[MergeICmps] Disable mergeicmps if the target does not want to handle memcmp expansion.

Summary: This is to avoid e.g. merging two cheap icmps if the target is not going to expand to something nice later.

Reviewers: dberlin, spatel

Subscribers: davide, nemanjai

Differential Revision: https://reviews.llvm.org/D38232

llvm-svn: 314970

1 files changed, 15 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Scalar/MergeICmps.cpp b/llvm/lib/Transforms/Scalar/MergeICmps.cpp
index a56cffcdfa0..c68cc53a518 100644
--- a/llvm/lib/Transforms/Scalar/MergeICmps.cpp
+++ b/llvm/lib/Transforms/Scalar/MergeICmps.cpp
@@ -28,6 +28,8 @@
 #include <vector>
 #include "llvm/ADT/APSInt.h"
 #include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/IntrinsicInst.h"
@@ -41,8 +43,6 @@ namespace {
 
 #define DEBUG_TYPE "mergeicmps"
 
-#define MERGEICMPS_DOT_ON
-
 // A BCE atom.
 struct BCEAtom {
   BCEAtom() : GEP(nullptr), LoadI(nullptr), Offset() {}
@@ -589,22 +589,30 @@ class MergeICmps : public FunctionPass {
   bool runOnFunction(Function &F) override {
     if (skipFunction(F)) return false;
     const auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
-    auto PA = runImpl(F, &TLI);
+    const auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
+    auto PA = runImpl(F, &TLI, &TTI);
     return !PA.areAllPreserved();
   }
 
  private:
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<TargetLibraryInfoWrapperPass>();
+    AU.addRequired<TargetTransformInfoWrapperPass>();
   }
 
-  PreservedAnalyses runImpl(Function &F, const TargetLibraryInfo *TLI);
+  PreservedAnalyses runImpl(Function &F, const TargetLibraryInfo *TLI,
+                            const TargetTransformInfo *TTI);
 };
 
-PreservedAnalyses MergeICmps::runImpl(Function &F,
-                                      const TargetLibraryInfo *TLI) {
+PreservedAnalyses MergeICmps::runImpl(Function &F, const TargetLibraryInfo *TLI,
+                                      const TargetTransformInfo *TTI) {
   DEBUG(dbgs() << "MergeICmpsPass: " << F.getName() << "\n");
 
+  // We only try merging comparisons if the target wants to expand memcmp later.
+  // The rationale is to avoid turning small chains into memcmp calls.
+  unsigned MaxLoadSize;
+  if (!TTI->enableMemCmpExpansion(MaxLoadSize)) return PreservedAnalyses::all();
+
   bool MadeChange = false;
 
   for (auto BBIt = ++F.begin(); BBIt != F.end(); ++BBIt) {
@@ -623,6 +631,7 @@ char MergeICmps::ID = 0;
 INITIALIZE_PASS_BEGIN(MergeICmps, "mergeicmps",
                       "Merge contiguous icmps into a memcmp", false, false)
 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 INITIALIZE_PASS_END(MergeICmps, "mergeicmps",
                     "Merge contiguous icmps into a memcmp", false, false)