riscv,x86: Stop using the arch Nop machine instruction unnecessarily.

That particular ExtMachInst is a convenient placeholder, but a value
of 0 in RISCV or a static uninitialized ExtMachInst (which will
therefore be all zeroes) on x86 works just as well, and removes the
need for an ISA specific constant.

Also, the idea of a universal Nop doesn't always make sense since it
could be that what, exactly, doesn't do anything depends on context
which would be lost on a constant value of an ExtMachInst. For
instance, the value of an ExtMachInst that makes sense might depend on
what mode the CPU was in, etc.

Change-Id: I1f1a43a5c607a667e11b79bcf6e059e4f7141b3f
Reviewed-on: https://gem5-review.googlesource.com/6825
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Gabe Black <gabeblack@google.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/src/arch/x86/isa/microops/base.isa b/src/arch/x86/isa/microops/base.isa
index dc36d0edb..42e7d2e33 100644
--- a/src/arch/x86/isa/microops/base.isa
+++ b/src/arch/x86/isa/microops/base.isa
@@ -59,8 +59,7 @@ let {{
             ''' + generatorNameTemplate + '''(StaticInstPtr curMacroop)
             {
                 static const char *macrocodeBlock = romMnemonic;
-                static const ExtMachInst dummyExtMachInst = \
-                    X86ISA::NoopMachInst;
+                static ExtMachInst dummyExtMachInst;
                 static const EmulEnv dummyEmulEnv(0, 0, 1, 1, 1);
 
                 Macroop * macroop = dynamic_cast<Macroop *>(curMacroop.get());