[BasicAA] Take attributes into account when requesting modref info for a call site

Differential Revision: https://reviews.llvm.org/D29989

llvm-svn: 296617

1 files changed, 31 insertions, 9 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 55f40a34839..d3898aad8a7 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -749,7 +749,11 @@ ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS,
   // as an argument, and itself doesn't capture it.
   if (!isa<Constant>(Object) && CS.getInstruction() != Object &&
       isNonEscapingLocalObject(Object)) {
-    bool PassedAsArg = false;
+
+    // Optimistically assume that call doesn't touch Object and check this
+    // assumption in the following loop.
+    ModRefInfo Result = MRI_NoModRef;
+
     unsigned OperandNo = 0;
     for (auto CI = CS.data_operands_begin(), CE = CS.data_operands_end();
          CI != CE; ++CI, ++OperandNo) {
@@ -761,20 +765,38 @@ ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS,
            OperandNo < CS.getNumArgOperands() && !CS.isByValArgument(OperandNo)))
         continue;
 
+      // Call doesn't access memory through this operand, so we don't care
+      // if it aliases with Object.
+      if (CS.doesNotAccessMemory(OperandNo))
+        continue;
+
       // If this is a no-capture pointer argument, see if we can tell that it
-      // is impossible to alias the pointer we're checking.  If not, we have to
-      // assume that the call could touch the pointer, even though it doesn't
-      // escape.
+      // is impossible to alias the pointer we're checking.
       AliasResult AR =
           getBestAAResults().alias(MemoryLocation(*CI), MemoryLocation(Object));
-      if (AR) {
-        PassedAsArg = true;
-        break;
+
+      // Operand doesnt alias 'Object', continue looking for other aliases
+      if (AR == NoAlias)
+        continue;
+      // Operand aliases 'Object', but call doesn't modify it. Strengthen
+      // initial assumption and keep looking in case if there are more aliases.
+      if (CS.onlyReadsMemory(OperandNo)) {
+        Result = static_cast<ModRefInfo>(Result | MRI_Ref);
+        continue;
+      }
+      // Operand aliases 'Object' but call only writes into it.
+      if (CS.doesNotReadMemory(OperandNo)) {
+        Result = static_cast<ModRefInfo>(Result | MRI_Mod);
+        continue;
       }
+      // This operand aliases 'Object' and call reads and writes into it.
+      Result = MRI_ModRef;
+      break;
     }
 
-    if (!PassedAsArg)
-      return MRI_NoModRef;
+    // Early return if we improved mod ref information
+    if (Result != MRI_ModRef)
+      return Result;
   }
 
   // If the CallSite is to malloc or calloc, we can assume that it doesn't