From 8dac7f572b2f490e16674cc7ed333dac5c93d280 Mon Sep 17 00:00:00 2001
From: Roger Chang <rogerycchang@google.com>
Date: Mon, 8 May 2023 14:30:04 +0800
Subject: [PATCH] arch-riscv: Refactor RVC decode flow when funct4==0b1001 and
 op==C2

the compressed instruction 0x901e (c.add zero, t2) should be decoded as
"c_add zero, t2" not c_ebreak

Change-Id: Ib2bd4b4d9739aa27ad290ead313e95b11b1727d1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70358
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
---
 src/arch/riscv/isa/decoder.isa | 28 +++++++++++-----------------
 1 file changed, 11 insertions(+), 17 deletions(-)

diff --git a/src/arch/riscv/isa/decoder.isa b/src/arch/riscv/isa/decoder.isa
index d89a0c9d5e..f22efb0bf0 100644
--- a/src/arch/riscv/isa/decoder.isa
+++ b/src/arch/riscv/isa/decoder.isa
@@ -365,27 +365,21 @@ decode QUADRANT default Unknown::unknown() {
                     Rc1 = rvSext(Rc2);
                 }});
             }
-            0x1: decode RC1 {
-                0x0: SystemOp::c_ebreak({{
-                    if (RC2 != 0) {
-                        return std::make_shared<IllegalInstFault>(
-                                "source reg x1", machInst);
-                    }
-                    return std::make_shared<BreakpointFault>(xc->pcState());
-                }}, IsSerializeAfter, IsNonSpeculative, No_OpClass);
-                default: decode RC2 {
-                    0x0: Jump::c_jalr({{
-                        if (RC1 == 0) {
-                            return std::make_shared<IllegalInstFault>(
-                                    "source reg x0", machInst);
-                        }
+            0x1: decode RC2 {
+                0x0: decode RC1 {
+                    0x0: SystemOp::c_ebreak({{
+                        return std::make_shared<BreakpointFault>(
+                            xc->pcState());
+                    }}, IsSerializeAfter, IsNonSpeculative, No_OpClass);
+                    default: Jump::c_jalr({{
                         ra = rvSext(NPC);
                         NPC = rvZext(Rc1);
                     }}, IsIndirectControl, IsUncondControl, IsCall);
-                    default: CompressedROp::c_add({{
-                        Rc1_sd = rvSext(Rc1_sd + Rc2_sd);
-                    }});
                 }
+                default: CompressedROp::c_add({{
+                    // RC1 == 0 is HINT
+                    Rc1_sd = rvSext(Rc1_sd + Rc2_sd);
+                }});
             }
         }
         format CompressedStore {