cpu-simple: Ignore writes to the "zero" register.

Rather than constantly overwriting the "zero" register to return its
value to zero, just ignore writes to it.

We assume here that the "zero" register is a standard RegVal type
register (ie not bigger than 64 bits) and is accessed as such.

Change-Id: I06029b78103019c668647569c6037ca64a4d9c76
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49709
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/minor/exec_context.hh

@@ -97,7 +97,6 @@ class ExecContext : public gem5::ExecContext
         pcState(*inst->pc);
         setPredicate(inst->readPredicate());
         setMemAccPredicate(inst->readMemAccPredicate());
-        thread.setIntReg(zeroReg, 0);
     }
 
     ~ExecContext()

src/cpu/simple/base.cc

@@ -307,9 +307,6 @@ BaseSimpleCPU::preExecute()
     SimpleExecContext &t_info = *threadInfo[curThread];
     SimpleThread* thread = t_info.thread;
 
-    // maintain $r0 semantics
-    thread->setIntReg(zeroReg, 0);
-
     // resets predicates
     t_info.setPredicate(true);
     t_info.setMemAccPredicate(true);

src/cpu/simple_thread.hh

@@ -405,6 +405,9 @@ class SimpleThread : public ThreadState, public ThreadContext
         auto &reg_file = regFiles[reg.classValue()];
         const auto &reg_class = reg_file.regClass;
 
+        if (reg.index() == reg_class.zeroReg())
+            return;
+
         DPRINTFV(reg_class.debug(), "Setting %s register %s (%d) to %#x.\n",
                 reg.className(), reg_class.regName(arch_reg), idx, val);
         reg_file.reg(idx) = val;
@@ -418,6 +421,9 @@ class SimpleThread : public ThreadState, public ThreadContext
         auto &reg_file = regFiles[reg.classValue()];
         const auto &reg_class = reg_file.regClass;
 
+        if (reg.index() == reg_class.zeroReg())
+            return;
+
         DPRINTFV(reg_class.debug(), "Setting %s register %d to %#x.\n",
                 reg.className(), idx, val);
         reg_file.reg(idx) = val;