[LoopStrengthReduce, x86] don't add cost for a cmp that will be macro-fused (PR35681)

In the motivating case from PR35681 and represented by the macro-fuse-cmp test:
https://bugs.llvm.org/show_bug.cgi?id=35681
...there's a 37 -> 31 byte size win for the loop because we eliminate the big base 
address offsets.

SPEC2017 on Ryzen shows no significant perf difference.

Differential Revision: https://reviews.llvm.org/D42607

llvm-svn: 324289

2 files changed, 11 insertions, 0 deletions

diff --git a/llvm/include/llvm/Analysis/TargetTransformInfo.h b/llvm/include/llvm/Analysis/TargetTransformInfo.h
index fe13e6e9894..b1abc792333 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfo.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfo.h
@@ -471,6 +471,11 @@ public:
   bool isLSRCostLess(TargetTransformInfo::LSRCost &C1,
                      TargetTransformInfo::LSRCost &C2) const;
 
+  /// Return true if the target can fuse a compare and branch.
+  /// Loop-strength-reduction (LSR) uses that knowledge to adjust its cost
+  /// calculation for the instructions in a loop.
+  bool canMacroFuseCmp() const;
+
   /// \brief Return true if the target supports masked load/store
   /// AVX2 and AVX-512 targets allow masks for consecutive load and store
   bool isLegalMaskedStore(Type *DataType) const;
@@ -978,6 +983,7 @@ public:
                                      Instruction *I) = 0;
   virtual bool isLSRCostLess(TargetTransformInfo::LSRCost &C1,
                              TargetTransformInfo::LSRCost &C2) = 0;
+  virtual bool canMacroFuseCmp() = 0;
   virtual bool isLegalMaskedStore(Type *DataType) = 0;
   virtual bool isLegalMaskedLoad(Type *DataType) = 0;
   virtual bool isLegalMaskedScatter(Type *DataType) = 0;
@@ -1197,6 +1203,9 @@ public:
                      TargetTransformInfo::LSRCost &C2) override {
     return Impl.isLSRCostLess(C1, C2);
   }
+  bool canMacroFuseCmp() override {
+    return Impl.canMacroFuseCmp();
+  }
   bool isLegalMaskedStore(Type *DataType) override {
     return Impl.isLegalMaskedStore(DataType);
   }
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
index 73e8aa22b3c..3bc73d52f2f 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
@@ -246,6 +246,8 @@ public:
                     C2.ScaleCost, C2.ImmCost, C2.SetupCost);
   }
 
+  bool canMacroFuseCmp() { return false; }
+
   bool isLegalMaskedStore(Type *DataType) { return false; }
 
   bool isLegalMaskedLoad(Type *DataType) { return false; }