Implement Hauser misa.C misalignment proposal (#187)

See https://github.com/riscv/riscv-isa-manual/commit/0472bcdd166f45712492829a250e228bb45fa5e7

- Reads of xEPC[1] are masked when RVC is disabled
- Writes to MISA are suppressed if they would cause a misaligned fetch
- Misaligned PCs no longer need to be checked upon fetch

4 files changed, 12 insertions, 6 deletions

diff --git a/riscv/decode.h b/riscv/decode.h
index 596a2ad..8fc8ada 100644
--- a/riscv/decode.h
+++ b/riscv/decode.h
@@ -206,7 +206,7 @@ private:
      } while(0)
 
 #define set_pc_and_serialize(x) \
-  do { reg_t __npc = (x); \
+  do { reg_t __npc = (x) & p->pc_alignment_mask(); \
        npc = PC_SERIALIZE_AFTER; \
        STATE.pc = __npc; \
      } while(0)
diff --git a/riscv/execute.cc b/riscv/execute.cc
index b302daa..9d1fb87 100644
--- a/riscv/execute.cc
+++ b/riscv/execute.cc
@@ -113,7 +113,6 @@ void processor_t::step(size_t n)
          default: abort(); \
        } \
        pc = state.pc; \
-       check_pc_alignment(pc); \
        break; \
      } else { \
        state.pc = pc; \
diff --git a/riscv/processor.cc b/riscv/processor.cc
index 7a5df90..548c649 100644
--- a/riscv/processor.cc
+++ b/riscv/processor.cc
@@ -450,6 +450,10 @@ void processor_t::set_csr(int which, reg_t val)
     case CSR_MCAUSE: state.mcause = val; break;
     case CSR_MTVAL: state.mtval = val; break;
     case CSR_MISA: {
+      // the write is ignored if increasing IALIGN would misalign the PC
+      if (!(val & (1L << ('C' - 'A'))) && (state.pc & 2))
+        break;
+
       if (!(val & (1L << ('F' - 'A'))))
         val &= ~(1L << ('D' - 'A'));
 
@@ -592,7 +596,7 @@ reg_t processor_t::get_csr(int which)
     }
     case CSR_SIP: return state.mip & state.mideleg;
     case CSR_SIE: return state.mie & state.mideleg;
-    case CSR_SEPC: return state.sepc;
+    case CSR_SEPC: return state.sepc & pc_alignment_mask();
     case CSR_STVAL: return state.stval;
     case CSR_STVEC: return state.stvec;
     case CSR_SCAUSE:
@@ -607,7 +611,7 @@ reg_t processor_t::get_csr(int which)
     case CSR_MSTATUS: return state.mstatus;
     case CSR_MIP: return state.mip;
     case CSR_MIE: return state.mie;
-    case CSR_MEPC: return state.mepc;
+    case CSR_MEPC: return state.mepc & pc_alignment_mask();
     case CSR_MSCRATCH: return state.mscratch;
     case CSR_MCAUSE: return state.mcause;
     case CSR_MTVAL: return state.mtval;
@@ -668,7 +672,7 @@ reg_t processor_t::get_csr(int which)
         return v;
       }
     case CSR_DPC:
-      return state.dpc;
+      return state.dpc & pc_alignment_mask();
     case CSR_DSCRATCH:
       return state.dscratch;
   }
diff --git a/riscv/processor.h b/riscv/processor.h
index ace86f9..3e67215 100644
--- a/riscv/processor.h
+++ b/riscv/processor.h
@@ -188,8 +188,11 @@ public:
     if (ext >= 'a' && ext <= 'z') ext += 'A' - 'a';
     return ext >= 'A' && ext <= 'Z' && ((state.misa >> (ext - 'A')) & 1);
   }
+  reg_t pc_alignment_mask() {
+    return ~(reg_t)(supports_extension('C') ? 0 : 2);
+  }
   void check_pc_alignment(reg_t pc) {
-    if (unlikely(pc & 2) && !supports_extension('C'))
+    if (unlikely(pc & ~pc_alignment_mask()))
       throw trap_instruction_address_misaligned(pc);
   }
   reg_t legalize_privilege(reg_t);