Have loop-rotate simplify instructions (yay instsimplify!) as it clones

them into the loop preheader, eliminating silly instructions like
"icmp i32 0, 100" in fixed tripcount loops.  This also better exposes the 
bigger problem with loop rotate that I'd like to fix: once this has been
folded, the duplicated conditional branch *often* turns into an uncond branch.

Not aggressively handling this is pessimizing later loop optimizations 
somethin' fierce by making "dominates all exit blocks" checks fail.

llvm-svn: 123060

Author: lattner

llvm/lib/Transforms/Scalar/LoopRotation.cpp

@@ -14,9 +14,10 @@
 #define DEBUG_TYPE "loop-rotate"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Function.h"
-#include "llvm/Analysis/LoopPass.h"
-#include "llvm/Analysis/DominanceFrontier.h"
 #include "llvm/Analysis/CodeMetrics.h"
+#include "llvm/Analysis/DominanceFrontier.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
@@ -205,9 +206,24 @@ bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
     // Otherwise, create a duplicate of the instruction.
     Instruction *C = Inst->clone();
 
-    C->setName(Inst->getName());
-    C->insertBefore(LoopEntryBranch);
-    ValueMap[Inst] = C;
+    // Eagerly remap the operands of the instruction.
+    RemapInstruction(C, ValueMap,
+                     RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
+    
+    // With the operands remapped, see if the instruction constant folds or is
+    // otherwise simplifyable.  This commonly occurs because the entry from PHI
+    // nodes allows icmps and other instructions to fold.
+    if (Value *V = SimplifyInstruction(C)) {
+      // If so, then delete the temporary instruction and stick the folded value
+      // in the map.
+      delete C;
+      ValueMap[Inst] = V;
+    } else {
+      // Otherwise, stick the new instruction into the new block!
+      C->setName(Inst->getName());
+      C->insertBefore(LoopEntryBranch);
+      ValueMap[Inst] = C;
+    }
   }
 
   // Along with all the other instructions, we just cloned OrigHeader's

llvm/test/Transforms/LoopRotate/phi-duplicate.ll

@@ -34,14 +34,13 @@ for.end:                                          ; preds = %for.cond
 
 ; CHECK:      define void @test
 ; CHECK-NEXT: entry:
-; CHECK-NEXT:   icmp slt i64
-; CHECK-NEXT:   br i1
+; CHECK-NEXT:   br i1 true, label %bb.nph, label %for.end
 ; CHECK-NOT:  :
 ; CHECK:      bb.nph:
 ; CHECK-NEXT:   br label %for.body
 ; CHECK-NOT:  :
 ; CHECK:      for.body:
-; CHECK-NEXT:   %j.02 = phi i64
+; CHECK-NEXT:   %j.01 = phi i64
 ; CHECK-NOT:    phi
 ; CHECK:        ret void
 ; CHECK-NEXT: }