[turbofan][x64] When spilling 32bit values, reload only 32 bits

When we spill a register that we know contains only 32 interesting bits
and then reload it from the spill slot, it's enough to reload its lower
half. This may save a few bytes, and guards against accidental changes
to the upper half (e.g. via pointer decompression).

Bug: v8:13216
Change-Id: I1d950d6e33d8ae94cf385af4f3e1db028bf333c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3854506Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82704}

src/compiler/backend/x64/code-generator-x64.cc

@@ -5231,7 +5231,19 @@ void CodeGenerator::AssembleMove(InstructionOperand* source,
     case MoveType::kStackToRegister: {
       Operand src = g.ToOperand(source);
       if (source->IsStackSlot()) {
-        __ movq(g.ToRegister(destination), src);
+        MachineRepresentation mr =
+            LocationOperand::cast(source)->representation();
+        if (mr == MachineRepresentation::kWord32 ||
+            mr == MachineRepresentation::kCompressed ||
+            mr == MachineRepresentation::kCompressedPointer) {
+          // When we need only 32 bits, move only 32 bits. Benefits:
+          // - Save a byte here and there (depending on the destination
+          //   register; "movl eax, ..." is smaller than "movq rax, ...").
+          // - Safeguard against accidental decompression of compressed slots.
+          __ movl(g.ToRegister(destination), src);
+        } else {
+          __ movq(g.ToRegister(destination), src);
+        }
       } else {
         DCHECK(source->IsFPStackSlot());
         XMMRegister dst = g.ToDoubleRegister(destination);