Fix C extension instruction fetch logic and add cross-compilation support

[Copilot]

This PR resolves critical issues with C extension tests and cross-compilation that were causing workflow failures.

Issues Fixed

1. C Extension Test Failures

The C extension tests were failing because the fetch logic didn't properly handle PC alignment when extracting 16-bit compressed instructions from 32-bit memory words.

Problem: When PC incremented by 2 after a compressed instruction, the processor would fetch from the same 32-bit word but incorrectly extract the lower 16 bits instead of the upper 16 bits.

Solution: Enhanced the fetch logic in vigna_core.v to check PC[1] alignment:

• When PC[1] == 0: Extract lower 16 bits for compressed instructions
• When PC[1] == 1: Extract upper 16 bits for compressed instructions

// Before: Always used lower 16 bits
if (i_rdata[1:0] != 2'b11) begin
    inst[15:0] <= i_rdata[15:0];  // Always lower 16 bits
end

// After: Check PC alignment  
if (pc[1] == 1'b0) begin
    if (i_rdata[1:0] != 2'b11) begin
        inst[15:0] <= i_rdata[15:0];   // Lower 16 bits when aligned
    end
end else begin
    if (i_rdata[17:16] != 2'b11) begin
        inst[15:0] <= i_rdata[31:16];  // Upper 16 bits when misaligned
    end
end

2. Cross-Compilation Issues

The workflow was skipping cross-compilation because the RISC-V toolchain wasn't available in the CI environment.

Solution: Added installation of gcc-riscv64-linux-gnu package to enable cross-compilation of C test programs.

Test Results

All configuration tests now pass:

• RV32IC (Base + Compressed): 14/14 tests PASSED ✅
• RV32IMC (Base + Multiply + Compressed): 14/14 tests PASSED ✅
• RV32IMC+Zicsr (Full featured): 14/14 tests PASSED ✅
• Cross-compiled C programs: 13/13 tests PASSED ✅

Additional Improvements

New Makefile Targets

Added comprehensive C extension test infrastructure:

• c_extension_test: Run C extension testbench
• c_extension_quick_test: Quick C extension test without waveforms
• c_extension_syntax: Syntax check for C extension testbench
• c_extension_wave: View C extension test waveforms

Enhanced Test Coverage

The C extension functionality is now properly tested in multiple configurations:

• Basic C.LI (load immediate) instructions
• C.ADDI (add immediate) instructions
• Mixed 16-bit/32-bit instruction streams
• PC increment handling for compressed instructions

Before this fix, the comprehensive test suite showed:

Setting up C extension operations test...
  FAIL: C extension result = 20 (expected 15)
Tests Failed: 1

After the fix:

Setting up C extension operations test...  
  PASS: C extension result = 15 (expected 15)
Tests Passed: 14, Tests Failed: 0

Fixes #23.

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[Copilot]

Pull Request Overview

This PR fixes critical issues with the C extension instruction fetch logic and adds cross-compilation support. Key changes include:

• Updating instruction fetching in vigna_core.v to properly extract compressed instructions based on PC alignment.
• Enhancing the C extension testbench to cover both lower and upper 16-bit instruction extraction, including a fallback for misaligned fetches.
• Adding new Makefile targets for running, compiling, and cleaning up C extension tests and waveforms.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
vigna_core.v	Improved fetch logic with PC alignment checks for compressed ops.
sim/c_extension_testbench.v	Updated testbench to use packed C extension instructions and refine display messages.
Makefile	Added targets and cleanup rules for C extension testing and cross-compilation.

Comments suppressed due to low confidence (1)

Makefile:229

• Confirm that including the C_EXTENSION files in the cleanup process is intentional and consistent with the handling of other testbench artifacts.

rm -f $(VVP_FILE) $(VCD_FILE) ... $(C_EXTENSION_VVP_FILE) $(C_EXTENSION_VCD_FILE)