From: Gabe Black <gabe.black@gmail.com>
Date: Sun, 1 Nov 2020 09:57:29 +0000 (-0800)
Subject: cpu: Access src and dest reg indexes using a pointer to member.
X-Git-Tag: develop-gem5-snapshot~454
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=9fc5344c0a82eb4f0cac3c06bfbd110759df6677;p=gem5.git

cpu: Access src and dest reg indexes using a pointer to member.

This will eventually let subclasses provide their own appropriately
sized storage for these indexes. By using a pointer to member instead of
a regular pointer, we ensure that even if the StaticInst is copied/moved
somewhere, it will still find its indexes correctly, without any
additional performance overhead or maintenance.

Unfortunately C++ has decided that arrays with known bounds are not
convertible/compatible with arrays with unknown bounds. I've found at
least two standards proposals in various stages of acceptance which say
that that's dumb and they should change that (because it's dumb and they
should change that), but in the mean time we can get everything to
compile by using the reinterpret_cast hammer. While this is
*technically* undefined behavior, it's basically not and should be
pretty safe.

Change-Id: Id747b0cf68d1a0b4809ebb66a32472187110d7d8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36876
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
---

diff --git a/src/cpu/static_inst.hh b/src/cpu/static_inst.hh
index 4f3dd04fe..22e55ca40 100644
--- a/src/cpu/static_inst.hh
+++ b/src/cpu/static_inst.hh
@@ -93,11 +93,16 @@ class StaticInst : public RefCounted, public StaticInstFlags
         MaxInstDestRegs = TheISA::MaxInstDestRegs       //< Max dest regs
     };
 
+    using RegIdArrayPtr = RegId (StaticInst:: *)[];
+
   private:
-    /// See destRegIdx().
-    RegId _destRegIdx[MaxInstDestRegs];
     /// See srcRegIdx().
     RegId _srcRegIdx[MaxInstSrcRegs];
+    RegIdArrayPtr _srcRegIdxPtr = nullptr;
+
+    /// See destRegIdx().
+    RegId _destRegIdx[MaxInstDestRegs];
+    RegIdArrayPtr _destRegIdxPtr = nullptr;
 
   protected:
 
@@ -236,15 +241,23 @@ class StaticInst : public RefCounted, public StaticInstFlags
 
     /// Return logical index (architectural reg num) of i'th destination reg.
     /// Only the entries from 0 through numDestRegs()-1 are valid.
-    const RegId& destRegIdx(int i) const { return _destRegIdx[i]; }
+    const RegId &destRegIdx(int i) const { return (this->*_destRegIdxPtr)[i]; }
 
-    void setDestRegIdx(int i, const RegId &val) { _destRegIdx[i] = val; }
+    void
+    setDestRegIdx(int i, const RegId &val)
+    {
+        (this->*_destRegIdxPtr)[i] = val;
+    }
 
     /// Return logical index (architectural reg num) of i'th source reg.
     /// Only the entries from 0 through numSrcRegs()-1 are valid.
-    const RegId& srcRegIdx(int i)  const { return _srcRegIdx[i]; }
+    const RegId &srcRegIdx(int i) const { return (this->*_srcRegIdxPtr)[i]; }
 
-    void setSrcRegIdx(int i, const RegId &val) { _srcRegIdx[i] = val; }
+    void
+    setSrcRegIdx(int i, const RegId &val)
+    {
+        (this->*_srcRegIdxPtr)[i] = val;
+    }
 
     /// Pointer to a statically allocated "null" instruction object.
     static StaticInstPtr nullStaticInstPtr;
@@ -283,10 +296,15 @@ class StaticInst : public RefCounted, public StaticInstFlags
     /// the fields that are meaningful for the particular
     /// instruction.
     StaticInst(const char *_mnemonic, ExtMachInst _machInst, OpClass __opClass)
-        : _opClass(__opClass), _numSrcRegs(0), _numDestRegs(0),
-          _numFPDestRegs(0), _numIntDestRegs(0), _numCCDestRegs(0),
-          _numVecDestRegs(0), _numVecElemDestRegs(0), _numVecPredDestRegs(0),
-          machInst(_machInst), mnemonic(_mnemonic), cachedDisassembly(0)
+        : _srcRegIdxPtr(
+                reinterpret_cast<RegIdArrayPtr>(&StaticInst::_srcRegIdx)),
+          _destRegIdxPtr(
+                reinterpret_cast<RegIdArrayPtr>(&StaticInst::_destRegIdx)),
+          _opClass(__opClass),
+          _numSrcRegs(0), _numDestRegs(0), _numFPDestRegs(0),
+          _numIntDestRegs(0), _numCCDestRegs(0), _numVecDestRegs(0),
+          _numVecElemDestRegs(0), _numVecPredDestRegs(0), machInst(_machInst),
+          mnemonic(_mnemonic), cachedDisassembly(0)
     { }
 
   public: