misc: When unused, set #MatRegClass registers to 0

This is working around an existing SMT issue [1].

The BaseO3CPU uses two physical matrix registers [2]. This is
enough for a single threaded CPU which as of now uses
1 architectural matrix only.

The problem arises when SMT is enabled.  As 2 architectural matrices
need to be supported by a single CPU, the O3CPU won't have any available
register in the freeList for renaming.  This causes the SMT O3CPU to
indefinitely stall renaming [3]

If the archtectural number of registers is seto to 0, the regclass won't
be taken into consideration when evaluating if we can rename
instructions.

This issue has been implicitly fixed for RISCV by a preceding PR [4]

[1]: gem5#668
[2]: https://github.com/gem5/gem5/blob/stable/src/cpu/o3/BaseO3CPU.py#L170
[3]: https://github.com/gem5/gem5/blob/stable/src/cpu/o3/rename.cc#L1228
[4]: gem5#83

Change-Id: I99bfdefff11a246b1f191251dc67689e95b3f0db
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/arch/mips/isa.cc

@@ -105,7 +105,7 @@ constexpr RegClass vecElemClass(VecElemClass, VecElemClassName, 2,
         debug::IntRegs);
 constexpr RegClass vecPredRegClass(VecPredRegClass, VecPredRegClassName, 1,
         debug::IntRegs);
-constexpr RegClass matRegClass(MatRegClass, MatRegClassName, 1, debug::MatRegs);
+constexpr RegClass matRegClass(MatRegClass, MatRegClassName, 0, debug::MatRegs);
 constexpr RegClass ccRegClass(CCRegClass, CCRegClassName, 0, debug::IntRegs);
 
 } // anonymous namespace

src/arch/power/isa.cc

@@ -57,7 +57,7 @@ RegClass vecRegClass(VecRegClass, VecRegClassName, 1, debug::IntRegs);
 RegClass vecElemClass(VecElemClass, VecElemClassName, 2, debug::IntRegs);
 RegClass vecPredRegClass(VecPredRegClass, VecPredRegClassName, 1,
         debug::IntRegs);
-RegClass matRegClass(MatRegClass, MatRegClassName, 1, debug::MatRegs);
+RegClass matRegClass(MatRegClass, MatRegClassName, 0, debug::MatRegs);
 RegClass ccRegClass(CCRegClass, CCRegClassName, 0, debug::IntRegs);
 
 } // anonymous namespace

src/arch/sparc/isa.cc

@@ -74,7 +74,7 @@ RegClass vecRegClass(VecRegClass, VecRegClassName, 1, debug::IntRegs);
 RegClass vecElemClass(VecElemClass, VecElemClassName, 2, debug::IntRegs);
 RegClass vecPredRegClass(VecPredRegClass, VecPredRegClassName, 1,
         debug::IntRegs);
-RegClass matRegClass(MatRegClass, MatRegClassName, 1, debug::MatRegs);
+RegClass matRegClass(MatRegClass, MatRegClassName, 0, debug::MatRegs);
 RegClass ccRegClass(CCRegClass, CCRegClassName, 0, debug::IntRegs);
 
 } // anonymous namespace

src/arch/x86/isa.cc

@@ -147,7 +147,7 @@ RegClass vecRegClass(VecRegClass, VecRegClassName, 1, debug::IntRegs);
 RegClass vecElemClass(VecElemClass, VecElemClassName, 2, debug::IntRegs);
 RegClass vecPredRegClass(VecPredRegClass, VecPredRegClassName, 1,
         debug::IntRegs);
-RegClass matRegClass(MatRegClass, MatRegClassName, 1, debug::MatRegs);
+RegClass matRegClass(MatRegClass, MatRegClassName, 0, debug::MatRegs);
 
 } // anonymous namespace