[PATCH] D146468: [LFTR] Use evaluateAtIteration in genLoopLimit [nfc]

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: nikic, fhahn.
Herald added subscribers: StephenFan, bollu, hiraditya, mcrosier.
Herald added a project: All.
reames requested review of this revision.
Herald added a project: LLVM.

Note that the comments being remove appear to be very out of sync with the actual code in question.

I am not entirely sure of this change, and am not strongly motivated to pursue it.  It mostly came out of discussion on https://reviews.llvm.org/D146457, and me going "why doesn't LFTR use this?".  I never found a good answer.


https://reviews.llvm.org/D146468

Files:
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp


Index: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -917,27 +917,13 @@
   assert(AR->getStepRecurrence(*SE)->isOne() && "only handles unit stride");
 
   // IVInit may be a pointer while ExitCount is an integer when FindLoopCounter
-  // finds a valid pointer IV. Sign extend ExitCount in order to materialize a
-  // GEP. Avoid running SCEVExpander on a new pointer value, instead reusing
-  // the existing GEPs whenever possible.
+  // finds a valid pointer IV.
   if (IndVar->getType()->isPointerTy() &&
       !ExitCount->getType()->isPointerTy()) {
-    // IVOffset will be the new GEP offset that is interpreted by GEP as a
-    // signed value. ExitCount on the other hand represents the loop trip count,
-    // which is an unsigned value. FindLoopCounter only allows induction
-    // variables that have a positive unit stride of one. This means we don't
-    // have to handle the case of negative offsets (yet) and just need to zero
-    // extend ExitCount.
-    Type *OfsTy = SE->getEffectiveSCEVType(IVInit->getType());
-    const SCEV *IVOffset = SE->getTruncateOrZeroExtend(ExitCount, OfsTy);
-    if (UsePostInc)
-      IVOffset = SE->getAddExpr(IVOffset, SE->getOne(OfsTy));
 
-    // Expand the code for the iteration count.
-    assert(SE->isLoopInvariant(IVOffset, L) &&
-           "Computed iteration count is not loop invariant!");
+    const SCEVAddRecExpr *ARBase = UsePostInc ? AR->getPostIncExpr(*SE) : AR;
+    const SCEV *IVLimit = ARBase->evaluateAtIteration(ExitCount, *SE);
 
-    const SCEV *IVLimit = SE->getAddExpr(IVInit, IVOffset);
     BranchInst *BI = cast<BranchInst>(ExitingBB->getTerminator());
     return Rewriter.expandCodeFor(IVLimit, IndVar->getType(), BI);
   } else {


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