type) ? kWasmI32 : type;
-    RegClass rc = reg_class_for(lowered_type);
-    // Initialize to anything, will be set in the loop and used afterwards.
-    LiftoffRegister reg = kGpCacheRegList.GetFirstRegSet();
-    LiftoffRegList pinned;
-    for (int pair_idx = 0; pair_idx < num_lowered_params; ++pair_idx) {
-      compiler::LinkageLocation param_loc =
-          descriptor_->GetInputLocation(input_idx + pair_idx);
-      // Initialize to anything, will be set in both arms of the if.
-      LiftoffRegister in_reg = kGpCacheRegList.GetFirstRegSet();
-      if (param_loc.IsRegister()) {
-        DCHECK(!param_loc.IsAnyRegister());
-        in_reg = LiftoffRegister::from_external_code(rc, type,
-                                                     param_loc.AsRegister());
-      } else if (param_loc.IsCallerFrameSlot()) {
-        in_reg = __ GetUnusedRegister(rc, pinned);
-        __ LoadCallerFrameSlot(in_reg, -param_loc.AsCallerFrameSlot(),
-                               lowered_type);
+    const bool needs_pair = needs_gp_reg_pair(type);
+    const ValueType reg_type = needs_pair ? kWasmI32 : type;
+    const RegClass rc = reg_class_for(reg_type);
+
+    auto LoadToReg = [this, reg_type, rc](compiler::LinkageLocation location,
+                                          LiftoffRegList pinned) {
+      if (location.IsRegister()) {
+        DCHECK(!location.IsAnyRegister());
+        return LiftoffRegister::from_external_code(rc, reg_type,
+                                                   location.AsRegister());
       }
-      reg = pair_idx == 0 ? in_reg
-                          : LiftoffRegister::ForPair(reg.gp(), in_reg.gp());
-      pinned.set(reg);
+      DCHECK(location.IsCallerFrameSlot());
+      LiftoffRegister reg = __ GetUnusedRegister(rc, pinned);
+      __ LoadCallerFrameSlot(reg, -location.AsCallerFrameSlot(), reg_type);
+      return reg;
+    };
+
+    LiftoffRegister reg =
+        LoadToReg(descriptor_->GetInputLocation(input_idx), {});
+    if (needs_pair) {
+      LiftoffRegister reg2 =
+          LoadToReg(descriptor_->GetInputLocation(input_idx + 1),
+                    LiftoffRegList::ForRegs(reg));
+      reg = LiftoffRegister::ForPair(reg.gp(), reg2.gp());
     }
     __ PushRegister(type, reg);
-    return num_lowered_params;
+
+    return needs_pair ? 2 : 1;
   }
 
   void StackCheck(WasmCodePosition position) {