[PeepholeOptimizer] Look through PHIs to find additional register sou…

…rces

Reintroduce r245442. Remove an overly conservative assertion introduced
in r245442. We could replace the assertion to use `shareSameRegisterFile`
instead, but in that point in `insertPHI` we already lost the original
Def subreg to check against. So drop the assertion completely.

Original commit message:

- Teaches the ValueTracker in the PeepholeOptimizer to look through PHI
instructions.
- Add findNextSourceAndRewritePHI method to lookup into multiple sources
returnted by the ValueTracker and rewrite PHIs with new sources.

With these changes we can find more register sources and rewrite more
copies to allow coaslescing of bitcast instructions. Hence, we eliminate
unnecessary VR64 <-> GR64 copies in x86, but it could be extended to
other archs by marking "isBitcast" on target specific instructions. The
x86 example follows:

A:
  psllq %mm1, %mm0
  movd  %mm0, %r9
  jmp C

B:
  por %mm1, %mm0
  movd  %mm0, %r9
  jmp C

C:
  movd  %r9, %mm0
  pshufw  $238, %mm0, %mm0

Becomes:

A:
  psllq %mm1, %mm0
  jmp C

B:
  por %mm1, %mm0
  jmp C

C:
  pshufw  $238, %mm0, %mm0

Differential Revision: http://reviews.llvm.org/D11197
rdar://problem/20404526

llvm-svn: 245479

llvm/include/llvm/Target/TargetInstrInfo.h

@@ -1303,6 +1303,33 @@ class TargetInstrInfo : public MCInstrInfo {
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;
 };
 
+/// \brief Provide DenseMapInfo for TargetInstrInfo::RegSubRegPair.
+template<>
+struct DenseMapInfo<TargetInstrInfo::RegSubRegPair> {
+  typedef DenseMapInfo<unsigned> RegInfo;
+
+  static inline TargetInstrInfo::RegSubRegPair getEmptyKey() {
+    return TargetInstrInfo::RegSubRegPair(RegInfo::getEmptyKey(),
+                         RegInfo::getEmptyKey());
+  }
+  static inline TargetInstrInfo::RegSubRegPair getTombstoneKey() {
+    return TargetInstrInfo::RegSubRegPair(RegInfo::getTombstoneKey(),
+                         RegInfo::getTombstoneKey());
+  }
+  /// \brief Reuse getHashValue implementation from
+  /// std::pair<unsigned, unsigned>.
+  static unsigned getHashValue(const TargetInstrInfo::RegSubRegPair &Val) {
+    std::pair<unsigned, unsigned> PairVal =
+        std::make_pair(Val.Reg, Val.SubReg);
+    return DenseMapInfo<std::pair<unsigned, unsigned>>::getHashValue(PairVal);
+  }
+  static bool isEqual(const TargetInstrInfo::RegSubRegPair &LHS,
+                      const TargetInstrInfo::RegSubRegPair &RHS) {
+    return RegInfo::isEqual(LHS.Reg, RHS.Reg) &&
+           RegInfo::isEqual(LHS.SubReg, RHS.SubReg);
+  }
+};
+
 } // End llvm namespace
 
 #endif