sign-extend rs2 for AMOMIN.W/AMOMAX.W operations

[zyk632]

fix(atomalu): sign-extend rs2 for AMOMIN.W/AMOMAX.W operations

Summary

Fix a hardware bug in AtomALU where rs2 is not properly sign-extended for .W atomic operations.

Background & Changes

In LSU.scala:181 (AtomALU), the following issues were identified:

• AMOMIN.W / AMOMAX.W: rs2 was not sign-extended from 32-bit to 64-bit
• AMOMINU.W / AMOMAXU.W: rs2 was not zero-extended for unsigned comparison

Example: 0x00000000ffffffff should be treated as -1 for AMOMIN.W, but the original RTL treated it as a positive 64-bit value (0xffffffff).

Fix applied:

• Signed word operations (AMOMIN.W/AMOMAX.W): sign-extend lower 32 bits of rs2
• Unsigned word operations (AMOMINU.W/AMOMAXU.W): zero-extend lower 32 bits of rs2
• .D operations: unchanged (64-bit behavior)

Test Results

Before fix:
a0 different: right = 0xffffffffffffffff, wrong = 0x0000000000000001
After fix:
HIT GOOD TRAP at pc = 0x800000d2, exit code = 0
All 35 existing NutShell cputest images pass DiffTest (failed=0).

Modified Files

• src/main/scala/nutcore/backend/fu/LSU.scala: Fix rs2 extension for .W AMO operations

Release Notes

Fix signed/unsigned comparison behavior for AMOMIN.W/AMOMAX.W/AMOMINU.W/AMOMAXU.W operations in AtomALU.

• I have tested these changes locally