Some bugfixes for CSR reading and setting FS for fflags updates (#43)

* csrrc[i] and csrrs[i] don't write CSRs if rs/zimm == 0

* Dirty fp state when setting new fp exceptions

* Set FS to dirty for all non-zero fflags writes.

riscv/decode.h

@@ -184,7 +184,10 @@ private:
 #define require_fp require((STATE.mstatus & MSTATUS_FS) != 0)
 #define require_accelerator require((STATE.mstatus & MSTATUS_XS) != 0)
 
-#define set_fp_exceptions ({ STATE.fflags |= softfloat_exceptionFlags; \
+#define set_fp_exceptions ({ if (softfloat_exceptionFlags) { \
+                               dirty_fp_state; \
+                               STATE.fflags |= softfloat_exceptionFlags; \
+                             } \
                              softfloat_exceptionFlags = 0; })
 
 #define sext32(x) ((sreg_t)(int32_t)(x))

riscv/insns/csrrc.h

@@ -1,4 +1,7 @@
-int csr = validate_csr(insn.csr(), true);
+bool write = insn.rs1() != 0;
+int csr = validate_csr(insn.csr(), write);
 reg_t old = p->get_csr(csr);
-p->set_csr(csr, old & ~RS1);
+if (write) {
+  p->set_csr(csr, old & ~RS1);
+}
 WRITE_RD(sext_xlen(old));

riscv/insns/csrrci.h

@@ -1,4 +1,7 @@
-int csr = validate_csr(insn.csr(), true);
+bool write = insn.rs1() != 0;
+int csr = validate_csr(insn.csr(), write);
 reg_t old = p->get_csr(csr);
-p->set_csr(csr, old & ~(reg_t)insn.rs1());
+if (write) {
+  p->set_csr(csr, old & ~(reg_t)insn.rs1());
+}
 WRITE_RD(sext_xlen(old));

riscv/insns/csrrs.h

@@ -1,4 +1,7 @@
-int csr = validate_csr(insn.csr(), insn.rs1() != 0);
+bool write = insn.rs1() != 0;
+int csr = validate_csr(insn.csr(), write);
 reg_t old = p->get_csr(csr);
-p->set_csr(csr, old | RS1);
+if (write) {
+  p->set_csr(csr, old | RS1);
+}
 WRITE_RD(sext_xlen(old));

riscv/insns/csrrsi.h

@@ -1,4 +1,7 @@
-int csr = validate_csr(insn.csr(), true);
+bool write = insn.rs1() != 0;
+int csr = validate_csr(insn.csr(), write);
 reg_t old = p->get_csr(csr);
-p->set_csr(csr, old | insn.rs1());
+if (write) {
+  p->set_csr(csr, old | insn.rs1());
+}
 WRITE_RD(sext_xlen(old));