enhance FoldOpIntoPhi in instcombine to try harder when a phi has

multiple uses.  In some cases, all the uses are the same operation,
so instcombine can go ahead and promote the phi.  In the testcase
this pushes an add out of the loop.

llvm-svn: 123568

1 files changed, 19 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 6d05466b9be..23e76ee499f 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -519,9 +519,17 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
     return 0;
   
   // We normally only transform phis with a single use, unless we're trying
-  // hard to make jump threading happen.
-  if (!PN->hasOneUse())
-    return 0;
+  // hard to make jump threading happen.  However, if a PHI has multiple uses
+  // and they are all the same operation, we can fold *all* of the uses into the
+  // PHI.
+  if (!PN->hasOneUse()) {
+    // Walk the use list for the instruction, comparing them to I.
+    for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end();
+         UI != E; ++UI)
+      if (!I.isIdenticalTo(cast<Instruction>(*UI))) 
+        return 0;
+    // Otherwise, we can replace *all* users with the new PHI we form.
+  }
   
   // Check to see if all of the operands of the PHI are simple constants
   // (constantint/constantfp/undef).  If there is one non-constant value,
@@ -628,6 +636,14 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
       NewPN->addIncoming(InV, PN->getIncomingBlock(i));
     }
   }
+  
+  for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end();
+       UI != E; ) {
+    Instruction *User = cast<Instruction>(*UI++);
+    if (User == &I) continue;
+    ReplaceInstUsesWith(*User, NewPN);
+    EraseInstFromFunction(*User);
+  }
   return ReplaceInstUsesWith(I, NewPN);
 }