[liftoff] Fix --trace-wasm-memory

With statically in-bounds memory accesses (implemented in
https://crrev.com/c/2919827) we would only have an offset but no index
register for {TraceMemoryOperation}. This CL fixes that situation.

R=thibaudm@chromium.org

Bug: chromium:1248024
Change-Id: I856b263a560cb71791c61e446e78dd99c9664190
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3149464Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76763}

src/wasm/baseline/liftoff-compiler.cc

@@ -2743,14 +2743,17 @@ class LiftoffCompiler {
     // Before making the runtime call, spill all cache registers.
     __ SpillAllRegisters();
 
-    LiftoffRegList pinned = LiftoffRegList::ForRegs(index);
+    LiftoffRegList pinned;
+    if (index != no_reg) pinned.set(index);
     // Get one register for computing the effective offset (offset + index).
     LiftoffRegister effective_offset =
         pinned.set(__ GetUnusedRegister(kGpReg, pinned));
-    // TODO(clemensb): Do a 64-bit addition here if memory64 is used.
     DCHECK_GE(kMaxUInt32, offset);
     __ LoadConstant(effective_offset, WasmValue(static_cast<uint32_t>(offset)));
-    __ emit_i32_add(effective_offset.gp(), effective_offset.gp(), index);
+    if (index != no_reg) {
+      // TODO(clemensb): Do a 64-bit addition here if memory64 is used.
+      __ emit_i32_add(effective_offset.gp(), effective_offset.gp(), index);
+    }
 
     // Get a register to hold the stack slot for MemoryTracingInfo.
     LiftoffRegister info = pinned.set(__ GetUnusedRegister(kGpReg, pinned));

test/mjsunit/regress/wasm/regress-1248024.js

+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --liftoff-only --trace-wasm-memory
+
+d8.file.execute('test/mjsunit/wasm/wasm-module-builder.js');
+
+const builder = new WasmModuleBuilder();
+builder.addMemory(16, 17);
+// Generate function 1 (out of 3).
+builder.addFunction('load', kSig_i_v)
+    .addBody([
+      // body:
+      kExprI32Const, 0,         // i32.const
+      kExprI32LoadMem8U, 0, 5,  // i32.load8_u
+    ])
+    .exportFunc();
+const instance = builder.instantiate();
+instance.exports.load();