[Bug?] DCache AMO store_data Returns Wrong Value

[canxin121]

Summary:

In the DCache module, the io.cpu.resp.bits.store_data field incorrectly returns the original store input data for Atomic Memory Operations (AMOs), instead of returning the computed result from the AMOALU.

Technical Details:

When an AMO instruction executes (e.g., amoadd, amoswap), the AMOALU computes the result and stores it in pstore1_storegen_data. However, the response field store_data was unconditionally assigned from pstore1_data (the original input from the CPU register), ignoring the AMO computation.

Location:

• File: src/main/scala/rocket/DCache.scala
• Line: ~975 (in unfixed version)

Impact:

This bug is latent in the current codebase because store_data is not actively used by downstream logic. However, it becomes visible when:

• Logging or tracing AMO operations that read this field
• Future features that rely on store_data to retrieve the actual stored value

Root Cause:

Missing runtime multiplexer to distinguish between AMO operations (which need the computed result) and regular STORE operations (which use the input data).

Fix:

Replace the unconditional assignment:

io.cpu.resp.bits.store_data := pstore1_data

With a conditional assignment based on operation type:

io.cpu.resp.bits.store_data := Mux(isAMO(pstore1_cmd), pstore1_storegen_data, pstore1_data)

This ensures AMO operations return the computed result while regular stores continue to return the input data.