commit 6f0ec20e8fa23e9588205ea0eaba8f3021abe1ac
author Chandler Carruth <chandlerc@gmail.com> Fri Jul 12 11:18:55 2013 +0000
committer Chandler Carruth <chandlerc@gmail.com> Fri Jul 12 11:18:55 2013 +0000
tree 4922e9ce6f2aed915948586459a599215edbc4b9
parent dd51a0c1e0b3cce8093244533b3505668d16f218

Revert "indvars: Improve LFTR by eliminating truncation when comparing
against a constant."

This reverts commit r186107. It didn't handle wrapping arithmetic in the
loop correctly and thus caused the following C program to count from
0 to UINT64_MAX instead of from 0 to 255 as intended:

  #include <stdio.h>
  int main() {
    unsigned char first = 0, last = 255;
    do { printf("%d\n", first); } while (first++ != last);
  }

Full test case and instructions to reproduce with just the -indvars pass
sent to the original review thread rather than to r186107's commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186152 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/IndVarSimplify.cpp

diff --git a/lib/Transforms/Scalar/IndVarSimplify.cpp b/lib/Transforms/Scalar/IndVarSimplify.cpp
index ddb5b27..df11e92 100644
--- a/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1612,29 +1612,10 @@
                << "  IVCount:\t" << *IVCount << "\n");
 
   IRBuilder<> Builder(BI);
-
-  unsigned CmpIndVarSize = SE->getTypeSizeInBits(CmpIndVar->getType());
-  unsigned ExitCntSize = SE->getTypeSizeInBits(ExitCnt->getType());
-  if (CmpIndVarSize > ExitCntSize) {
-    const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
-    const SCEV *ARStart = AR->getStart();
-    const SCEV *ARStep = AR->getStepRecurrence(*SE);
-    if (isa<SCEVConstant>(ARStart) && isa<SCEVConstant>(IVCount)) {
-      const APInt &Start = cast<SCEVConstant>(ARStart)->getValue()->getValue();
-      const APInt &Count = cast<SCEVConstant>(IVCount)->getValue()->getValue();
-
-      APInt NewLimit;
-      if (cast<SCEVConstant>(ARStep)->getValue()->isNegative())
-        NewLimit = Start - Count.zext(CmpIndVarSize);
-      else
-        NewLimit = Start + Count.zext(CmpIndVarSize);
-      ExitCnt = ConstantInt::get(CmpIndVar->getType(), NewLimit);
-
-      DEBUG(dbgs() << "  Widen RHS:\t" << *ExitCnt << "\n");
-    } else {
-      CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
-                                      "lftr.wideiv");
-    }
+  if (SE->getTypeSizeInBits(CmpIndVar->getType())
+      > SE->getTypeSizeInBits(ExitCnt->getType())) {
+    CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
+                                    "lftr.wideiv");
   }
 
   Value *Cond = Builder.CreateICmp(P, CmpIndVar, ExitCnt, "exitcond");